VERY low temperatures are quite bearable in calm conditions. You feel relatively comfortable because a layer of nearly motionless air encapsulates your body; its innermost layers adapt to the body temperature, and the envelope insulates you like the "dead air" space between the panes of a double-glazed window.
When a breeze develops, however, it disturbs and ultimately blows away this insulating layer, and thereafter each molecule of air carries away with it the heat it gathers as it touches you.
It was thoughts like these that led the American meteorologist Paul Sipple, working in Antarctica in 1939, to devise the concept of wind chill. Although quoted as a "temperature", it is more a measure of the influence of the wind on the perceived temperature. It is in a sense a measure of discomfort; it answers the question: "How cold would it have to be if there were no wind blowing, for me to feel as uncomfortable as I do now?"
The idea is very popular, mainly because, at a superficial level, it is easy to understand, and we have all felt chilled by a stiff breeze when standing in the cold. It is appropriate enough to think in terms of wind chill in any circumstances where humans or animals are exposed to the relevant conditions, or when studying the problem of heat-loss from buildings in cold weather.
Indeed the concept of wind chill can be applied in any situation where warm objects are exposed simultaneously to both wind and low temperatures; where, quite simply, the cold wind carries the heat away.
But it is wrong to apply the concept to unheated, inanimate objects. The temperature of the radiator of a stationary car, of an oil storage tank, or of growing plants - if they are dry - will not drop below the local air temperature no matter how strongly the wind blows.
If they happen to be wet, their temperature may drop slightly below that of the surrounding air because of heat lost through evaporation, but the concept of wind chill in such cases is quite meaningless.
And even in the case of human beings, the wind chill equivalent temperature quoted on the media should not be taken too literally. The extent to which we feel the cold depends on many other factors besides wind and temperature: it depends on how much clothing we have on, on our age and physical condition, on whether we are physically active at the time or not, and indeed on whether the wind comes from behind or blows directly on to the face.