Q: How do humidity and wind chill make us feel warmer or colder?

A: Our perception of temperature is due to the transfer of heat between our bodies and the environment. It has to do with the body's need to maintain a constant temperature by regulating dissipation of the metabolic heat it generates. Heat and temperature are closely related but distinct concepts. Temperature is a measure of our perception of hotness or coldness. The faster we lose heat to a substance, the colder it feels to us. Heat is energy in the form of molecular motion which moves spontaneously only from high temperature to low temperature. Dissipation of metabolic heat to the air is by convection, conduction, radiation, and latent heat. Of these, only radiation depends entirely on the temperature of the air. The others are affected by moving air and humidity. Because dry air is a poor conductor we exchange only small amounts of heat with it by conduction. Humid air is a slightly better conductor so when it is cold and humid we lose heat more rapidly than we would otherwise and it feels colder than it really is. Moving air feels colder because it removes the heated air from around the skin's surface. The effect of this convective heat loss is measured by the wind chill factor: the temperature at which still, dry air would conduct heat from unclothed skin at the same rate as the wind does at the ambient temperature. A heat transfer problem arises when exertion increases the metabolic rate. To keep its temperature from rising too high the body must exhaust this excess heat. Another problem arises when the air is warmer than the skin. Heat flows only from hot to cold so the body cannot lose heat to the warmer air by conduction, convection or radiation. The solution to both problems is the evaporation of perspiration from the skin which absorbs heat. This latent heat loss is faster in warm air, but slower in humid air. Warm, humid, air feels warmer than it really is because we are not losing heat to it rapidly enough. Cold, damp air feels colder than it really is because we are losing heat to it too rapidly. Moving air feels cool because it both carries away heat and increases evaporation. The combination of trade winds and relatively dry air give Hawaii its natural air conditioning and keep us cool, but most of the time not too cold.

Richard Brill is assistant professor of science at Honolulu Community College. Send questions to Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817, fax to 239-5152 or email to rickb@hcc.hawaii.edu

Heat & Humidity ©1996 Richard C. Brill