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J. Perelman
"Entertaining physics". Book 2.
Chapter 7. Thermal phenomena

THAT'S WHY WHEN THE WIND IS COLDER?

Everyone knows, of course, that in calm weather, frost is transferred much more easily than with the wind. But not all of us realize clearly the cause of this phenomenon. More cold when the wind is felt only living beings; the thermometer does not fall below when wind blows. Feeling a sharp cold in the windy cold weather is due primarily to the fact that from the face (and body) is taken away much more heat than in calm weather, when the air heated by the body, not so quickly replaced by a new portion of the cold air. Than the wind is stronger, the greater the mass of air time for each minute to come in contact with the skin, and therefore, the more heat is taken every minute from our body. This one is already enough to cause a cold feeling.

But there is another reason. Our skin is always evaporates moisture, even in the cold air. For evaporation requires heat; it is taken away from our body and from the layer of air that the body fits. If the air is stationary, the evaporation takes place slowly, as adjacent to the skin layer of air is soon filled pairs (saturated with moisture the air is not intense evaporation). But if the air is moving and skin replenished by new his portion, the evaporation of all time is supported by very abundant, and it requires a large consumption of heat, which is taken from our body.

But how great cooling effect of the wind? It depends on his speed and air temperature; in General it is much larger than usually think. Here is an example which gives an idea of what happens this fall. Let the temperature is +4°, and there is no wind. The skin is our body under such conditions, has a temperature of 31°. If the breeze is blowing, barely driving flags and not cheveley foliage (speed 2 m/s), the skin is cooled to 7°; when the wind, causing the flag to polecats (speed 6 m/s), the skin is cooled to 22°: temperature it drops to 9°! These data are taken from the book N. N. Kalitina "Fundamentals of atmospheric physics as applied to medicine"; the curious reader will find in it many interesting details.

So, how will be felt by us cold, we can't judge by only one temperature, and must take into account wind speed. The same frost is transferred to Leningrad on average worse than in Moscow, because the average wind speed on the shores of the Baltic sea is 5 - 6 m/sec, and in Moscow - only 4.5 m/sec. Even easier to frost in Transbaikalia, where the average wind speed of 1.3 m Famous Arctic frost felt not so cruel as to think we are accustomed in Europe to relatively strong winds; Eastern Siberia characterized by almost complete calm, especially in the winter.

Entertaining physics J. Perelman

 




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