Liza Cohen, age 12, of San Francisco, Calif., for her question:
WHAT IS HUMIDITY?
The word humidity belongs to the weatherman, who uses it almost every day. He may tell us that the humidity is high, low or medium. Or he may announce the precise percentage of relative humidity. We know from experience that high humidity goes with hot muggy weather. But naturally there is much more to the story behind the scenes.
Daily weather reports are based on local readings of temperature and pressure, wind and precipitation and relative humidity. It is no big problem to check temperature, pressure, wind data and precipitation with the help of a few weather gadgets. But the day's humidity is a very tricky job. It calls for a sample of the day's air, plus its temperature and vapor content. This data is checked on a precise chart of relative humidity.
Basically, humidity is the quota of vapor in the air. This is relative because there is a limit to how much vapor the air can hold at a certain temperature. Warm air can hold more vapor than cool air. And a relative humidity of 100% means that air at this or that temperature has reached its saturation point.
The whole thing is very complicated because the relative humidity changes when the vapor content of a given air sample stays the same and the temperature rises or falls. The standard sample is a cubic meter of average outdoor air and its vapor content is given in grams.
For example, suppose the air sample is 86 deg. and the cubic meter contains 30.4 grams of vapor. This is saturation point and the relative humidity is 100%. If the same air cools to 68 deg., its saturation point is 17.31 grams. It now has a surplus of 3.65 grams of gaseous water which must be converted into drops or droplets of liquid water.
When the vapor content of our 86 deg. sample is 17.31, its relative humidity is 57%. It can hold 3.65 more grams before saturation point. At 50 deg. our cubic meter of air reaches 100% humidity with a vapor content of 9.41 grams. At a freezing 32 deg., the saturation limit is 4.85 grams.
Obviously the relative humidity is the trickiest item in the daily weather report. It is hardest to figure and, what's more, it often changes from moment to moment. The sudden arrival of a moist or dry wind changes the percentages. And the relative humidity rises and falls with falling and rising temperatures.