Glossary of Terms¶

Absolute Humidity¶

The actual mass of water vapor present in a given volume of air, usually expressed in grams per cubic metre.

It states how much water is present rather than comparing it to any maximum, but it changes when air expands or compresses even if no water was added or removed.

Example: Very cold or desert air may hold under 1 g/m³; humid tropical air may hold 30 g/m³.

Contrast with: Relative Humidity

Absolute Zero¶

The lowest possible temperature, at which atomic motion reaches its minimum, equal to −273.15 °C or −459.67 °F.

Nothing can be colder, because atoms cannot move less than not at all. Laboratories have approached within billionths of a degree but the point itself cannot be reached.

See also: Kelvin Scale, Kinetic Energy Of Atoms

Acceleration¶

The rate at which velocity changes, measured in metres per second squared.

Seismic shaking is acceleration: the ground repeatedly speeds up and slows down in different directions. Ground acceleration is often expressed as a fraction of gravity, which is 9.81 metres per second squared.

Example: A ground acceleration of 0.5 g means a sideways force equal to half the weight of the building experiencing it.

Accelerometer¶

A sensor that measures acceleration by detecting the displacement of a suspended mass relative to its housing.

See also: MEMS Accelerometer, Inertial Mass

Accuracy¶

How close a measurement is to the true value of the quantity being measured.

Accuracy is one of the two independent quality measures used throughout this book. A sensor can be accurate on average while still being unreliable from one reading to the next.

Example: A thermometer reading 21.0 °C when the true air temperature is 21.1 °C is accurate. One reading 25.0 °C is not, even if it reads 25.0 °C every single time.

See also: Precision, Measurement Uncertainty, Calibration

Agricultural Planning¶

The use of environmental measurements to decide what to plant, when to plant it, when to irrigate, and when to harvest.

Most of it runs on accumulated warmth rather than elapsed days, since crops develop according to the total heat they receive.

Example: Summing daily average temperatures above a crop-specific base value gives growing degree days, which predict flowering and harvest more reliably than a calendar.

See also: Evapotranspiration, Air Temperature

Air Mass¶

A large volume of air, often hundreds of kilometres across, in which temperature and moisture are roughly uniform throughout.

Air masses take on the character of the region where they form and carry those properties with them when they move. Most day-to-day change at a monitoring station is one air mass replacing another.

Example: Air that has sat over the Gulf of Mexico becomes warm and moist; air that has sat over northern Canada becomes cold and dry.

Air Quality¶

The degree to which the air is free of pollutants harmful to health or visibility.

A weather station influences air quality assessment without measuring pollution directly, because wind disperses pollutants and calm conditions let them accumulate.

Example: A temperature inversion traps vehicle exhaust near the ground and produces a severe smog episode.

Air Temperature¶

The temperature of the air itself, measured in shade with free airflow at a standard height.

This is the quantity reported in weather forecasts. It differs from the temperature of any nearby surface, sometimes by tens of degrees.

See also: Surface Temperature, Stevenson Screen

Albedo¶

The fraction of incoming solar radiation that a surface reflects, ranging from zero for a perfect absorber to one for a perfect reflector.

Example: Fresh snow reflects about 85 percent of sunlight; asphalt reflects under 10 percent, which is why dark pavement grows so hot.

See also: Solar Radiation, Urban Heat Island

Altimeter¶

An instrument that indicates altitude by measuring atmospheric pressure and converting it using an assumed standard atmosphere.

Because real pressure differs from the standard, pilots set the current local sea level pressure before landing. An uncorrected altimeter flown from high pressure into low pressure reads higher than the aircraft actually is.

See also: Pressure Altitude, Sea Level Pressure

Analog Signal¶

An electrical signal that varies smoothly and can take any value within its range.

Example: The voltage produced by a thermocouple changes continuously as the temperature changes, passing through every intermediate value.

Contrast with: Digital Signal

Analog To Digital Conversion¶

The process of sampling a smoothly varying signal at particular instants and representing each sample as a number.

Two approximations happen at once: sampling discards what occurred between instants, and quantization rounds each sample to one of a fixed set of levels.

Example: A 12-bit converter covering 0 to 3.3 volts has 4,096 available levels, so the smallest change it can report is about 0.8 millivolts.

See also: Analog Signal, Digital Signal, Resolution

Anemometer¶

An instrument that measures wind speed.

Example: A cup anemometer on a school roof spins faster as the wind rises, and its rotation rate is converted to metres per second.

See also: Cup Anemometer, Sonic Anemometer

Aneroid Barometer¶

A barometer that measures atmospheric pressure using a sealed metal capsule with most of the air removed, whose flexible walls flex as pressure changes.

Invented by Lucien Vidi in 1844, it replaced the mercury column with something portable, which made barometers common aboard ship and eventually made weather forecasting possible.

See also: Mercury Barometer, Barometric Pressure

Apparent Temperature¶

The temperature a person perceives, accounting for the combined effect of air temperature, humidity, wind, and sometimes sunlight.

It is what a forecast means by "feels like." In cold conditions it is dominated by wind; in hot conditions by humidity.

Atmosphere¶

The layer of gases held around a planet by gravity.

Earth's atmosphere is about 78 percent nitrogen and 21 percent oxygen, with water vapor and other gases making up the remaining one percent. It has no upper boundary; it simply thins with height.

Example: About three quarters of the atmosphere's mass lies below 12 kilometres, in the layer where all weather occurs.

See also: Atmospheric Layer, One Atmosphere

Atmospheric Layer¶

One of the bands into which the atmosphere is divided by height, each with distinct temperature behaviour.

From the ground upward these are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. All weather occurs in the lowest one.

Example: The troposphere extends to roughly 12 kilometres and contains about three quarters of the atmosphere's mass.

Atomic Clock¶

A clock that keeps time by counting the extremely regular oscillations of atoms, typically caesium or rubidium.

Atomic clocks drift by less than a second over millions of years. Each satellite in a navigation system carries one, and the accuracy of the whole system depends on them.

See also: GPS Satellite, Coordinated Universal Time

Aviation Safety¶

The set of practices and measurements that keep aircraft operating without accident, several of which depend directly on atmospheric data.

Altimeters are barometers, so aircraft separation depends on pressure. Crosswind limits determine whether a runway can be used, and sudden changes in wind with height are hazardous during takeoff and landing.

Example: A pilot resets the altimeter to the airport's reported sea level pressure before beginning an approach.

Axis Labeling¶

The practice of stating what each axis of a chart shows, in what units, and over what range.

A truncated vertical axis is not dishonest in itself, since some scales have no meaningful zero. The test is whether the chosen range matches the claim being made and whether the scale is disclosed clearly enough that a reader cannot mistake it.

Example: The same week of temperature data looks alarming on a 0.8-degree axis and flat on a 40-degree one.

See also: Line Chart, Science Communication

Barometric Forecasting¶

Predicting weather from the direction and rate of change of atmospheric pressure.

The rate of change carries more information than the value itself. A rapid fall indicates approaching bad weather hours before there is anything visible in the sky.

Example: A fall of more than three hectopascals in three hours indicates deteriorating conditions.

See also: Pressure Tendency, Barometric Pressure

Barometric Pressure¶

The pressure exerted at a point by the weight of the atmosphere above it.

Also called atmospheric pressure. At sea level it averages about 1013.25 hectopascals, and it falls with height because less air remains overhead.

Example: A station at sea level typically reads between 980 and 1040 hectopascals; values outside that band suggest either a storm or a sensor problem.

See also: Pressure, Sea Level Pressure, One Atmosphere

Base Station¶

A computer at a fixed, convenient location that collects data from one or more remote stations.

Having mains power, a reliable network, and no weather exposure, a base station can hold the full archive, run charts, alert an operator when a station goes silent, and serve as one of the required backup copies.

Example: A Raspberry Pi indoors collects hourly uploads, charts them, and emails an alert if a station goes silent.

Battery Capacity¶

The quantity of charge a battery can store, expressed in milliamp-hours or amp-hours.

Usable capacity is always less than the rating: batteries should not be fully discharged, and capacity falls substantially in cold weather, which arrives in the same season as the lowest solar input.

Example: A 20,000 mAh lithium battery yields about 16,000 mAh usable, and less again at 0 °C.

See also: Power Budget, Charge Controller

Beaufort Scale¶

A thirteen-point scale from 0 to 12 relating wind speed to observable effects on land and at sea.

Written by Francis Beaufort in 1805 and adopted by the Royal Navy in 1838, it solved the problem of making observations comparable worldwide without requiring any instrument at all.

Example: Force 7, described as whole trees in motion and walking made difficult, corresponds to 13.9 to 17.1 metres per second.

BME280 Sensor¶

A combined environmental sensor from Bosch Sensortec measuring temperature, barometric pressure, and relative humidity in a package about 2.5 millimetres square.

It contains three sensing elements using three different mechanisms. Its temperature element also compensates the other two, so a program reading pressure must read temperature first.

See also: Piezoresistive Effect, Capacitive Humidity Sensor, Silicon Diode Sensor

Boiling Point Of Water¶

The temperature at which water changes from liquid to gas throughout its volume, which at standard atmospheric pressure is 100 °C or 212 °F.

It is one of the two fixed points used to define the Celsius and Fahrenheit scales, and it varies with pressure, occurring at lower temperatures at high altitude.

See also: Celsius Scale, Freezing Point Of Water

Bolometer¶

An instrument that measures radiant energy by detecting the change in electrical resistance of a blackened absorber as it warms.

Invented by Samuel Langley in 1880, it was sensitive enough to detect temperature changes of a ten-thousandth of a degree and allowed the solar spectrum to be mapped in detail.

Boundary Layer¶

The thin film of air held against a surface, which insulates that surface from the air beyond it.

The concept explains wind chill: moving air strips away the warmed film against skin and replaces it with cold air, increasing the rate of heat loss.

Breadboard¶

A plastic block containing spring-loaded holes that grip inserted wires, allowing circuits to be assembled without soldering.

Holes are connected in fixed groups: long power rails along the edges, and short rows of five in the main area that do not connect across the central channel.

Building Code¶

A set of legal requirements governing how structures must be designed and built.

Codes specify a design wind speed and a seismic design category for each region, both derived from statistical analysis of decades of instrument records. Environmental measurement becomes law at this point.

Example: A roof in coastal Florida must withstand a far higher design wind speed than one in Ohio, because the anemometer records differ.

See also: Wind Load, Magnitude Versus Intensity

Calibration¶

The process of comparing an instrument against a reference standard and correcting the difference, either by adjusting the instrument or by recording an offset to apply to its readings.

