Page:Handbook of Meteorology.djvu/74

 62 quickly are regarded as the “most favorable” nuclei. Were it not for this feature of condensation, gentle rains would become sporadic cloudbursts. The measurement of the dust content of the air is not yet a part of the scope of weather observations, but the importance of it is universally recognized.

Conditions of Condensation and Precipitation.—In another chapter the relation of temperature to the amount of water vapor has been discussed. The absolute water vapor content of the air is the gross amount of water it contains. This is usually estimated in grains per cubic foot or in milligrams per cubic decimeter. Between the twenty-fifth and fiftieth parallels of latitude the amount of water per cubic foot averages roughly from 1 to 3 grains in winter and from 5 to 7 grains in summer—north to south. The proportion varies, however. Sea winds are wet winds; land winds are usually dry. The higher the temperature, the greater the possible absolute content of water vapor.

Condensation does not begin until the temperature of the air has reached the degree below which only a certain proportion of vapor can exist—that is, below the temperature of saturation, or dew-point. Any excess is condensed and appears in one or another of the forms noted.

Relative Humidity.—The water vapor content of the air which is not condensed is so important to life and to human comfort that its measurement is an essential part of Weather Bureau observations. The higher the temperature of the air, the greater the amount of water vapor it may contain—about 4 times as much at 70° F (21° C) as at 32 F (0° C), and 10 times as much at 100° F (38° C); hence the term relative humidity. This is expressed in terms of the per cent of water vapor necessary to saturation. Thus, if the relative humidity is 50 per cent, half the vapor necessary for saturation at the observed temperature is present.

Ordinarily, the humidity is highest in early morning, when the temperature is lowest; it is usually lowest at the warmest part of the day. On dewy and frosty mornings the humidity at ground level is 100 per cent; a few feet above ground it is probably at 96 per cent; during the hottest part of the day it may be as low as 30 per cent, or even lower; on cloudy days it may not vary materially during the day. During foggy