|COMPARISON OF VERY DRY AND VERY MOIST AIR
METEOROLOGIST JEFF HABY
Dry air, in relation to the weather, is air that has a low amount of moisture. The air is typically not completely dry but at very cold
temperatures or at warm temperatures with a low relative humidity, the air has only a small amount of moisture. Air that is dry will
have either a low relative humidity or a small value of moisture. At very cold temperatures, air can have a high relative humidity
but have a small amount of moisture. This air though often has less moisture than warm air with a low relative humidity.
Moist air, in relation to the weather, is air that has a high amount of moisture. Dewpoints above 60 F and mixing ratios above 15 g/kg
are indications the air has a significant amount of moisture. For air to have a high amount of moisture, it is more important to
observe the dewpoint than the relative humidity. Relative humidity can be high at any temperature, but only when temperatures are
warm does a high relative humidity indicate the air has a significant amount of moisture.
The contrast I would like to make is between saturated winter-like air to saturated tropical-like air. Suppose the winter-like air has
a temperature of 17 F and the air is saturated and the tropical-like air has a temperature of 77 F and the air is saturated. In
the winter-like air, there is around 2 g/kg of moisture in the air while in the tropical air there is around 20 g/kg of moisture
in the air. Thus, the tropical air has 10 times as much moisture even though the relative humidity is 100% in both the winter-like
and tropical-like air.
The reduced amount of moisture in winter air is responsible for indoor air being so dry (high static, dry skin, chapped lips). Many homes
require the use of a humidifier to reduce the problem of indoor dry air. The increased amount of moisture in tropical
air is responsible for indoor air being humid. Problems such as mold and mildew can result from humid indoor air. In this
case a dehumidifier can take some moisture out of the air.