How and why?

Windchill is most commonly felt when a living being is out of doors in moving air, but how does it work?

There are three cases to consider when studying the cooling effects of moving air versus still air, given the same ambient temperature. These are:

  1. An object at ambient temperature.
  2. An object above ambient temperature, cooling to the ambient level.
  3. An object above ambient temperature, but attempting to replenish its own heat.

At Ambient Temperature

The first category is the simplest to deal with. Any amount of movement of the surrounding air will not cause an object to drop below ambient temperature. A thermometer, for example, will read the same in still or moving air.

Cooling To Ambient Temperature

Example 2 could be a cup of coffee. In still air, the coffee will cool slowly, since an insulating pocket of warm air will form around it. Of course, in real life, this air will convect away, and cause a moving-air situation, but we will ignore that for the purpose of this examination.

If we move air past the cup of coffee, the heat will be taken away more quickly, and the cup will cool quicker - as if it is in cooler still air. That is until it reaches close to ambient temperature, which it will not go below.

Replenishing Heat

Now we examine example 3. This could be a person, or a house with the heating turned on. The person can be clothed - exposed skin is not a requirement. We have already established that when something is warmer than the ambient temperature, it loses heat to its surroundings. However, the metabolism (in the person), or the boiler (in the house) work to keep the temperature high. It is the balance between heat loss and replenishment which is the important point here. With moving air around the object, heat is carried away quickly, and the heat loss is greater. More work is required to replenish the heat, or else the temperature of the object will drop slightly. This is perceived as a lower ambient temperature - the crux of windchill.

Corollary

An interesting corollary to this is to examine the effects when ambient temperature is above the temperature of the body. Normally (in temperate climates), the temperature is below that of the human body. A fan can be used to reduce your body temperature by carrying heat away more quickly. If the ambient temperature is above that of your body, no amount of fanning will help - you need to drink cold water to carry heat from your body, or use an air conditioner to reduce the temperature of the moving air, and therefore the ambient temperature.

Measuring Windchill

Windchill is notoriously difficult to measure. A thermometer will record only the ambient temperature. A wet and dry thermometer is affected by humidity as well as windchill. The windchill is also related to surface area, mass and heat capacity of the body.

A general idea may be gained by placing an object in still air of the correct ambient temperature, and heating it. It should be heated until at a steady input of power, its steady temperature is the same as the body under test - for example, the blood temperature of a human.

Once it reaches the correct steady-state temperature, the power input of the heater should be noted. The experiment should be repeated in the moving air, with the heater set to exactly the same power as before. The steady state temperature will be seen to be lower, and this will be related to the windchill - unfortunately, the relationship will not be trivial.