Skip to content

Apparent Temperature Explorer

Run the Apparent Temperature Explorer MicroSim Fullscreen
Edit in the Chart.js Playground

About This MicroSim

Wind chill and heat index are almost always taught as two unrelated lookup tables. They are not. They are two regimes of one question: how fast can this body shed heat? Drawing them on one axis makes that visible.

Air temperature runs left to right across the whole chart, from −40 to 50 °C. The vertical axis switches with the regime, because the variable that matters switches too: wind speed in the cold, relative humidity in the heat. Between them sits a grey band from 10 to 27 °C where neither correction applies, and it is labelled as such. Clicking into the far side of the chart switches regimes and says so.

Colour is the difference from the actual air temperature, so a cell that feels colder and a cell that feels warmer are visibly different things.

Two features target specific misconceptions:

  • What is being cooled? Switch from Person to Car or Water pipe and the whole surface goes flat. Every column collapses to the air temperature, because that is what actually happens. Wind chill is a rate of heat loss from a warm body, not a temperature the air reaches. A pipe freezes when the air hits 0 °C, and wind only changes how quickly it gets there.
  • The wet bulb ceiling. In the hot regime a dark region marks where wet bulb temperature exceeds 35 °C. Inside it, evaporative cooling has stopped working: no shade, no water and no fan help, and the condition is survivable for only a few hours even at rest.

Four presets set up the arguments. Minnesota January at −20 °C in 8 m/s feels like −32.4 °C. Gulf Coast August at 33 °C and 80 per cent feels like 48.1 °C. Sahara noon at 45 °C and 10 per cent feels like 42.1 °C — cooler than the thermometer, because sweat evaporates freely. And British autumn lands squarely in the grey band, where the answer is that nothing is happening at all.

The cross-section view fixes the air temperature and sweeps the other variable, which is where the diminishing returns of wind speed become obvious. The pin holds one condition while you probe another and reports the difference between them.

Sources are printed under the chart: the 2001 NWS and Environment Canada wind chill index, the Rothfusz heat index regression, and Stull (2011) for wet bulb.

How to Use

  • Click or drag anywhere on the heatmap to probe a condition.
  • Try a preset, then move one variable at a time and watch which one moves the answer.
  • Set What is being cooled? to Car. This is the single most useful thing here.
  • Change the wind unit and re-read the same point in m/s, km/h, mph and knots.
  • Pin this point, then probe a second condition to compare them directly.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://iowerx.github.io/pi-env-monitor/sims/apparent-temperature-explorer/main.html"
        height="532px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Grade Level

6-12

Duration

20-25 minutes

Bloom's Taxonomy Level

Apply (L3)

Prerequisites

  • Chapter 8 on relative humidity and wet bulb temperature
  • Chapter 10 sections on wind chill and heat index

Activities

  1. Find the crossover (6 min): Starting from −20 °C, raise the air temperature until the wind stops mattering. Where does it happen, and why is the band drawn where it is?
  2. Cool a car (5 min): Set the target to Car at −5 °C in 15 m/s. Write down what the readout says and explain it to someone who thinks the car reaches the wind chill value.
  3. Two ways to be dangerous (8 min): Pin Gulf Coast August, then probe Sahara noon. Both are dangerous. Explain, using wet bulb, why only one of them is dangerous in a way that shade cannot fix.

Assessment

  • Calculates apparent temperature from air temperature plus one secondary variable.
  • Explains why wind dominates in the cold and humidity in the heat.
  • States correctly that objects cool to the air temperature, not to the wind chill.

References

  1. NWS: Wind Chill Chart and the 2001 index - the formula and the frostbite times used here.
  2. Wikipedia: Wind chill - Siple and Passel, and why their 1945 formula was replaced.
  3. NWS Weather Prediction Center: the heat index equation - the Rothfusz regression and its two adjustments.
  4. Wikipedia: Wet-bulb temperature - the 35 °C limit and what it means physiologically.