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How does wind chill work / Why does wind make stuff cold?

  • I've been wondering for a while now how wind chill works and actually causes you to feel colder? Another example is blowing on hot food, why does that cool things down? Thanks

  • Answer:

    It is to do with thermodynamics and contact time. The greater the temperature difference between 2 objects the faster the temperature of the objects will change as they try to balance to become in equilibrium (its logarithmic). E.g. if you have a glass of hot water in a room what happens is the cooler room temperature reduces the temperature of the glass of water and the glass of water actually raises the temperature of the room (only slightly). In still air, the air local to the glass of water will be a local hot spot i.e. slightly warmer than the room thus reducing the difference between the glass and surrounding air (ignoring the effects of convection). When you add a breeze to the equation (wind chill) you replace the local hot spot with new air. This keeps the temperature difference between the air and object towards the maximum. Because the temperature difference is kept large the rate of change of temperature of the object remains fast. Things will actualy cool down but only to match the temperature of the air (or wind). They will not cool down indefinately. Our problem is the surrounding air is usually less than the bodies ideal temperature (37C core and approx 21C external to body). The problem for people is made worse becuase we lose sensible heat (sensible heat is normal heat loss) and latent heat (latent heat is in this case heat energy used to change liquid [sweat] to gas [sweat evaporate vapour]). High wind speeds add an extra element because your clothes normally create a layer of trapped air that your body heats up and this acts as a layer of thermal insulation. High winds will suck this layer away (entrainment) and effectively remove a layer of clothing. You can feel this when a vehicle passes you at high speed. Another more complex aspect is the relative humidity of the air will affect the amount of moisture the air can take up. If the take up is reduced (as in a high humidity e.g. rainforrest) your sweat can't escape your body as easily and you lose more heat because waters heat capicity is far greater than that of air.

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We have an invisible shell of warm air around us, kept that way by our body heat, when it is blown away our body gets cooler by trying to keep that shell warm.

Ian r

Blowing on hot food simply moves fast the particles of air that are colder than the food and therefore remove faster the heat of it. Wind chill is something else. Water has to take energy in form of heat in order to go from solid (ice) to liquid (water) and from liquid, to gas. The opposite is also true: from going to gas to liquid, or liquid to solid, it gives away energy in form of heat. That is the reason we sweat: our body tries to cool down by letting water on our skin to evaporate. If we are for example in saturated atmosphere, like in the fog, we won't gain anything from sweating; the air can't take more moisture, it is already 100 percent saturated. The dryer the air, i.e. the lower the relative humidity, the more we will feel the effect of the sweating, or drying of our skin if, e.g. wet on the beach.

Michel Verheughe

Air around the body becomes warmed from the body. In still air this acts as an insulator. When this still air is replaced by cooler air from say a wind then the insulation effect is lost. This obviously makes you and the food cool. This is not a lot of hot air which I am preaching but a scientific fact.

Mickyd

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