Calibration links an inexpensive sensor to the worldwide system of measurement standards through a chain of comparisons.

Example: Placing a thermometer in an ice-water bath, finding it reads 0.4 °C, and recording a −0.4 °C offset to subtract from every future reading.

See also: Reference Standard, Accuracy, Sensor Drift

Capacitive Humidity Sensor¶

A humidity sensor in which a moisture-absorbing polymer film between two electrodes changes capacitance in proportion to relative humidity.

Introduced by Vaisala in 1973, this design is fast, small, low-power, and manufacturable on a chip, and it is used in essentially every electronic humidity sensor sold today.

See also: Capacitive Sensing, Relative Humidity

Capacitive Sensing¶

A transduction method that detects a physical change by measuring the change it causes in the ability of two conductors to store electric charge.

Example: A polymer film between two metal plates absorbs water vapor from the air, changing how much charge the gap can hold.

See also: Capacitive Humidity Sensor, Transduction

Cellular Data Link¶

A connection to a remote station carried over the mobile telephone network.

It works anywhere with coverage and frees a station's location from proximity to a building, at the cost of a data plan and higher power consumption.

Example: A station in a field with no nearby building can still report every hour through the mobile network.

Celsius Scale¶

A temperature scale on which water freezes at 0 degrees and boils at 100 degrees under standard atmospheric pressure.

Proposed by Anders Celsius in 1742 with the values reversed from their modern arrangement, and inverted to the present form shortly after his death.

See also: Fahrenheit Scale, Kelvin Scale, Temperature Conversion

Charge Controller¶

A device placed between a solar panel and a battery that regulates charging to prevent damage.

It protects against overcharge and over-discharge, blocks current flowing back through the panel at night, and applies the correct charging profile for the battery chemistry. It is not optional.

Example: Connecting a panel straight to a lithium battery without one risks overcharging and fire.

Chilled Mirror Hygrometer¶

An instrument that measures dew point directly by cooling a mirror until condensation forms on it and recording the temperature at that instant.

These are the most accurate humidity instruments available and serve as reference standards against which cheaper sensors are calibrated.

Citizen Science¶

Scientific work carried out wholly or partly by members of the public, often in collaboration with professional researchers.

Several environmental networks accept amateur data and feed it into professional products, including official forecasts and earthquake catalogues.

Example: CoCoRaHS was founded after a 1997 flash flood that a sparse official gauge network entirely missed, and its volunteer measurements now inform official drought monitoring.

See also: Data Sharing, Science Communication

Climate¶

The pattern of weather at a place averaged over a long period, conventionally thirty years.

Climate describes what usually happens rather than what is happening. It cannot be measured directly by any instrument; it is computed from decades of weather observations.

Example: Expecting a July afternoon in Arizona to be hot and dry is a statement about climate; the rain that falls on your picnic is weather.

Contrast with: Weather

Climate Record¶

A long series of environmental measurements used to describe conditions over decades or centuries.

The global instrumental record begins around 1850. Building a usable record requires detecting and correcting every instrument change, station relocation, and sensor drift, which is what makes a century of differing instruments comparable at all.

Example: The global instrumental temperature record begins around 1850, when enough standardized stations existed to compute a meaningful average.

Climatologist¶

A scientist who studies weather patterns over decades, centuries, or longer.

Because the instrumental record is short, climatologists also read proxy records such as tree rings, ice cores, and lake sediments.

Example: A climatologist might read tree rings or ice cores to reconstruct temperatures from centuries before thermometers existed.

Contrast with: Meteorologist

Cloud Formation¶

The process by which rising, cooling air reaches saturation and its water vapor condenses into visible droplets on airborne particles.

Example: Sun-warmed air rises, cools as it expands with height, and condenses at the altitude where it can no longer hold its vapor, producing a cloud base at a distinct level.

See also: Condensation, Convection, Dew Point

Command Line Interface¶

A means of controlling a computer by typing text commands rather than manipulating graphical objects.

It consumes almost no power, works well over slow or unreliable network links, and is how a station with no screen is operated.

See also: Shell Command, Operating System

Condensation¶

The change of water from vapor to liquid, occurring when air is cooled past the point at which it can sustain all the vapor it contains.

Condensation releases the energy that evaporation absorbed, warming the surrounding air. That released energy is what powers thunderstorms and hurricanes.

Contrast with: Evaporation

Condensation Nuclei¶

Microscopic airborne particles, such as dust, salt, or pollen, onto which water vapor condenses to form cloud droplets.

Without such particles, water vapor can remain in the air well past the point at which it would otherwise condense.

Conditional Statement¶

A programming construct that runs different code depending on whether a stated condition is true.

Python evaluates conditions in order and runs the first that is true, so the order of tests changes the result.

Example: Testing whether a pressure reading falls outside a plausible range before trusting it.

Convection¶

The transfer of heat by the bulk movement of a fluid, in which warmed fluid rises and cooler fluid sinks to replace it.

Convection is the mechanism that lifts warm moist air to the altitude where clouds form.

See also: Heat Transfer, Cloud Formation

Coordinate System¶

An agreed set of rules for describing a position using numbers, comprising an origin, directions, and units.

Example: Latitude and longitude form a coordinate system with the equator and the prime meridian as its two reference lines.

See also: Latitude, Longitude, WGS 84 Datum

Coordinated Universal Time¶

The world's primary time standard, having no time zones and no daylight saving adjustment, maintained by a worldwide network of atomic clocks.

Remote stations record timestamps in this standard because local time produces duplicate hours, missing hours, and records that will not sort correctly.

Example: A reading stamped 2026-08-25T14:30:00Z means the same instant to a reader in any country.

Coriolis Effect¶

The apparent deflection of moving air caused by the Earth's rotation beneath it, to the right in the northern hemisphere and to the left in the southern.

It is why weather systems appear as spirals rather than as air rushing straight from high pressure to low.

Correlation¶

A measure of how strongly two variables move together, summarized by a coefficient ranging from −1 to +1.

Correlation never establishes causation. Two quantities may move together because one drives the other, because both respond to a third factor, or by coincidence.

Example: Temperature and relative humidity typically correlate near −0.8, because warming air raises its saturation capacity without adding water.

CSV File¶

A plain text file in which each line is one record and fields within a line are separated by commas.

Its virtues for long-term monitoring are that anything can read it, a person can inspect it directly, appending requires no rewriting, corruption is contained to damaged lines, and it will still open decades from now.

Example: A file whose first line reads timestamp,temperature_c,pressure_hpa and whose remaining lines are readings.

Cup Anemometer¶

A wind speed instrument with three or four hemispherical cups on horizontal arms that spin about a vertical shaft.

Invented by Thomas Romney Robinson in 1846, it works regardless of wind direction. It over-reads in gusty conditions because the cups have inertia, and it stalls entirely below about half a metre per second.

Current¶

The rate at which electric charge flows past a point, measured in amperes.

Current is drawn by a device rather than applied to it: a device takes what it needs from a supply capable of providing more.

Example: A BME280 draws under one milliamp, so a pin capable of 50 milliamps has ample headroom.

Data Backup¶

The keeping of copies of data in locations other than the station that produced it.

The common guidance is three copies, on two kinds of media, with one in a different physical location. A backup that has never been restored and inspected is not yet a backup.

Example: The card in the station, a copy on a school computer, and a copy in a repository satisfies the three-copy rule.

Data Field¶

One individual value within a data record.

Example: In a record containing a timestamp, temperature, and pressure, the temperature value is one field.

Data Logging¶

The automatic recording of measurements over time, with each reading stored alongside the time it was taken.

Example: Appending a timestamped row to a CSV file once a minute, unattended, for a year.

See also: Sampling Interval, Time Series

Data Record¶

One complete observation, comprising all readings taken at a single moment together with the time they were taken.

Example: One line of a log file containing a timestamp and five sensor values.

Data Storage¶

Where and how collected data physically resides.

On a small computer this usually means a memory card, whose limiting characteristics are finite write endurance and a tendency to fail suddenly and completely rather than degrading gracefully.

Example: A year of one-minute readings occupies about 32 megabytes, so card reliability rather than capacity is the limiting factor.

Data Type¶

The kind of value a variable holds, which determines what operations are valid on it.

Example: The value 21.4 is a floating point number and can be added to; the value "21.4" in quotation marks is a string of text and cannot.

Data Validation¶

The process of checking whether readings are physically plausible and identifying those that are not.

Two checks catch most faults: whether a value falls within a physically possible range, and whether it changed faster than the quantity physically can.

Example: Rejecting a pressure reading of 500 hectopascals because no sea-level station has ever recorded one.

Database¶

A structured system for storing, indexing, and querying data efficiently.

A database answers questions without scanning every record, at the cost of being binary rather than human-readable. For a monitoring station it is worth adding only once a specific problem demands it.

Example: Finding every hour in five years when pressure fell more than 3 hectopascals takes seconds in a database and minutes scanning files.

Contrast with: CSV File

Depth Of Discharge¶

The proportion of a battery's stored charge that is drawn out before recharging.

Running a battery deeper than its chemistry tolerates shortens its life sharply, so usable capacity is always less than the rated capacity.

Example: Lithium cells are commonly limited to 80 percent depth of discharge; lead-acid cells to about 50 percent.

Dew And Frost Formation¶

The condensation of water vapor onto surfaces that have radiated heat to the clear night sky and cooled below the dew point.

If the dew point is above freezing the result is dew; if below, vapor deposits directly as ice crystals and the result is frost.

Example: Dew appears on an open lawn but not under a tree, because the sheltered ground cannot radiate its heat to the sky.

Dew Point¶

The temperature to which air must be cooled, at constant pressure, for it to become saturated and begin condensing.

Dew point tracks the actual water content, so it stays constant while temperature and relative humidity swing through the day. It predicts how humid a day feels far better than relative humidity does.

Example: A dew point above 21 °C feels oppressive regardless of what the humidity percentage reads.

See also: Relative Humidity, Fog Formation

Digital Signal¶

An electrical signal that has only a limited set of possible values, usually two, representing on and off.

Digital signals survive noise and copying without degrading, which is why sensor data travels this way.

Example: The data line of an I2C bus is either near 3.3 volts or near 0 volts, and a small amount of electrical noise does not change which it is.

Contrast with: Analog Signal

Dilution Of Precision¶

A number summarizing how much satellite geometry degrades a position fix.

Satellites spread widely across the sky produce a sharp fix; satellites clustered together produce a smeared one, because the distance spheres intersect at a shallow angle.

Diurnal Cycle¶

The daily pattern of variation in an environmental quantity, driven by the Earth's rotation changing the angle of the Sun.

Air temperature follows the cycle with a lag of two to three hours behind peak sunlight, because the ground must warm first and then warm the air.

