Re: Does relative humidity affect how cold a person feels?

I'm afraid I have to agree with your Icelandic friend. You're right in saying greater relative humidity makes a person feel warmer on a hot day...however, greater relative humidity does not necessarily make a person feel warmer on a cold day...and in fact usually does just the opposite. The reasons for this have to do with the body's response to environmental temperatures.

Three physical processes determine the heat exchange between the body and the environment. 1) Radiation - heat absorbed from walls, or heaaters (if their temperature is higher than that of the body), or from the sun, sky, and ground. The body emits radiation, or loses heat, at the same time. We notice this most when temperatures in our environment are considerably cooler than our bodies. 2) Convection - heat transferred by air motions...wind or drafts. 3) Evaporation - the one we're concerned with in this question.

In hot weather, the body perspires and this water evaporates from the skin, heat loss occurs, and the skin is cooled. More water will evaporate if the air is dry rather than humid....making us feel relatively cooler. On really hot days...the sweat cannot evaporate as fast...the layer of air next to our skin is already so close to saturation that our sweat cannot be evaporated as fast as it is produced and we grow uncomfortably warm. The degree of cooling depends on the area of skin exposed, the velocity of the air around us, and the relative humidity. Even when the body is not sweating, there is some heat loss through breathing Relatively dry cool air is carried into the lungs where evaporation and warming occur.

On cold days...there is much less actual water vapor in the atmosphere. than at the same relative humidity on a warm air. That's why it's called RELATIVE humidity. (Check out USA TODAY's page on relative humidity.) Less water vapor is needed to reach saturation at lower temperatures than at warmer temperatures.

Even on cold days our bodies are perspiring. The sweat is evaporated into the layer of air next to our skins...cooling it's temperature and making us feel colder. Drier air allows this evaporation to happen quickly almost unnoticed by us. The addition of our sweat to the layer of air next to our skin is so small as to produce little noticeable cooling. In higher humidities, the moisture lingers, dampens our clothing...and the evaporational cooling is much more noticeable. Thus we feel colder rather than warmer.

Windy days can feel colder than calm days of the same temperature. Air is such a poor conductor that if no wind is blowing and we are not moving, a thin layer of warm air forms next to the skin and we may feel quite comfortable.