It's Not the Heat, It's the Humidity

Consumers' Research Magazine, July 1994 | Go to article overview

It's Not the Heat, It's the Humidity


Hazy, hot, humid: these words are so easy to say (short, simple, and alliterative) and they not only typically describe today's weather but also the weather for the remainder of the week (and probably for the remainder of the summer, for that matter). But what do these simple and often used words really mean? Well:

Hazy--when you see the air, it's hazy (the visibility is down); Hot--when you see a lot of red on your thermometer, it's hot (the temperature is up); Humid--when it feels sticky, the relative humidity is high--or is it?

Relative humidity is the measure of the amount of water vapor in the air compared to how much it can possibly hold at that temperature. A relative humidity of 50% means that the air contains only one-half as much water as it potentially could contain.

The capacity of air to hold water is strongly dependent on the temperature of the air, and this relationship is not linear. The warmer the air, the more water it can hold. Air at 95[degrees]F can hold four times as much water as air at 50[degrees]F. For this reason, relative humidity can be a misleading measure of air "stickiness." For example, as the temperature of air varies, the relative humidity also varies, even if the actual amount of moisture in the air remains the same. On a typical day, the relative humidity is much higher (near 100%) in the early morning, when the temperature is the lowest, than it is in the afternoon, when the temperature is at its highest. Yet, it feels much "stickier" in the middle of the afternoon.

Another example: during a snowstorm in January, the relative humidity is 100%. The air contains as much mosture as it can hold, and, as a matter of fact, there is a surplus of moisture, which is why it is snowing. Usually, there are not many complaints about it feeling sticky and "humid." On the other hand, on a summer afternoon when the air temperature is 95[degrees]F and the relative humidity is somewhere around 50%, the stickiness of the air is the leading topic of conversation.

That Awful, Sticky Feeling. So what exactly is going on? What is it that produces that sticky feeling if it is not the relative humidity? Well, at temperatures below about 75[degrees]F, your body can cool itself through radiation and conduction. The air surrounding your body is cool enough to allow your body to get rid of most of its excess heat.

When the temperature begins to climb into the 80s and 90s, though, the air is too warm to allow you to cool passively, so your body begins to try to cool itself actively, and you begin to sweat. You then cool through the evaporation of perspiration. This method of cooling works as long as the sweat can evaporate.

When the relative humidity is low, the air is well below saturation, and evaporation takes place rapidly. As the relative humidity rises, the evaporation slows because the air begins to near saturation with respect to water vapor. On a day that is in the upper 90s, your body has to work hard to cool itself and, thus, you sweat a lot. Even at a seemingly low relative humidity, like 40%, evaporation does not take place fast enough to keep up with the sweat production and you become very wet and uncomfortable. …

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