166. Water Vapor in the Air.—The amount of water vapor present in the air has a marked effect upon the weather and the climate of a locality. The study of the moisture conditions of the atmosphere, or hygrometry, is therefore a matter of general interest and importance. The water vapor in the atmosphere is entirely due to evaporation from bodies of water, or snow, or ice. In the discussion of evaporation, it is described as due to the gradual escape of molecules into the air from the surface of a liquid. This description fits exactly the conditions found by all careful observers. Since the air molecules are continually striking the surface of the liquid, many of them penetrate it and become absorbed. In the same manner many vapor molecules reenter the liquid, and if enough vapor molecules are present in the air so that as many vapor molecules reenter the liquid each second as leave it, the space above the liquid is said to be saturated as previously described. (See Art. 18.)

167. Conditions for Saturation.—If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the retarding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated.

Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.

168. The Formation of Dew.—If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the cold surface as dew. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of grass and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, frost forms instead of dew.

169. Formation of Fog.—If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This constitutes a fog. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, e.g., moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a cloud is formed. If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and rain results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that hail is said to be formed.

170. The Dew Point.—The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the dew point. This is often determined in the laboratory by partly filling a polished metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point.

171. The Humidity of the Atmosphere.—After the dew point has been obtained, one may compute the relative humidity or degree of saturation of the atmosphere, from the table given below. This is defined as the ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature.

For example, if the dew point is 5°C. and the temperature of the air is 22°C., we find the densities of the water vapor at the two temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly. Determinations of humidity may give indication of rain or frost and are regularly made at weather bureau stations. They are also made in buildings such as greenhouses, hospitals, and schoolhouses to see if the air is moist enough. For the most healthful conditions the relative humidity should be from 40 per cent. to 50 per cent.

Weight of Water (w) in Grams Contained in 1 Cubic Meter of Saturated Air at Various Temperatures (t°)C.

t°C. w
-10 2.1
- 9 2.4
- 8 2.7
- 7 3.0
- 6 3.2
- 5 3.5
- 4 3.8
- 3 4.1
- 2 4.4
- 1 4.6
0 4.9
1 5.2
2 5.6
3 6.0
4 6.4
5 6.8
6 7.3
7 7.7
8 8.1
9 8.8
10 9.4
11 10.0
12 10.6
13 11.3
14 12.0
15 12.8
16 13.6
17 14.5
18 15.1
19 16.2
20 17.2
21 18.2
22 19.3
23 20.4
24 21.5
25 22.9
26 24.2
27 25.6
28 27.0
29 28.6
30 30.1