See also: Seasonal Variation, Solar Zenith Angle

Duty Cycling¶

Switching a device off, or into a low-power state, during the intervals when it is not needed.

It is the most effective way to reduce a station's consumption, because a station reading once a minute is idle for almost all of that minute.

Example: Disabling unused video output and status lights can save a fifth of a small station's daily energy with no loss of function.

Earthquake¶

The sudden release of accumulated elastic energy when a fault slips.

The point underground where slip begins is the hypocentre; the point on the surface directly above it is the epicentre. Depth matters greatly, since a shallow event causes far more surface damage than a deep one of the same magnitude.

Example: A magnitude 6 event 10 kilometres deep can devastate a city, while the same magnitude 300 kilometres down may barely be felt.

Earthquake Early Warning¶

A system that detects an earthquake already in progress and alerts distant locations before the damaging shaking reaches them.

It is not prediction. It works because the fast, harmless P wave outruns the destructive S wave, and because radio signals travel far faster than either. A blind zone near the epicentre receives no useful warning.

See also: P Wave, S Wave, Seismic Network

Electromagnetic Spectrum¶

The full range of electromagnetic radiation ordered by wavelength, from radio waves through infrared, visible light, and ultraviolet to X-rays and gamma rays.

All of it is the same phenomenon differing only in wavelength. Human vision responds to a narrow band near the middle.

See also: Visible Light, Infrared Radiation, Ultraviolet Radiation

Electrostatic Discharge¶

The sudden flow of accumulated static charge between two objects at different electrical potentials.

Damage to electronic components can occur at voltages far below the threshold a person can feel, and the resulting harm is often latent, appearing days or weeks later.

Example: A shock you can feel is around 3,000 volts, but a chip can be damaged at 100 volts, which nobody notices.

Elevation¶

Height above a reference surface, usually mean sea level.

Elevation is required rather than optional for a monitoring station, because atmospheric pressure falls roughly twelve hectopascals per hundred metres and readings cannot be compared without it.

Example: A station 300 metres up reads about 36 hectopascals lower than one at sea level in identical weather.

See also: Sea Level Pressure, Coordinate System

Energy Demand¶

The electricity a region requires at a given moment, forecast largely from temperature.

Because electricity cannot be stored at scale, generation must match demand continuously, and a forecast error of a degree across a large region corresponds to hundreds of megawatts.

Example: Air conditioning demand peaks across a whole city on the same hot afternoon, which is when grids are most likely to fail.

Enhanced Fujita Scale¶

A scale rating tornado intensity from EF0 to EF5, based on the damage caused and the wind speed required to produce it.

Created by Tetsuya Fujita in 1971 and revised in 2007. It works backwards from damage because tornadoes destroy the instruments that would otherwise measure them directly.

See also: Wind Load, Saffir Simpson Scale

Environmental Monitoring¶

The repeated measurement of conditions in the natural environment, in the same place, over an extended period.

Each part of that description matters. A single reading is an observation; the same reading repeated for years is a record that can answer questions no single measurement can.

Example: A station that records temperature every minute for five years can show whether a valley is warming; one reading cannot.

Epicenter¶

The point on the Earth's surface directly above the place where an earthquake's fault slip begins.

Evaporation¶

The change of water from liquid to gas at a surface, occurring at any temperature and not only at boiling.

Evaporation cools what it leaves behind, because the fastest-moving molecules escape and lower the average energy of those remaining.

Example: Sweat cools skin by evaporating; the wet bulb of a psychrometer reads lower than the dry bulb for the same reason.

Contrast with: Condensation

Evapotranspiration¶

The combined loss of water from a surface by evaporation from soil and transpiration from plants.

Calculating it requires temperature, humidity, wind speed, and solar radiation together, which is why irrigation scheduling uses a full weather station rather than a thermometer.

Example: Warmer, drier, windier, and sunnier conditions all raise the daily water loss a field must have replaced.

See also: Evaporation, Agricultural Planning

Exception Handling¶

A programming technique for catching errors when they occur and deciding how to respond, rather than allowing a program to stop.

For an unattended station this is what turns a momentary sensor fault into one missing reading instead of weeks of lost data. Errors should be caught specifically and always recorded.

Example: Catching an OSError from a failed sensor read costs one reading instead of stopping the station.

Fahrenheit Scale¶

A temperature scale on which water freezes at 32 degrees and boils at 212 degrees, with 180 degrees between those points.

Proposed by Daniel Gabriel Fahrenheit in 1724, it was the first temperature scale that different instruments could reproduce, which made readings comparable between observers.

See also: Celsius Scale, Temperature Conversion

Fault¶

A fracture in the Earth's crust along which masses of rock have moved relative to one another.

Faults occur where tectonic plates meet, and they are where the strain that produces earthquakes accumulates.

File Permissions¶

Settings controlling which users may read, write, or execute each file.

Every file carries separate permissions for its owner, a group, and all other users.

Example: A program file without execute permission will not run, producing a failure that resembles a bug in the code.

File Rotation¶

The practice of periodically closing the current data file and beginning a new one, usually named by date.

Rotation keeps individual files small and quick to open, contains any corruption to a single period, and makes copying only new data straightforward.

Example: Files named readings-2026-08-25.csv change automatically at midnight and sort chronologically as plain text.

File System¶

The way an operating system organizes and stores files.

Linux arranges everything into a single tree beginning at the root. Hardware devices appear within that tree as files, which is why programs read sensors using the same operations they use for data files.

Example: The I2C bus appears in the tree as /dev/i2c-1, and a program reads a sensor by opening that path.

See also: Operating System, File Permissions

Flood Warning¶

An alert issued when rising water is expected to threaten an area, based on precipitation measurements, soil moisture, and river levels.

Flash flood warning depends particularly on dense rainfall measurement, because an intense storm over a small area can be missed entirely by a sparse gauge network.

Example: An intense storm over a few square kilometres can produce a flash flood that a sparse gauge network never records.

See also: Precipitation, Citizen Science

Fog Formation¶

Condensation occurring throughout a layer of air rather than on a surface, producing suspended droplets that reduce visibility.

Fog is a cloud that forms at ground level. It requires a clear sky, light wind, a small gap between temperature and dew point, and enough time to cool.

Freezing Point Of Water¶

The temperature at which liquid water becomes ice, which at standard atmospheric pressure is 0 °C or 32 °F.

It provides a free reference standard: an ice-water bath sits reliably at 0.0 °C and can be used to check a thermometer.

Function¶

A named block of code that performs a task, may accept inputs, and may return a result.

Functions avoid repetition, can be tested in isolation, and give a name to an idea, which makes a program readable.

Example: A function that converts Celsius to Fahrenheit can be checked once and then used everywhere.

GNSS¶

The general term for all global navigation satellite systems, covering GPS, GLONASS, Galileo, and BeiDou together.

Receivers that listen to several constellations obtain better satellite geometry and therefore a better position, particularly where terrain or buildings block part of the sky.

GPIO Pin¶

A connection on a single-board computer whose behaviour is controlled by software, able to read an applied voltage or apply one.

On the Raspberry Pi these operate at 3.3 volts and are not tolerant of 5 volts.

GPS¶

The Global Positioning System, a satellite navigation system allowing a receiver anywhere on Earth to determine its position and the exact time.

Approved in 1973 and fully operational in 1995. Until May 2000 the civilian signal was deliberately degraded, and switching that policy off improved civilian accuracy from about 100 metres to under 10.

GPS Satellite¶

A satellite that continuously broadcasts its own position and the current time from an onboard atomic clock.

A receiver derives its distance from each satellite by comparing the broadcast time against the arrival time and multiplying by the speed of light.

Ground Connection¶

The reference point in a circuit defined as zero volts, against which all other voltages are measured, and the return path completing every circuit.

A missing ground connection is the most common wiring fault in sensor projects, and it produces symptoms that resemble software failure.

Growing Degree Days¶

An accumulated measure of warmth above a crop-specific base temperature, used to predict plant development.

Example: Maize develops above a base of about 10 °C, so a day averaging 22 °C contributes 12 growing degree days.

Hair Hygrometer¶

An instrument that measures relative humidity using the fact that a human hair changes length as it absorbs and releases moisture.

Built by Horace-Bénédict de Saussure in 1783, it was the first humidity instrument good enough to be read as a number. A hair lengthens by roughly two percent between very dry and fully saturated air, and a lever amplifies that change to move a pointer.

Example: Degreased blonde hair was found to work best, and such instruments remained standard for well over a century because they need no power and survive outdoors for years.

See also: Psychrometer, Relative Humidity

Header Row¶

The first line of a data file, naming each field.

Field names must carry their units. Without them, a stranger — including the author some years later — cannot tell whether a temperature is Celsius or Fahrenheit, or whether a pressure is corrected to sea level.

Example: timestamp,temperature_c,pressure_hpa,humidity_pct

Heat Index¶

Apparent temperature in hot conditions, combining air temperature with relative humidity.

High humidity slows the evaporation of sweat, so the body cannot cool itself effectively. The effect is large: at 32 °C, the heat index is about 31 °C at 40 percent humidity and about 41 °C at 80 percent.

Example: At 32 °C the heat index is about 31 °C at 40 percent humidity and about 41 °C at 80 percent.

See also: Apparent Temperature, Wet Bulb Temperature

Heat Transfer¶

The movement of thermal energy from a warmer object or region to a cooler one, by conduction, convection, or radiation.

Heat always flows in that direction unaided. There is no separate substance called cold; cold is the absence of thermal energy.

See also: Convection, Infrared Radiation

Hectopascal¶

A unit of pressure equal to one hundred pascals, and the standard unit in international meteorological reports.

One hectopascal equals one millibar exactly, which is why weather services could adopt SI units without changing any published numbers.

Example: Standard sea level pressure is 1013.25 hectopascals.

Hexadecimal¶

A number system with sixteen digits, written 0 through 9 and then A through F, conventionally used for device addresses and register values.

Example: The address written 0x76 in hexadecimal is 118 in ordinary decimal counting.

High Pressure System¶

A region where atmospheric pressure is higher than in the surrounding area, produced by air sinking from above.

Sinking air is compressed and warms, moving away from saturation, so clouds dissipate. High pressure is associated with fair, settled weather.

Contrast with: Low Pressure System

Humidity¶

A general term for the amount of water vapor present in the air.

Three precise measures exist, each answering a different question: absolute humidity, relative humidity, and dew point.

Example: Two days can both report 50 percent humidity while holding four times different amounts of water.

See also: Absolute Humidity, Relative Humidity, Dew Point

Hydrostatic Pressure¶

The pressure produced at a point by the weight of a fluid at rest above it.

