Wind chill, a pleasant breeze, and severe dehydration
Air speed is a multiplier on heat exchange, and the same breeze can be a comfort or a danger depending on the temperature. Why wind chill does not actually make the air colder.
Continuing our series on thermal neutrality, this article covers air speed: what it is, and what it does to a body.
How air speed is measured
Air speed is expressed in metres per second, or feet per minute in imperial. Indoors it typically ranges from effectively zero up to about 1.5 m/s. Outdoors it can be many times that.
Faster air means faster heat exchange
The body generates heat continuously and has to hold a core temperature of around 37 °C. In an environment cooler than that, the surrounding air is actively taking heat from your skin by convection and conduction. Above 37 °C, the air is adding heat to you instead.
Neither is inherently good or bad; it depends where you are on the balance. When heat loss outpaces heat production you feel cold, and if it continues, core temperature falling below 35 to 36 °C means hypothermia. When heat gain outpaces your ability to shed it, core temperature above 39 °C brings its own serious consequences.
Air speed acts as a multiplier on that exchange. It does not change the temperature of anything. It changes the rate.
That produces three quite different situations.
1. Wind chill
With little or no air movement — say 0 to 0.2 m/s — the air immediately against your skin is warmer than the air a metre away, because it has already absorbed some of your heat. That warmed layer acts as insulation.
Eventually it drifts away and is replaced by fresh, cooler air, which takes more heat from you because the temperature gradient is steeper.
Raise the air speed and that replacement happens continuously. The insulating layer never gets a chance to form, heat leaves you faster, and it feels colder than the thermometer says.
Here is the part that surprises people: wind chill does not lower the air temperature. The air is exactly as warm as it was. You can verify this — water does not freeze because the wind chill figure reads below zero. Wind chill is a statement about the rate at which you lose heat, not about the air. Cold, after all, is not a thing; it is the perception of heat leaving.
2. A pleasant breeze
Between roughly 28 °C and body temperature, the air is still absorbing heat from you, but less effectively — as the temperature gradient narrows, so does the flow.
The body compensates. Blood vessels near the skin dilate, bringing warm blood closer to the surface, and you begin to sweat. Evaporating sweat carries away a great deal of heat.
Increase air speed here and the same multiplier applies, but now it works in your favour: replacing the humid, warmed air next to your skin speeds up both convective loss and evaporation. This is why a fan feels wonderful in summer, and why the car window goes down.
Same physics as wind chill. Opposite verdict.
3. When it becomes dangerous
Above about 37 °C, the sign flips.
Air hotter than you cannot absorb your heat. It adds heat instead. Worse, it supplies the energy driving evaporation, so you sweat faster while getting less cooling from it — and you dehydrate rapidly.
Raising air speed in these conditions accelerates the dehydration without delivering cooling. This is why an electric fan stops helping once air temperature exceeds body temperature, and starts being a health risk. It is a genuinely counter-intuitive result, and it kills people in heatwaves.
In summary
Air speed multiplies the rate of heat exchange between a body and its surroundings:
- Cold air: faster loss, the sensation of cold, and eventually a real risk.
- Warm air below body temperature: faster loss, and a welcome one.
- Air above body temperature: no cooling available, accelerated dehydration, and a hazard.
Next in the series: what all this means for air speed in the built environment, and how it should shape the way a building is heated.
