Why is wind cold?

Mammals lose heat by evaporation of moisture from the skin. You HAVE noticed that you sweat when it's hot, have you not?

Wind increases the speed of evaporation.

 

But it's a good one. When you are sweating, the breeze can cool you through evaporation, even though the air temperature is not cool. Evaporation can cool you...

 

Even without evaporation, it blows off the warm air layer just next to your skin. Your body is about 37 degrees Celcius, which is usually hotter than the atmosphere next to it - except in a desert in summer, where a "breeze" is by no means cool.

 

If a surface is at a higher temperature then the surrounding air, then it heats the air that is close to the surface. For example, the air that touches the surface may be at the same temperature as the surface. There is a boundary layer near the surface where the temperature of the air and the temperature of the surface is nearly the same. Heat flow through that thermal layer will be much smaller than the heat flow that would result if the there was no thermal layers. The heated air near the surface acts a lot like an insulator.
The wind reduces the boundary layer. The wind carries away the heated air, which forms the thermal layer, by convection. The wind replaces the thermal layer by cooler air from far away. The flow of heat through the cooler layer is greater then it would be for a warmer layer.
Thus, one could say that the wind blows away the thermal layer of air that serves as a sort of insulator.
Stationary air is a good insulator. However, moving air conducts heat much better than stationary air.
The moving air does heat the surface by friction. However, the heat energy generated by friction is very small. The heat carried away by convection is much larger than the flow of heat generated by friction.
The case may be different if the atmosphere was the exact same temperature as the surface. Then, there would be no thermal layer. The friction could make a difference. It may heat the wall, a little. However, the thermal layer is always important if there is a big difference between the temperature of the air and the temperature of the surface.

The apparent temperature of the air is characterized by the wind chill factor. The process that I just described is convection of heat from the surface by the wind.
http://en.wikipedia.org/wiki/Wind_chill
“The human body loses heat through convection, evaporation, conduction, and radiation.[1] The rate of heat loss by a surface through convection depends on the wind speed above that surface. As a surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts the boundary layer, allowing for new, cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.
For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects. Thus, the attempt to maintain a given surface temperature in an environment of faster heat loss results in both the perception of lower temperatures and an actual greater heat loss increasing the risk of adverse effects such as frostbite, hypothermia, and death.”

http://en.wikipedia.org/wiki/Convection
“Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids.[5] Heat is the entity of interest being advected (carried), and diffused (dispersed). This can be contrasted with conductive heat transfer, which is the transfer of energy by vibrations at a molecular level through a solid or fluid, and radiative heat transfer, the transfer of energy through electromagnetic waves.
Heat is transferred by convection in numerous examples of naturally occurring fluid flow, such as: wind, oceanic currents, and movements within the Earth's mantle. Convection is also used in engineering practices to provide desired temperature changes, as in heating of homes, industrial processes, cooling of equipment, etc.”

http://engineering.dartmouth.edu/~cushman/courses/engs44/insulation.pdf [Broken]
“Air is an excellent insulator (R = 5.5/inch), as long as it does not move.
It has poor conduction but good convection properties.”

 

This, by the way, is why things like styrofoam and rock wool are good thermal insulators. They capture air, which is a good insulator, and make it very difficult for it to move.

 

And hair/ fur, among other things...

 

Your body cools by evaporation, true, but there is also the concept of convection to worry about. If your skin temperature and the air temperature are not the same temperature, there will be heat transfer between them. As the speed of the wind increases, in general, the speed of heat transfer also increases. Usually the air is cooler than your body so you feel this as a cooling effect. To a point, you still feel it cooling you even when the wind is warmer than your body because even though there is net head transfer into your body from the air, you are still losing more heat through evaporation.