It depends on depth and fluid density, not on the shape of the container, and it acts equally in all directions.

Example: The pressure at the bottom of a narrow ten-metre pipe of water equals that at the bottom of a ten-metre-deep lake.

Hypocenter¶

The point underground where an earthquake's fault slip begins, also called the focus.

Its depth strongly affects surface shaking: a shallow event causes far more damage than a deep one of equal magnitude.

Example: An event at 10 kilometres depth causes far stronger surface shaking than one at 300 kilometres.

I2C Bus¶

A two-wire communication bus allowing one controlling device to exchange data with many peripheral devices.

The two wires carry data and a clock signal, and both require pull-up resistors. Its economy of pins makes it the standard choice for connecting sensors.

See also: I2C Device Address, Pull Up Resistor

I2C Device Address¶

A number, usually seven bits, uniquely identifying a device on an I2C bus.

The controller broadcasts an address and only the matching device responds. No two devices on the same bus may share an address.

Example: The BME280 answers at either 0x76 or 0x77, selected by how its address pin is wired.

Inches Of Mercury¶

A unit of pressure equal to the pressure supporting a column of mercury one inch high, used in United States weather reports and in aviation.

Example: Standard sea level pressure is 29.92 inches of mercury, equal to 1013.25 hectopascals.

See also: Mercury Barometer, Hectopascal

Inertial Mass¶

The property of matter that resists any change in its state of motion.

This property solves the problem of measuring motion from inside a moving frame: a suspended mass tends to stay still while the frame moves with the ground, and the relative displacement is the measurement.

Example: A heavy mass hanging on a soft spring stays nearly still while the frame bolted to the ground moves beneath it.

Infrared Radiation¶

Electromagnetic radiation with wavelengths longer than visible red light, from about 700 nanometres to one millimetre.

Every object above absolute zero emits it. William Herschel discovered it in 1800 by placing a thermometer just beyond the red end of a spectrum and finding it read higher than any visible colour.

See also: Electromagnetic Spectrum, Heat Transfer

Ingress Protection Rating¶

A standardized measure of how effectively an enclosure excludes solid objects and liquids, written as IP followed by two digits.

Example: IP65 indicates dust-tight construction that also resists water jets, which is the usual target for an outdoor station enclosure.

Insolation¶

Solar energy accumulated over a period of time on a unit area, usually expressed in kilowatt-hours per square metre per day.

Insolation is the area under an irradiance curve. It is the figure that determines whether a solar panel can meet a given demand.

Example: A mid-latitude site might receive 6 kilowatt-hours per square metre on a clear summer day and 1.5 in December.

Intermittent Connectivity¶

A network connection that works at some times and fails at others, unpredictably.

This is the normal condition for a remote station. Software must therefore record readings locally first and treat transmission as a separate optional step, since network outages correlate with the storms that make the data most valuable.

Example: A cellular link that drops during heavy rain, which is precisely when the readings matter most.

Interpolation¶

Estimating values between known measurements by assuming the quantity changed smoothly between them.

Interpolated values look identical to measurements in a data file, so they must be labelled. Filling a long gap by interpolation invents data that was never recorded.

Example: Filling a six-hour gap with a straight line produces values indistinguishable from real measurements unless they are labelled.

Irradiance¶

The power of radiation arriving per unit area of surface, measured in watts per square metre.

Irradiance is a rate. Accumulated over time it becomes insolation.

Example: Clear noon sunlight on a horizontal surface at mid-latitudes delivers roughly 900 watts per square metre in summer.

See also: Insolation, Watts Per Square Meter

Isobar¶

A line on a weather map connecting points of equal atmospheric pressure.

Isobars are read like contour lines on a topographic map: closely spaced lines indicate a steep pressure gradient and therefore strong wind.

Example: Isobars packed tightly together on a chart indicate strong wind, even if the central pressure is unremarkable.

Jumper Wire¶

A short wire with a connector at each end, used to make temporary electrical connections without soldering.

Colour carries no electrical meaning but conventionally red indicates power and black indicates ground, which makes a wiring harness readable months later.

Kelvin Scale¶

A temperature scale whose zero is absolute zero and whose degree is the same size as a Celsius degree.

Because its zero represents an absence of atomic motion rather than an arbitrary reference, ratios on this scale are meaningful: 200 K genuinely represents twice the average kinetic energy of 100 K.

Example: Water freezes at 273.15 K and boils at 373.15 K.

See also: Absolute Zero, Measurement Scale

Kinetic Energy Of Atoms¶

The energy of motion possessed by the atoms or molecules in a substance, whose average value is what temperature measures.

Hot means atoms are moving quickly; cold means they are moving slowly. Heat is not a substance that flows into objects.

Knot Unit¶

A unit of speed equal to one nautical mile per hour, about 0.514 metres per second, used in marine and aviation contexts worldwide.

Because a nautical mile is one minute of latitude, speed in knots relates directly to position on a chart, which is why the unit has persisted.

Latitude¶

The angular distance of a place north or south of the equator, measured in degrees from 0 at the equator to 90 at each pole.

Latitude has a natural zero fixed by the Earth's rotation, and one degree of latitude corresponds to about 111 kilometres everywhere.

Contrast with: Longitude

Line Chart¶

A chart plotting values against a continuous variable, joining consecutive points with straight segments.

Joining points asserts that the quantity existed continuously between readings. That assertion is false across a gap in the data, so the line must be broken there.

Example: Plotting a week of temperature readings against time reveals the daily cycle at a glance.

Liquid In Glass Thermometer¶

A thermometer consisting of a sealed glass tube with a bulb of liquid at one end and a narrow bore into which the liquid expands.

The narrow bore acts as a mechanical amplifier: a small change in the bulb's volume produces a large, readable movement in the tube.

Logarithmic Scale¶

A measurement scale on which each step represents multiplication by a fixed factor rather than addition of a fixed amount.

Logarithmic scales are used when the quantity being measured spans many orders of magnitude and a linear scale would compress most values into an unreadable band.

Example: On the moment magnitude scale, each whole step upward represents about ten times the ground motion and about thirty-two times the energy.

See also: Order Of Magnitude, Moment Magnitude Scale

Longitude¶

The angular distance of a place east or west of the prime meridian, measured in degrees from 0 to 180 in each direction.

Longitude has no natural zero; its reference was decided by international agreement. One degree of longitude shrinks from about 111 kilometres at the equator to nothing at the poles.

See also: Prime Meridian, Longitude Problem

Longitude Problem¶

The historical difficulty of determining east–west position at sea, unsolved for centuries and responsible for many shipwrecks.

The underlying principle was understood: the Earth turns fifteen degrees per hour, so comparing local time against a reference time gives longitude. The obstacle was building a clock that could keep reference time through a sea voyage.

Loop¶

A programming construct that repeats a block of code.

A monitoring station is essentially one long loop: read the sensors, record the values, wait, and repeat.

Example: A while True: loop reads the sensors, writes a row, sleeps for a minute, and repeats indefinitely.

See also: Conditional Statement, Function

Low Pressure System¶

A region where atmospheric pressure is lower than in the surrounding area, produced by air rising.

Rising air expands and cools, moving toward saturation, so its moisture condenses. Low pressure is associated with cloud, wind, and precipitation.

Contrast with: High Pressure System

Magnitude Versus Intensity¶

The distinction between how large an earthquake was and how strongly it was felt at a particular place.

An earthquake has exactly one magnitude regardless of where anyone stands, and many intensities that vary with distance, depth, and local ground conditions.

Example: Magnitude is the wattage of a light bulb; intensity is how bright it looks from where you are sitting.

See also: Moment Magnitude Scale, Mercalli Intensity Scale

Marine Chronometer¶

A clock accurate and rugged enough to keep reference time throughout a long sea voyage, enabling longitude to be determined.

John Harrison's fourth design, completed in 1761, lost only about five seconds across an 81-day voyage to Jamaica.

Measurement¶

The act of comparing a physical property against an agreed standard and reporting the result as a number with a unit.

Example: Comparing a table against a metre stick and reporting 2 metres is a measurement; saying the table is long is not.

See also: Physical Property, Unit Of Measure, Quantitative Data

Measurement Range¶

The span between the smallest and largest values an instrument can measure at all.

Outside its range an instrument does not give a poor reading; it gives no usable reading. Checking that expected values sit comfortably inside the range is the first specification to verify before choosing a sensor.

Example: A sensor rated from 300 to 1100 hectopascals cannot be used where pressures fall below 300.

Measurement Scale¶

The set of rules defining what the numbers in a measurement mean, including where zero sits and which comparisons are valid.

Whether ratios are meaningful depends entirely on the scale. Scales with an arbitrary zero support subtraction but not division.

Example: 20 °C is not twice as warm as 10 °C, because the Celsius zero marks the freezing point of water rather than an absence of thermal energy.

See also: Kelvin Scale, Logarithmic Scale

Measurement Uncertainty¶

An honest statement of how far a reported measurement could be from the true value, usually written with a plus-or-minus range.

Reporting a number without an uncertainty claims more knowledge than the instrument supports.

Example: Writing 21.3 ± 0.5 °C states that the true value is very likely between 20.8 and 21.8 °C.

See also: Accuracy, Precision, Resolution

MEMS Accelerometer¶

An accelerometer built as a micro-electro-mechanical system, with a microscopic proof mass on silicon springs whose displacement changes a measured capacitance.

Developed for automotive airbag sensing around 1991. Individually far less sensitive than a research seismometer, but cheap and numerous enough to form dense networks that a sparse network of precise instruments cannot match.

MEMS Sensor¶

A sensor built with microscopic mechanical structures etched onto a silicon chip using semiconductor manufacturing methods.

The abbreviation stands for Micro-Electro-Mechanical System. Mass production for automotive airbags in the 1990s drove the cost down far enough that these sensors became common in ordinary devices.

Example: The accelerometer that rotates a phone screen contains a proof mass on silicon springs a few micrometres across.

See also: MEMS Accelerometer, Semiconductor

Mercalli Intensity Scale¶

A scale rating the shaking experienced at a particular location from I to XII, based on observed effects on people, objects, and buildings.

Developed by Giuseppe Mercalli in 1902 and revised in 1931. Roman numerals are used deliberately to prevent confusion with magnitude values.

Mercury Barometer¶

An instrument that measures atmospheric pressure by the height of a mercury column the atmosphere can support.

Built by Evangelista Torricelli in 1643, it stands about 760 millimetres tall at sea level. Mercury was chosen because water would require a column over ten metres high.

See also: Torricelli Experiment, Aneroid Barometer

Metadata¶

Information describing a dataset: what was measured, where, when, with which instrument, and under what conditions.

