Heat is required or lost anytime a substance changes state (solid to liquid, liquid to gas, etc) For evaporation to take place, heat is required to change a liquid to a gas. This is why the body uses the mechanism of sweat to cool the body. When the liquid sweat from the body comes in contact with moving air, evaporation takes place. This draws heat from the body to facilitate the change in state from liquid to gas. While this works perfectly well during hot or warm conditions, evaporation is not a good thing when hypothermia is a danger.
Further loss of heat in cold conditions from evaporation is common when the following occurs. A hiker is working hard going up a slope and begins to sweat because the body is burning a lot of fuel and needs to remove some of that extra heat. The hiker then stops to rest a bit, and the moving ambient air then comes in contact with the sweaty hiker. Evaporation then takes place. The hiker feels an immediate chill. This is the most common form of evaporative danger when it comes to hypothermia for the climber and/or hiker, etc.
HOW TO LESSEN THE DANGERS OF COLD FROM EVAPORATION:
1) Proper attention to clothing is critical here. You do not want to get soaked by excessive sweating when hiking
. This really adds to heat loss due to evaporation, and even more so with the next topic we are going to discuss. This is why layering is so important when is comes to clothing. Better to feel good when hiking and stay dry, then to add more clothing just so you can stay warm when taking a short break. If you get your clothing all wet from sweating, it will be very difficult to get it dry under cold and/or damp conditions in the mountains or any other outdoor location.
2) Just like with convection, using a windbreaker type shell will greatly slow down heat loss due to evaporation. Also, to add a little sense to the first point just above, if you are done for the day, or if you are going to take an extended break, you need to adjust your clothing by removing anything that is wet, and adjust your layering so that you are warm enough to be comfortable.