Page:Handbook of Meteorology.djvu/227

 saturation condensation takes place in the form of dew; or, if below 32° F (0° C) in the form of frost.

The table, p. 280, shows the amount of moisture at different temperatures which may be present mingled with the air. Thus, at 30° F a little less than 2 grains per cubic foot can be present before condensation begins; while at 70° F there may be nearly 8 grains. In other words, if 2 grains per cubic foot were present when the temperature was 30° F, condensation would be taking place; while, if the temperature were 70°, the air would be very dry, because only one-quarter of the moisture required for saturation is present. Sensible moisture, therefore, is relative, requiring measurement of temperature and absolute humidity at the same time.

The measurement of absolute humidity by direct methods is not ordinarily required in weather observations. It may be determined by aspirating a measured quantity of air very slowly through a hygroscopic substance, weighing the substance before and after. It may be determined more easily, however, by ascertaining the relative humidity.

The Measurement of Humidity; Wet-Dry-Bulb Hygrometers.—Various methods of determining the humidity of the air have been devised. Some of them are merely hygroscopes. Thus, a slip of paper moistened with a solution containing gelatine and cobaltic chloride is pink in moist air and blue in dry air. A better hygroscope is the toy chalet from which a woman emerges in dry weather, while a man with an umbrella stands in the door during damp weather. The manikins are suspended by a short piece of catgut which, twisting in the one case and untwisting in the other, because of changing moisture content, indicates roughly the changes in the humidity of the air.

A curled piece of vegetable fiber, one end fastened to a base, the other carrying an index hand, has become popular as a hygroscope. It has practically no value for quantitative determinations, but is not without value in indicating conditions of moisture not at once apparent to the senses but at the same time necessary to bodily comfort.

For the quantitative measurement of humidity hygrometers are now practically reduced to two types, dry-wet-bulb thermometers, and hair hygrometers. The United States Weather Bureau provides several kinds of the first-named type.