One of the most misunderstood things we have to understand
is the “Wind Chill Factor.” We hear it every time we listen we listen to
a weather forecast without the slightest idea of what the forecaster is babbling about
this time.
To explain the wind chill factor let us suppose that your
body is a radiating
body that is a heat source. If the
wind is still only a small amount of heat will be radiated from your body, but
as the wind speed
increases the heat your body is putting out is removed faster as a result of
the moving air.
The reason for wearing clothes is directly connected to the
wind chill factor. On a warm day we need
just enough clothes to stay out of jail.
As the temperature drops and the speed of the wind increases we require
more clothes. If the wind is blowing at
a given temperature the amount of heat removed from our body increases by the
velocity of the wind. The harder the
wind blows the more clothes we wear.
The author has experienced -62 degrees Fahrenheit in a wind
that was blowing about 15 mph, it was cold.
In this kind of weather conditions you actually dress in layers with the
outer layer being made of some kind of windproof fabric or some kind of natural
fur. I should imagine that the wind chill factor that day was around -80 Fahrenheit if not more.
That is a rather extreme example of the wind chill factor,
but it is calculated by using what the temperature would be if the wind was
still and not blowing. It is entirely
possible to have a wind chill factor of “O” at a temperature that is above
freezing +32 Fahrenheit if the wind is blowing hard enough.
We all know that the freezing point of water is +32 F but
even though O F is below the freezing point the water doesn’t freeze. This is because the wind blowing over water
makes waves, and water is at its densest at +34 F. Indeed the water is chilled to its densest,
but on the forward edge of the wave the chilled water sinks into warmer water
that is below it. On the backside of the
wave the pressure difference between it and the water under it is less, and it
draws the warmer water up into the wave where it is cooled by the action of the
wind. This creates a continuous cycle of
warmer water replacing colder water that continues until the wind velocity dies
down until the water is finally calm.
Then the water returns to a state of equilibrium. A human body exposed to the same conditions
would find itself uncomfortably cold.
References:
Wind Chill, Wikipedia, http://en.wikipedia.org/wiki/Wind_chill
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