Sunday, June 17, 2012

The Wind Chill Factor:

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.


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