Because our skin temperature is closer to the air temperature when it is windy, we feel that the wind makes it colder outside. When we feel that it is cold outside, we are actually sensing the temperature of our skin. Humans do not sense the temperature of the air. When there is wind, the thermal resistance of the boundary layer is smaller, the heat loss is higher and the temperature of the skin is closer to the air temperature. The insulation of the boundary layer depends on its thickness. Because the outer layers of still air are blown off more easily than the ones closer to the skin, when it is nearly calm, a small increase in wind speed causes a much greater thinning of the boundary layer thickness than the same increase in wind speed when the wind is already strong.Ĭonvective heat loss is really conduction through an insulating boundary layer. The stronger the wind, the thinner the layer. Add a wind, and the only still air that remains would be the air in the immediate vicinity of some surface, like the skin. In a perfect calm, if free convection could be suppressed (as it is in microgravity), the boundary layer would be infinitely thick. If you do the same experiment in a hot sauna, instead of feeling cool, the spot you blow on can feel painfully hot, because you have blown away the boundary layer of sauna air that had been cooled by the skin and allowed the heat of the sauna to reach the skin more easily. If you blow on your arm, it can feel cool even though your breath is relatively warm because you have blown away the warm boundary layer air that was insulating the skin. The boundary layer insulates your skin from the environment. As a result, near the skin or any surface there is a zone of relatively still air that may be several millimetres thick. Because air has some internal stickiness (viscosity), there is drag between the adhering air and the air molecules farther away from the skin. Right at the surface of the skin, the adhering air is still. We live in an ocean of air, and air, like water, can be said to “wet” the skin or any other object that is immersed in it. If you dive into the ocean and come back out again, a thin layer of water will adhere to your skin. To understand wind chill, one must have some appreciation of the concept of a thermal boundary layer. ![]() See Wikipedia's guide to writing better articles for suggestions. Specific concerns may be found on the talk page. The tone or style of this article or section may not be appropriate for Wikipedia.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |