Wind chill is winter’s version of summer’s heat index and they both deal with dangerous temperatures, cold and hot. Hot temperatures on a humid day can feel much hotter than the actual temperature because the body is cooled through evaporation and that action is stifled due to the moisture in the air. Cold temperatures on a humid day, it would seem, should also feel warmer because having a higher humidity makes our body evaporate less water, resulting in less cooling. In fact cold, dry air will most times feel warmer than cold, humid air at the same temperature. Many have experienced cold, dry air and compared it to experiencing cold, humid air and know first hand that a cold day in the southeast U.S. feels colder than a cold day in the southwest U.S.
The naked truth of why this is has to do with the clothes we wear. A person without clothes in cold, humid air may feel slightly warmer than in cold, dry air at the same temperature. As the humidity increases in cold air, the clothing we wear to keep warm insulates less. The moisture in the fibers and the wicking effect of the clothes can accelerate the heat loss through clothing. Even though the clothes do not appear wet, the humidity in the air may create a thin layer of moisture on the fibers and result in greater conduction, or transfer, of heat through the clothing.