Without it a data file is a set of numbers that cannot be interpreted. A pressure reading cannot be understood without the station's elevation, and a temperature reading cannot be judged without knowing whether the sensor was shielded.

Example: Recording that pressure is uncorrected station pressure prevents a reader mistaking 995 hPa for an approaching storm.

Meteorologist¶

A scientist who studies and forecasts weather over periods from hours to about two weeks.

Meters Per Second¶

The SI unit of speed, and the standard unit for reporting wind speed in scientific work.

Example: 10 metres per second is 36 kilometres per hour, about 22 miles per hour, or roughly 19 knots.

Millibar¶

A unit of pressure equal to one thousandth of a bar, formerly the standard meteorological unit.

It is numerically identical to the hectopascal, so charts drawn in millibars remain correct when relabelled.

Missing Data¶

A gap in a record where readings should exist but do not.

Gaps must remain visible rather than being silently closed. A gap accompanied by a matching entry in a station log is diagnosable; a gap with no explanation is not.

Example: Six hours with no rows, matched by a log entry recording a power failure, is a documented gap rather than a mystery.

Moment Magnitude Scale¶

A magnitude scale computed from the physical size of a fault rupture, specifically the rock rigidity, the area that slipped, and the average slip distance.

Developed by Kanamori and Hanks in 1979 to replace the Richter scale for large events, because it does not saturate. It is what modern reports mean by an earthquake's magnitude.

See also: Richter Scale, Logarithmic Scale

Moving Average¶

A smoothing technique that replaces each value with the mean of it and its neighbours within a window of fixed length.

Choosing the window length determines what is revealed. A window matching one full cycle removes that cycle almost entirely, so a 24-hour window strips out the daily rhythm and leaves the slower weather changes.

Example: A ten-minute average of one-minute readings removes sensor noise while preserving the shape of the day.

Nautical Mile¶

A unit of distance equal to one minute of latitude, about 1852 metres, from which the knot is derived.

One Atmosphere¶

A unit of pressure defined as the standard average pressure at sea level, equal to 1013.25 hectopascals or 29.92 inches of mercury.

Example: Descending about ten metres underwater adds roughly one additional atmosphere of pressure.

See also: Sea Level Pressure, Barometric Pressure

Operating System¶

The software that manages a computer's hardware and provides services that other programs use.

It handles hardware access, runs and schedules programs, organizes files, controls permissions, and presents an interface to a user.

Example: Ubuntu Server manages the Raspberry Pi's memory, storage, and network so a logging program does not have to.

See also: Ubuntu Server, Raspberry Pi OS

Order Of Magnitude¶

A factor of ten, used to compare quantities when relative size matters more than exact value.

Example: The atmosphere is roughly three orders of magnitude thicker than the tallest building, meaning about a thousand times.

See also: Scientific Notation, Logarithmic Scale

Outlier¶

A data point that differs markedly from those around it.

An outlier is a question rather than a conclusion. The most useful test is whether other measured channels moved in a physically consistent way: a real event moves several at once, while an instrument fault usually moves one.

Example: A single 47 °C reading among neighbours of 21 °C, with no other channel moving, is almost certainly electrical noise.

P Wave¶

The compressional seismic wave, in which rock is alternately squeezed and stretched along the direction of travel.

P waves travel at about 6 kilometres per second in the crust, arrive first, cause little damage, and can travel through liquid.

Contrast with: S Wave

Partial Pressure Of Vapor¶

The portion of total atmospheric pressure contributed by water vapor alone.

John Dalton established around 1801 that each gas in a mixture exerts pressure independently, so the total is the sum of the parts.

Example: Of 1013 hectopascals of atmospheric pressure, water vapor might contribute anywhere from 1 to 40 hectopascals.

See also: Saturation Vapor Pressure, Pressure

Pascal Unit¶

The SI unit of pressure, defined as one newton of force per square metre of area, named for Blaise Pascal.

It is a small unit for atmospheric work: standard sea level pressure is 101,325 pascals, which is why meteorology uses hectopascals instead.

Photodiode¶

A semiconductor device that produces an electric current proportional to the light falling on it.

Photodiodes are fast, small, and inexpensive, but they respond unevenly across wavelengths, so their readings shift as the spectrum of arriving light changes.

See also: Photoelectric Effect, Pyranometer

Photoelectric Effect¶

The release of electrons from a material when light strikes it.

Explained by Albert Einstein in 1905, the effect depends on the colour of the light rather than its brightness. It is the basis of photodiodes and solar cells.

Example: Light falling on a piece of silicon frees electrons, producing a current proportional to how much light arrives.

See also: Photodiode, Photovoltaic Effect

Photovoltaic Effect¶

The generation of a voltage or current in a material when it absorbs light.

Discovered by Edmond Becquerel in 1839 and made practical in silicon by Bell Laboratories in 1954.

Example: A solar panel in sunlight produces a voltage across its terminals with no moving parts involved.

See also: Solar Cell, Photoelectric Effect

Physical Property¶

A feature of an object or place that can be described by a number obtained through comparison against a standard.

Temperature, pressure, length, and mass are physical properties. Qualities such as beauty are not, because no instrument can compare them against a standard.

See also: Measurement, Quantitative Data

Piezoresistive Effect¶

The change in electrical resistance of a material when it is mechanically deformed.

Example: A thin silicon diaphragm flexes under air pressure, and the resistance of elements formed into it changes measurably as it bends.

Pin Header¶

The row of metal pins along the edge of a single-board computer that provides the physical connection point for wires.

Position Fix Accuracy¶

How close a satellite navigation receiver's reported position is to its true position.

Accuracy varies with satellite geometry, atmospheric delay, signal reflection from nearby surfaces, and obstruction of the sky. Vertical accuracy is consistently worse than horizontal, because no satellites lie beneath the receiver.

Example: Vertical accuracy is typically two to three times worse than horizontal, because no satellites lie beneath the receiver.

Power Budget¶

An accounting of how much energy a system consumes against how much it can collect or store.

For a solar-powered station the computer and the radio dominate consumption while the sensors are negligible, and the budget must be balanced against the worst month of the year rather than the average.

Example: A station consuming 3,600 mAh per day against a panel yielding 2,100 in December will run flat.

See also: Insolation, Duty Cycling, Battery Capacity

Precipitation¶

Water falling from the atmosphere to the ground as rain, snow, sleet, or hail.

It occurs when droplets within a cloud collide and merge until they are too heavy to remain suspended. The form that reaches the ground depends on the temperatures it falls through.

Example: Whether a cloud delivers rain, sleet, or snow depends on the temperatures the droplets fall through.

Precision¶

How close repeated measurements of an unchanging quantity are to one another.

Precision and accuracy are independent. A consistently wrong sensor is precise and inaccurate, and that fault can be corrected; a randomly scattering sensor cannot be corrected the same way.

Example: Readings of 25.1, 25.1, 25.0, 25.1 °C are highly precise regardless of whether the true temperature is 25 °C or 21 °C.

Contrast with: Accuracy

Pressure¶

Force distributed over an area, calculated as force divided by the area it acts upon.

Example: The same push applied through a drawing pin's tip produces enormous pressure because the area is tiny.

See also: Barometric Pressure, Hydrostatic Pressure

Pressure Altitude¶

Altitude calculated from a barometric pressure reading by assuming a standard atmosphere.

Because real pressure differs from the standard, the calculation carries an error that must be corrected using the current local sea level pressure.

Example: An aircraft altimeter converts the measured pressure into a height using a standard atmosphere assumption.

Pressure Gradient¶

The rate at which atmospheric pressure changes with horizontal distance.

Wind speed depends on the steepness of this gradient rather than on how low the pressure is. On a weather map, closely spaced isobars indicate a steep gradient and strong wind.

Example: A shallow low with tightly packed isobars produces stronger wind than a deep low with widely spaced ones.

Pressure Tendency¶

The change in barometric pressure over a defined period, conventionally three hours.

Example: A tendency of −5 hectopascals over three hours indicates rapidly deteriorating conditions and likely storms.

See also: Barometric Forecasting, Time Series

Prime Meridian¶

The meridian defined as zero degrees longitude, from which all longitudes are measured.

Fixed at Greenwich by the International Meridian Conference in 1884, chosen largely because most of the world's shipping already used charts based on it.

Example: Twenty-five nations met in Washington in 1884 and chose Greenwich, largely because most shipping charts already used it.

Proof Mass¶

The small suspended mass inside an accelerometer or seismometer whose inertia provides a reference against which motion of the housing is measured.

See also: Inertial Mass, MEMS Accelerometer

Psychrometer¶

An instrument that determines humidity using two thermometers, one with its bulb wrapped in a wet wick.

Evaporation cools the wet bulb, and the drier the air the greater the cooling. The difference between the two readings gives the humidity.

See also: Wet Bulb Temperature, Evaporation

Pull Up Resistor¶

A resistor connecting a signal line to the positive supply voltage, holding the line high whenever nothing is actively driving it low.

Without one, an unconnected input floats and reports meaningless fluctuating values. The I2C bus requires pull-up resistors, though most sensor breakout boards include them.

Puy De Dome Experiment¶

The 1648 experiment in which Florin Périer, at Blaise Pascal's request, carried a barometer up the Puy de Dôme in France and recorded the mercury column falling with altitude.

A second barometer was left at the base under observation as a control. The result demonstrated that the atmosphere has weight and settled a two-thousand-year argument.

See also: Torricelli Experiment, Barometric Pressure

Pyranometer¶

An instrument that measures total solar irradiance arriving on a horizontal surface from the whole sky, combining direct beam and scattered light.

The classic design places a blackened thermopile beneath a glass dome. Because a thermopile responds almost equally to all wavelengths, it measures total energy accurately.

Pyrheliometer¶

An instrument that measures direct beam solar radiation from the Sun's disc alone, excluding light scattered by the sky.

It views through a narrow tube and must be mounted on a tracker that follows the Sun. Claude Pouillet built the first in 1838 and used it to estimate the solar constant.

Contrast with: Pyranometer

Python¶

A programming language designed for readability, first released in 1991.

It is well suited to this work because it is already installed on the relevant systems, has libraries for nearly every sensor, and requires no manual memory or type management.

Example: Seven lines of Python with a sensor library replace several hundred lines of register handling.

See also: Python Library, Script Execution

Python Library¶

A collection of pre-written code that a program can use rather than reimplementing.

Libraries are what make sensor work practical for a beginner: the register reads, calibration constants, and compensation arithmetic for a sensor become a few lines instead of several hundred.

