Dew Point Temperature

Ask the Weather Guys: How is humidity measured?

Q: How is humidity measured?

A: As the spring finally comes after a cool second half of April, it is not only the temperature that rises but also the humidity.

Humidity is a measure of the amount of invisible water vapor that is in the air. This amount can be measured in a variety of ways and is easily the most highly variable part of the air throughout the year.

One measure of the water vapor content is known as the mixing ratio. The mixing ratio is, as its name suggests, a ratio of the mass of water vapor in the air (measured in grams) to the mass of the dry air sample that contains the water vapor, measured in kilograms. The dry air is the mixture of every other chemical in the sample except water vapor.

In the dead of winter, the mixing ratio may be as low as 0.5 grams/kilogram while during the middle of summer it can be as large as 20 grams/kilogram. Naturally, the larger the amount of water vapor, the "muggier" the air feels and the more likely it is that thunderstorms with torrential rains may develop.

Another, perhaps more popular, measure of the humidity is the dew point temperature. The dew point temperature is the temperature to which the air must be cooled (at constant pressure) before the water vapor contained within the air condenses into liquid water.

As you might guess, the higher the dew point temperature, the higher the water vapor content of the air and the "muggier" the air feels. For this reason, the dew point temperature is often a reliable gauge of human comfort with the usual high dew points of summer leaving most of us feeling rather uncomfortable.

Steven A. Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) the last Monday of each month at 11:45 a.m.