CHAPTER IX
THE MOISTURE OF THE AIR: PRECIPITATION
Dew and frost are commonly regarded as condensation rain, snow and hail are classed as precipitation. So much of the rain and snow results from the adiabatic cooling of the air—that is, cooling by an updraught of warm air—that this may be considered the normal cause of precipitation.
A mass of air composing an updraught is cooled at the rate of i degree Fahrenheit for each 183 feet of ascent (about 10.7 degrees centigrade per kilometer) and this rate does not vary much in the first 10,000 feet. When the rising mass has reached the level where it is at the temperature of saturation, condensation begins; rain-clouds form; and, from the coalescence of cloud matter, rain-drops or snowflakes fall.[1]
The most remarkable example of updraught, adiabatic cooling, condensation and precipitation is the equatorial cloud-belt. It is the updraught of the Trade Winds and consists of a zone of cumulus clouds several hundred miles in width. In sailors’ vernacular, it is the belt of the Doldrums. Throughout most of its width rain is of almost daily occurrence; greasy-appearing clouds, with sharp edges, hover above the horizon about noon, and steadily mount the sky. Wherever they are- ↑ Rain-drops vary in size from approximately 0.0004 inch (0.01 mm.) to about 0.25 inch (6.5 mm.) in diameter. Drops varying in size from very large to very small frequently fall in the course of one shower. Ordinarily the drops are about 0.1 inch (approximately 3 mm.) in diameter. At best, however, these dimensions are only approximate. Large drops are shattered by a stiff wind, and drops a quarter of an inch in diameter are apt to be shattered before reaching the ground. The largest drops occur in connection with thunder-storms. A shower composed of fine drops much diffused is usually termed drizzle. Very fine drops—droplets that are heavy enough to fall—are properly called mist. These droplets are larger than those of fog, the latter being floating and not falling matter. A thin fog is also called mist, in Weather Bureau nomenclature.