Example: Importing adafruit_bme280 turns a complex sequence of register reads into sensor.temperature.

Qualitative Observation¶

A description of something recorded without a number.

Qualitative observations are how people notice that something is worth measuring, but they cannot be graphed or compared across years.

Example: "The sky is overcast" and "the wind is strong" are qualitative observations.

Contrast with: Quantitative Data

Quality Flag¶

A field added to a data record marking a reading as suspect, without removing it from the file.

Flagging preserves the original evidence while allowing analysis to exclude questionable values, and it lets a later reader reach a different conclusion.

Example: A row marked suspect_range records that a value fell outside plausible limits but keeps the value itself.

Quantitative Data¶

Information recorded as a number together with a unit.

A number without a unit is not quantitative data, because nobody can interpret it or compare it to anything else.

Example: "Wind speed is 24 kilometres per hour" is quantitative data; "wind speed is 24" is not.

See also: Unit Of Measure, Qualitative Observation

Raspberry Pi¶

A family of low-cost single-board computers created to give students hands-on experience with hardware.

The first board shipped in 2012 and over sixty million have since been produced.

Example: A Raspberry Pi Zero 2 W costs about as much as a pizza and runs the same operating system family as a web server.

See also: Single Board Computer, Raspberry Pi Zero 2 W

Raspberry Pi OS¶

The official Linux distribution for Raspberry Pi computers, based on Debian, with particularly strong support for that hardware.

See also: Operating System, Ubuntu Server

Raspberry Pi Zero 2 W¶

A small, low-power member of the Raspberry Pi family with built-in wireless networking, measuring about 65 by 30 millimetres.

Its size, low power draw, and standard 40-pin header make it well suited to a battery-powered remote station.

See also: Raspberry Pi, Single Board Computer

Reference Standard¶

The physical object or reproducible procedure that defines a unit and against which instruments are ultimately compared.

Standards have moved historically from objects somebody owns toward procedures anyone can repeat, because an object can be damaged or can drift.

Example: The metre is defined as the distance light travels in 1/299,792,458 of a second, a procedure any suitably equipped laboratory can reproduce.

Relative Humidity¶

The ratio of the actual water vapor pressure to the maximum the air can sustain at its current temperature, expressed as a percentage.

Because the maximum rises steeply with temperature, relative humidity changes when the temperature changes even if no water enters or leaves the air. This is why it peaks before dawn and falls in the afternoon.

Example: Fifty percent at 32 °C holds more than four times the water of fifty percent at 5 °C.

See also: Saturation Vapor Pressure, Dew Point

Remote Station¶

A monitoring station that operates unattended, away from mains power and often away from any controlled network.

Its design goal is not needing to be visited, since every trip is costly and sometimes impossible in bad weather.

Example: A station on a post in a field, reachable only by a three-hour round trip.

Resistance Thermometer¶

A temperature sensor that works because the electrical resistance of a metal rises predictably as it warms.

Platinum is the usual metal because its response is unusually linear and stable, which makes these instruments suitable as reference standards.

Resolution¶

The smallest change an instrument can detect and report.

Resolution describes the fineness of the display, not the correctness of the reading. An instrument can report many digits and still be badly wrong.

Example: A thermometer displaying 21.3 °C has a resolution of 0.1 °C and will not register a rise of 0.04 °C.

See also: Accuracy, Measurement Uncertainty

Richter Scale¶

The first earthquake magnitude scale, published by Charles Richter with Beno Gutenberg in 1935, derived from the maximum amplitude recorded on a seismogram.

It saturates above about magnitude 7, understating the largest events, and has been superseded by moment magnitude for serious use despite continuing to appear in news reports.

See also: Moment Magnitude Scale, Logarithmic Scale

S Wave¶

The shear seismic wave, in which rock moves perpendicular to the direction of travel.

S waves travel at about 3.5 kilometres per second, arrive after P waves, cause most earthquake damage, and cannot travel through liquid. Their absence on the far side of the planet is how the liquid outer core was discovered.

Example: The rolling motion that topples furniture arrives after the initial thump of the P wave.

Contrast with: P Wave

Saffir Simpson Scale¶

A scale rating hurricanes from category 1 to 5 by their maximum sustained wind speed.

Because wind force rises with the square of speed, a category 5 exerts roughly four times the force of a category 1 rather than five times.

See also: Sustained Wind Speed, Wind Load

Sampling Interval¶

The time between consecutive readings.

Sampling too fast produces large files, higher power consumption, and redundant readings; sampling too slowly misses events. Dense data can always be thinned later, but detail never captured cannot be recovered.

Example: One reading per minute produces 1,440 rows per day and 525,600 per year.

See also: Sensor Response Time, Time Series

Saturation Vapor Pressure¶

The maximum partial pressure of water vapor that air can sustain at a given temperature, beyond which the excess condenses.

It rises steeply with temperature, roughly doubling every ten degrees Celsius. This single relationship drives dew, fog, frost, cloud, and rain.

Example: At 0 °C the limit is about 6.1 hectopascals; at 30 °C it is about 42.4.

See also: Relative Humidity, Partial Pressure Of Vapor

Scatter Plot¶

A chart showing the relationship between two variables by plotting one against the other as unconnected points.

Unlike a line chart, both axes are measurements and time does not appear. Points are not joined, because no claim is made about intermediate combinations.

Example: Plotting temperature against relative humidity for a week reveals their inverse relationship as a downward-sloping cloud of points.

Science Communication¶

Explaining scientific work to people who were not involved in it.

Effective communication leads with the finding rather than the method, presents one clearly labelled chart, gives the result a comparison that makes its size meaningful, and states plainly what was not measured.

Example: "The car park runs 6 °C hotter than the field" is a finding; a description of how the sensor was wired is background.

Scientific Notation¶

A way of writing numbers as a value between one and ten multiplied by a power of ten.

Example: 101325 is written as 1.01325 × 10⁵, and 0.000001 is written as 1 × 10⁻⁶.

Script Execution¶

Running a program stored as a file rather than typing statements one at a time.

Example: Running python3 logger.py from the command line starts the program.

Sea Level Pressure¶

The pressure a station would report if it were located at sea level, computed from its measured pressure and its known elevation.

Every pressure value in a weather report is a sea level pressure. Without this correction, stations at different heights cannot be compared, because elevation changes pressure far more than weather does.

Example: A station at 300 metres reading 977 hectopascals corresponds to about 1013 hectopascals at sea level, an entirely ordinary day.

See also: Elevation, Barometric Pressure

Seasonal Variation¶

The annual pattern of change in environmental quantities, driven by the tilt of Earth's rotational axis relative to its orbital plane.

The tilt changes both the Sun's maximum height in the sky and the length of the day, so the two effects compound. Seasons are not caused by changes in distance from the Sun.

See also: Solar Zenith Angle, Diurnal Cycle

Seismic Network¶

A set of seismic stations whose combined recordings locate earthquakes and characterize ground motion across a region.

A network does two things a single station cannot: locate an epicentre by trilateration from three or more stations, and reject false alarms by requiring that several stations agree.

Seismic Wave¶

A wave of energy travelling through the Earth, released by an earthquake or other sudden disturbance.

The delay between the arrival of the fast P wave and the slower S wave grows with distance, so a single station can estimate its distance from the source at roughly eight kilometres per second of separation.

Seismograph¶

An instrument that produces a permanent record of ground motion over time.

The classic design attaches a pen to a suspended mass and passes a rotating drum, mounted on the moving frame, beneath it. John Milne and colleagues built the first true seismograph in Japan around 1880.

Example: A pen on a suspended mass traces ground motion onto a drum that moves with the frame.

Seismometer¶

The sensing element of a seismograph, comprising the suspended mass, its suspension, and the mechanism that detects relative motion.

Modern broadband instruments use force feedback, electrically holding the mass centred and reporting the current required to do so.

Seismoscope¶

An instrument that detects and indicates that ground motion has occurred, without producing a continuous record of it.

Zhang Heng built one around 138 AD: a bronze vessel ringed with dragon heads, each holding a ball that dropped audibly into a bronze toad below when a distant tremor arrived.

Contrast with: Seismograph

Selective Availability¶

The former United States policy of deliberately degrading the civilian GPS signal, discontinued on 2 May 2000.

Ending it improved civilian position accuracy from roughly 100 metres to under 10 without any change to receivers.

Semiconductor¶

A material whose ability to conduct electricity falls between that of a conductor and an insulator, and which changes conductivity in response to heat, pressure, or light.

Silicon is the semiconductor on which almost all modern sensing depends. Its responsiveness is what makes it useful, and its manufacturability is what makes it cheap.

See also: Silicon Diode Sensor, Piezoresistive Effect, MEMS Sensor

Sensor¶

A device that detects a physical property and converts it into a signal that can be recorded, displayed, or acted upon.

Example: A microphone is a sensor that converts sound pressure into a voltage.

See also: Transduction, Physical Property

Sensor Averaging¶

Taking several readings of the same quantity and reporting their mean, in order to reduce random variation.

Random variation falls in proportion to the square root of the number of readings averaged, so improvements diminish quickly as the count rises.

Example: Averaging sixteen readings reduces random noise to about one quarter of its single-reading value.

See also: Sensor Filtering, Moving Average

Sensor Datasheet¶

The manufacturer's technical document describing a sensor's capabilities, requirements, and limitations.

Seven specifications matter before wiring anything: measurement range, accuracy, resolution, supply voltage, current draw, interface, and response time.

Example: Checking a datasheet reveals that a sensor is accurate only to ±1 °C, which settles whether a 0.4 °C discrepancy is worth investigating.

See also: Measurement Range, Sensor Response Time

Sensor Drift¶

A slow, systematic change in a sensor's readings over time that does not reflect any real change in what is being measured.

Drift is difficult to detect because no single day's readings ever look wrong. It is found by comparison against a reference or a neighbouring station over months.

Example: A humidity sensor whose overnight peaks fall from 99 to 93 percent across two months has drifted, not detected drier air.

Sensor Exposure¶

How a sensor is presented to the quantity it measures, including shielding, ventilation, height, and surroundings.

Exposure requirements conflict between sensors: a temperature sensor needs shade while a solar sensor needs an unobstructed sky, so they cannot occupy the same position.

Example: A thermometer in afternoon sun reports its own temperature rather than the air's, sometimes by 10 °C or more.

See also: Sensor Siting, Stevenson Screen

Sensor Filtering¶

Any technique that removes unwanted variation from a signal while preserving the part of interest.

Filtering cannot distinguish unwanted noise from a genuine rapid change, so heavy filtering removes real events along with the noise.

