Absolutely dry air is a thing unknown in the natural world. The atmosphere is like a great sponge. It greedily takes up water and gives it back only when it has more than it can hold. Very few people realize that water in the form of vapor is much lighter than air, and that air containing a large proportion of water vapor weighs considerably less than the same bulk of perfectly dry air.

The amount of water vapor in the air varies of course with the temperature. In every cubic foot of air at 40° below the Fahrenheit zero, there is 1/20 of a gram of water. When the atmosphere contains as much moisture as it will hold it is said to be saturated, and its humidity is 100%. If it contains only one-fourth of what it can absorb the humidity is 25%. The average humidity of the United States varies from 80% along the coasts to less than 40% in Arizona and New Mexico. A relative humidity of less than 50% indicates a comparatively dry climate, while a humidity of only 35% indicates the dryness of the desert.

The percentage of water in the air is measured by an instrument called a hygrometer (Fig. 55).

Fig. 55. - Hygrometer.