See also: Sensor Averaging, Moving Average

Sensor Response Time¶

How long a sensor takes to reflect a change in the quantity it measures, usually quoted as the time to reach 63 percent of the way to a new value.

Response time sets a floor on useful sampling: reading faster than the sensor can settle produces records of the sensor catching up rather than new information.

Example: A humidity element that needs about one second to settle should not be read ten times per second.

Sensor Siting¶

The choice of where to place a station and its sensors.

The convention is a clearance from any obstruction of at least four times that obstruction's height, over a natural surface, at standard measurement heights. A siting error cannot be corrected after the fact.

Example: A station beside a five-metre building needs twenty metres of clearance to meet the four-times-height convention.

See also: Sensor Exposure, Urban Heat Island

Serial UART¶

A two-wire point-to-point communication method with no shared clock, in which both ends must be configured to the same speed.

The abbreviation stands for Universal Asynchronous Receiver/Transmitter. Unlike a bus, it connects exactly two devices.

Severe Weather Warning¶

An alert issued to the public that dangerous conditions are occurring or imminent.

The distinction from a watch matters: a watch means conditions favour severe weather and one should prepare, while a warning means it is happening and one should act.

Example: A pressure fall of more than six hectopascals in three hours, combined with rising wind, triggers a warning rather than a watch.

See also: Weather Forecasting, Pressure Tendency

Sextant¶

A navigational instrument that measures the angle between two objects, typically a star and the horizon, using mirrors and a graduated arc spanning one sixth of a circle.

Developed in the 1730s, it allowed latitude to be determined at sea to within a few kilometres.

Shell Command¶

An instruction typed at a command line interface, consisting of a command name with optional settings and arguments.

Example: tail -f readings.csv prints the end of the data file and keeps printing new rows as they arrive.

See also: Command Line Interface, Script Execution

SI Prefix¶

A syllable attached to a unit that multiplies it by a power of ten.

Example: The prefix "hecto" means one hundred, so one hectopascal is one hundred pascals.

SI Units¶

The International System of Units, the worldwide standard for measurement, built on seven base units from which all others are derived.

Example: The pascal is a derived SI unit, defined as one newton of force per square metre of area.

See also: Unit Of Measure, Standardization

Silicon Diode Sensor¶

A temperature sensor that works because the forward voltage across a silicon diode falls predictably as the diode warms.

The shift is roughly two millivolts per degree Celsius. It is small, consistent, and easily measured, and it fits alongside other sensing elements on a single chip.

SIM7600A Module¶

A combined cellular data and satellite navigation module, communicating over serial UART using text instructions called AT commands.

In this project it provides both the mobile data link for telemetry and the GNSS receiver that supplies position and precise time.

See also: Serial UART, Cellular Data Link

Single Board Computer¶

A complete computer built on one circuit board, with processor, memory, and connectors soldered in place.

The design trades upgradeability for small size, low power consumption, and the ability to connect directly to electronic components.

Software Package¶

A bundle containing a program, its configuration, and a description of what else it requires in order to run.

Package managers install those requirements automatically, which is why a single install command can pull in several supporting components.

Example: sudo apt install i2c-tools installs the utility that confirms whether a sensor is wired correctly.

See also: Operating System, Python Library

Solar Cell¶

A device that converts sunlight directly into electricity by the photovoltaic effect, used for power generation rather than measurement.

Modern commercial silicon panels convert roughly twenty percent of the solar energy striking them.

See also: Solar Panel, Photovoltaic Effect

Solar Constant¶

The solar irradiance measured at the top of Earth's atmosphere on a surface perpendicular to the Sun's rays at Earth's average orbital distance, about 1361 watts per square metre.

The name overstates its constancy. It varies by roughly 0.1 percent across the eleven-year sunspot cycle and by about 6.9 percent annually because Earth's orbit is elliptical.

Solar Energy Generation¶

The production of electricity from sunlight using the photovoltaic effect.

Site selection depends on measured insolation, since two sites a hundred kilometres apart can differ by a fifth in annual energy, which decides whether a project is viable.

Example: Two candidate sites a hundred kilometres apart can differ by a fifth in annual insolation, deciding whether a project is viable.

Solar Noon¶

The moment each day when the Sun reaches its highest point in the sky at a given location, which is when the solar zenith angle is smallest.

Solar noon rarely coincides with clock noon, because time zones cover wide bands of longitude.

See also: Solar Zenith Angle, Diurnal Cycle

Solar Panel¶

An assembly of solar cells that converts sunlight into electrical power.

Panels are rated under standardized bright conditions, so real output is considerably lower and must be estimated from local insolation with an allowance for angle, dirt, temperature, and conversion losses.

Example: A 10 W panel at a mid-latitude site yields about 10.5 watt-hours on a December day and four times that in June.

See also: Charge Controller, Power Budget

Solar Radiation¶

The electromagnetic energy emitted by the Sun and received at the Earth.

Roughly half arrives as infrared, somewhat over forty percent as visible light, and about seven percent as ultraviolet. It is the energy source driving temperature, evaporation, and wind.

Example: Turn off the Sun and temperature cycles, evaporation, cloud, and wind all stop within weeks.

See also: Irradiance, Electromagnetic Spectrum

Solar Zenith Angle¶

The angle between the Sun's position and the point directly overhead.

Irradiance on a horizontal surface falls with the cosine of this angle, because a beam striking at a slant is spread over a larger area. This relationship produces both the daily and the seasonal cycles.

Example: At a zenith angle of 60 degrees, a horizontal surface receives half the irradiance it would at zero.

See also: Diurnal Cycle, Seasonal Variation

Sonic Anemometer¶

A wind speed instrument that times ultrasonic pulses travelling between fixed transducers, using the difference in transit time with and against the wind.

It has no moving parts, no stall speed, and a response fast enough to resolve turbulence, but it is expensive and disrupted by ice or heavy rain.

Contrast with: Cup Anemometer

SPI Bus¶

A communication bus using four or more wires — data out, data in, a clock, and a separate select line for each device.

It is considerably faster than I2C and can send in both directions simultaneously, at the cost of requiring more pins.

Contrast with: I2C Bus

Standardization¶

The process of establishing a single agreed definition or procedure and getting many parties to adopt it.

Standardization is a social achievement rather than a scientific discovery, and it is what makes measurements taken by different people in different places comparable.

Example: The 1884 agreement to place the prime meridian at Greenwich allowed charts from different countries to place the same island at the same longitude.

Stevenson Screen¶

A louvered white box that houses meteorological instruments, shielding them from direct sunlight and precipitation while allowing air to circulate freely.

Designed by Thomas Stevenson in 1864. Its standardized construction and mounting height are what make temperature readings comparable between stations worldwide.

See also: Air Temperature, Sensor Exposure

Store And Forward¶

A design in which a reading is written to local storage first and transmitted afterward as a separate, optional step.

This prevents a network failure from causing data loss, which matters most during storms when links fail and the measurements are most valuable.

Example: A reading is appended to the local CSV file first; the upload is attempted afterward and may fail harmlessly.

See also: Intermittent Connectivity, Telemetry

Surface Temperature¶

The temperature of a solid surface such as soil, grass, asphalt, or a roof.

It can differ enormously from the air temperature above it, and that difference is the mechanism behind urban heat islands.

Example: On a clear summer afternoon with 30 °C air, asphalt can exceed 60 °C.

Contrast with: Air Temperature

Sustained Wind Speed¶

The average wind speed over a defined period, conventionally two minutes in the United States and ten minutes under World Meteorological Organization standards.

Because the averaging period differs between standards, the same wind can produce different reported sustained speeds and therefore different storm categories.

See also: Wind Gust, Saffir Simpson Scale

Systemd Service¶

A program managed by the operating system's service manager, which starts it automatically at boot, restarts it if it fails, and keeps it running after an operator disconnects.

Without one, a logger stops when its terminal session ends and does not return after a power interruption.

Example: Running systemctl enable station-logger makes the program start automatically after a power cut.

See also: Operating System, Script Execution

Telemetry¶

The automatic transmission of measurements from a remote location to somewhere they can be collected.

Telemetry turns a data recorder into a monitored instrument, because a failure becomes apparent within hours rather than at the next visit.

Example: An hourly upload lets an operator notice within hours that a station has stopped reporting.

See also: Cellular Data Link, Wi-Fi Network, Base Station

Temperature¶

A measure of the average kinetic energy of the atoms or molecules in a substance.

Temperature describes the average energy per particle, not the total energy present, which is why a spark and a bath at the same temperature affect skin so differently.

See also: Kinetic Energy Of Atoms, Heat Transfer

Temperature Conversion¶

The arithmetic of translating a temperature between the Fahrenheit, Celsius, and Kelvin scales.

Because the scales differ in both degree size and zero point, conversion of a temperature requires both a ratio and an offset. Conversion of a temperature difference uses the ratio only.

Example: A rise of 5 °C is a rise of 9 °F, not 41 °F.

See also: Unit Conversion, Measurement Scale

Temperature Inversion¶

A condition in which warmer air lies above cooler air, the reverse of the usual arrangement.

An inversion suppresses convection and traps pollution and moisture near the ground, and it is responsible for most severe urban smog episodes.

Text Editor¶

A program for creating and modifying plain text files.

Program code, configuration, and service definitions are all plain text.

Example: Opening nano logger.py allows the logging program to be edited directly on the station.

Thermistor¶

A temperature sensor made from a semiconductor whose resistance changes sharply with temperature.

Its resistance changes far more per degree than a metal's, making small changes easy to detect, but the relationship is strongly curved rather than linear.

Example: A thermistor's resistance may change several percent per degree, far more than platinum's, but along a curved rather than straight line.

See also: Resistance Thermometer, Semiconductor

Thermocouple¶

A temperature sensor made of two dissimilar metal wires joined at one end, producing a voltage that depends on the temperature at the junction.

Thermocouples produce only microvolts per degree but survive extreme temperatures, working from below −200 °C to above 1700 °C.

Thermoelectric Effect¶

The appearance of a voltage across a junction of two dissimilar metals when the two ends of the circuit are at different temperatures.

Discovered by Thomas Seebeck in 1821, it is the principle behind the thermocouple.

Thermopile¶

A set of thermocouples connected in series so that their individual voltages add to produce a measurable signal.

A single thermocouple produces only microvolts. Wiring many in series is what makes thermal measurement of sunlight practical.

Thermoscope¶

An early instrument that indicates temperature changes without providing a numerical value.

Galileo built one around 1592. Because its tube was open to the air, its reading also responded to atmospheric pressure, which made it doubly unreliable.

Example: Galileo's device showed that the room had become warmer without indicating by how much.

Contrast with: Liquid In Glass Thermometer

Time Constant¶

The time an instrument takes to complete about 63 percent of its response to a step change, used as the standard measure of response speed.

Time Series¶

A sequence of measurements of the same quantity taken at successive points in time.

Order and spacing both carry meaning, and neighbouring values are related, which is what makes an anomalous reading detectable at all.

Example: Shuffling the rows of a temperature log destroys the information entirely, because order carries meaning.

Time Zone¶

A region that shares a common clock time, usually defined as a whole-hour offset from the prime meridian.

Time zones are political rather than physical constructs. Several countries use half-hour or 45-minute offsets, some span many geographic zones under one clock, and daylight saving rules change by jurisdiction and occasionally by year.

See also: Coordinated Universal Time, Prime Meridian

Timestamp¶

A record of the date and time at which a measurement was taken, stored alongside the measurement.

Written in ISO 8601 format in Coordinated Universal Time, timestamps sort chronologically when sorted as plain text, because their fields run from largest to smallest with fixed widths.

Example: 2026-08-25T14:30:00Z

See also: Coordinated Universal Time, Time Series

Torricelli Experiment¶

The 1643 experiment in which Evangelista Torricelli inverted a mercury-filled sealed tube into a dish and observed the column fall to a stable height, leaving an empty space above it.

It demonstrated both that a vacuum can exist and that the atmosphere exerts measurable pressure.

Traceability¶

The property of a measurement being connected to a reference standard through an unbroken chain of comparisons.

Traceability is what allows a reading taken in a back garden to be compared meaningfully with one taken anywhere else on Earth.

Example: A school thermometer checked against a psychrometer, itself checked against a laboratory standard, is traceable.

See also: Calibration, Reference Standard

Transduction¶

The conversion of one form of energy or signal into another, and specifically in a sensor the conversion of a physical property into an electrical property.

Every sensor in this book works by finding a material whose electrical behaviour changes reliably when the property of interest changes.

Example: A microphone transduces sound pressure into a voltage; a speaker transduces a voltage back into sound pressure.

Trend¶

A persistent long-term direction in a time series, distinguishable from short-term variation.

Establishing a trend requires enough data to separate it from noise. A single school year of readings describes weather and cannot establish a climate trend.

Example: A 24-hour moving average strips out the daily cycle and leaves the slower movement of weather systems.

See also: Moving Average, Climate Record

Trilateration¶

Determining a position from measured distances to several points of known location.

Distances alone are used; angles are not, which distinguishes it from triangulation. A satellite navigation receiver needs four satellites rather than three, because its own clock error is a fourth unknown to be solved alongside the three position coordinates.

Example: Knowing the distance to three known points narrows a position on a plane to a single place.

Tsunami Warning¶

An alert issued when an undersea earthquake may have displaced enough water to generate destructive waves.

The warning outruns the wave because seismic waves travel through rock at kilometres per second and alerts travel at the speed of light, while a tsunami crosses deep ocean at around 700 kilometres per hour.

Example: Seismic detection issues an initial alert within minutes; deep-ocean pressure buoys then confirm or cancel it.

See also: Earthquake Early Warning, Seismic Network

Ubuntu Server¶

A Linux distribution intended for servers, shipping without a graphical interface and offering long-term security support.

Example: Long-term support releases receive security updates for five years, which suits a station meant to run untouched.

See also: Operating System, Raspberry Pi OS

Ultraviolet Radiation¶

Electromagnetic radiation with wavelengths shorter than visible violet light, from about 10 to 400 nanometres.

It carries more energy per photon than visible light, enough to break chemical bonds, which is why it damages skin, degrades plastics, and fades pigments.

See also: UV Index, Electromagnetic Spectrum

Unit Conversion¶

Changing a measurement from one unit to another without changing the quantity it represents.

Multiplying by a fraction equal to one lets the unwanted units cancel, and watching them cancel confirms the calculation was set up correctly before any arithmetic is done.

Example: 12 inches × (2.54 centimetres / 1 inch) = 30.48 centimetres, with inches cancelling.

See also: Unit Of Measure, Temperature Conversion

Unit Of Measure¶

An agreed quantity of a physical property against which other quantities of that property are compared.

Example: Saying a table is two metres long states that it is twice as long as the agreed length called a metre.

See also: SI Units, Standardization, Quantitative Data

Urban Heat Island¶

A built-up area that is significantly warmer than the surrounding countryside, commonly by two to five degrees Celsius and occasionally by more than ten.

The causes are all measurable: dark surfaces with low albedo, thermal mass storing and re-releasing heat, loss of cooling from vegetation, reduced airflow between buildings, and waste heat from air conditioning and vehicles.

Example: The effect peaks after dark rather than at noon, because stored heat is released overnight.

UV Index¶

A standardized measure of sunburn-causing ultraviolet radiation, weighted by how strongly each wavelength damages skin.

Developed by Environment Canada in 1992 and adopted worldwide. It cannot be inferred from brightness or warmth, because cloud reduces visible light and infrared far more than it reduces ultraviolet.

Example: A cool, hazy day can carry a UV index of 7, which burns unprotected skin.

Vacuum¶

A space containing no matter.

Aristotle argued that a vacuum was impossible, and the belief stood for nearly two thousand years until Torricelli produced one at the top of a mercury column.

Example: The empty space at the top of Torricelli's inverted mercury tube was the first vacuum anyone had deliberately produced.

Variable¶

A named place in a program where a value is stored for later use.

Including the unit in a variable's name is a cheap defence against the class of error in which two parts of a system disagree about what a number means.

Example: pressure_hpa cannot be silently mistaken for a pressure in pascals.

Visible Light¶

The portion of the electromagnetic spectrum that human eyes detect, roughly 400 to 700 nanometres.

The Sun's output peaks in this band, which is the reason eyes evolved to use it.

Voltage¶

Electrical pressure, the force driving charge to move, measured in volts between two points.

Voltage is applied to a device and must match its rating. Applying a higher voltage than a component is rated for typically destroys it.

Example: A sensor rated for 3.3 volts connected to a 5 volt pin will usually be destroyed.

Water Vapor¶

Water in its gas phase, present in the atmosphere as individual molecules mixed among the other gases and completely invisible.

Clouds and steam are not water vapor; they are liquid droplets, which is why they can be seen.

Watts Per Square Meter¶

The SI unit of irradiance, expressing power arriving per unit area.

Example: The solar constant is about 1361 watts per square metre; an overcast day at ground level may deliver 150.

Weather¶

The state of the atmosphere at one place at one time.

Weather is what a station measures directly: temperature, pressure, humidity, wind, and precipitation at this moment.

Example: The temperature, wind, and cloud outside your window right now.

Contrast with: Climate

Weather Forecasting¶

The prediction of future atmospheric conditions from current measurements.

Modern forecasting solves the physics of the atmosphere forward in time, which requires an accurate picture of the present at as many points as possible. Every additional observation improves that starting point.

Example: Thousands of surface stations, balloons, buoys, and satellites feed a model that solves the atmosphere forward in time.

See also: Barometric Forecasting, Meteorologist

Weather Front¶

The boundary between two air masses of different temperature and humidity.

A cold front produces sharp lift, brief intense rain, and an abrupt temperature drop; a warm front produces gradual lift and long steady rain.

Example: A cold front passing a station shows a pressure minimum, an abrupt wind shift, and several degrees of temperature fall within an hour.

See also: Air Mass, High Pressure System

Weather Vane¶

An instrument that indicates wind direction by pivoting freely so that a large tail surface is pushed downwind and a pointer aims into the wind.

The design is ancient; the Tower of the Winds in Athens carried one around 50 BC.

Weatherproof Enclosure¶

A sealed box protecting electronics from rain, dust, insects, and sunlight.

A completely sealed enclosure fails: trapped air expands and contracts with daily temperature swings, drawing in moist air that condenses inside and cannot escape. A membrane vent is required, and it also allows a pressure sensor to read the outside atmosphere.

Example: A sealed box with no vent draws in moist air each night and condenses it onto the circuit board.

See also: Ingress Protection Rating, Sensor Exposure

Wet Bulb Temperature¶

The lowest temperature achievable by evaporating water into the air at constant pressure, measured by a thermometer whose bulb is kept wet.

In saturated air no evaporation is possible, so the wet bulb equals the ordinary temperature. A sustained wet bulb temperature near human body temperature is survivable only briefly, because sweating can no longer cool the body.

WGS 84 Datum¶

The mathematical model of the Earth's shape and size used as the reference for GPS coordinates.

Coordinates from different datums can place the same physical location more than a hundred metres apart, so mixing coordinate sets without checking the datum introduces errors that resemble sloppy surveying.

Wi-Fi Network¶

A wireless local network, used to carry data from a station within range of an access point.

It requires no additional hardware on a board with built-in wireless, but its limited range ties a station's position to a building, which usually conflicts with good siting.

Example: Convenient within about 100 metres of a building, which usually conflicts with good sensor siting.

Wildfire Risk¶

An assessment of how readily fire will start and spread, combining relative humidity, temperature, wind speed, recent precipitation, and time since rain.

Utilities pre-emptively disconnect power lines when defined thresholds are crossed, so this measurement triggers an operational decision affecting many people.

Example: Hot, very dry, and windy at once is the combination that prompts utilities to shut off power lines.

Wind¶

The horizontal movement of air relative to the Earth's surface, driven by differences in atmospheric pressure.

Air flows from higher pressure toward lower pressure, deflected by the Earth's rotation into the spiral patterns visible on satellite images.

Wind Chill¶

Apparent temperature in cold conditions, describing the increased rate at which wind removes heat from exposed skin.

Wind chill applies to warm bodies only. It does not cool objects below the actual air temperature; a pipe in a −20 °C wind freezes at −20 °C regardless of the wind chill value.

See also: Apparent Temperature, Boundary Layer

Wind Direction¶

The compass direction from which the wind is blowing, reported in degrees clockwise from true north.

By convention direction names the source, not the destination: a north wind blows from the north toward the south.

Wind Gust¶

A brief peak in wind speed, reported when it exceeds the sustained wind speed by a defined margin, conventionally about 2.6 metres per second.

Because a gust is defined relative to the sustained speed, a visible spike does not always qualify.

Wind Load¶

The force that wind exerts on a structure, which rises with the square of wind speed.

Doubling the wind speed quadruples the force. This relationship is why building codes specify design wind speeds by region and why modest increases in storm strength cause disproportionate damage.

See also: Building Code, Enhanced Fujita Scale

Wind Speed¶

How fast air is moving past a fixed point.

Example: A wind of 10 metres per second is force 5 on the Beaufort scale, enough to sway small trees.

See also: Anemometer, Meters Per Second, Beaufort Scale