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 
Cumulus, fracto-cumulus, and strato-cumulus (right center). Notice the typical “anvil” in center.
South of the equator the zone of constant rains is not so well marked as it is north of the equator; moreover, the belt of easterly winds at times covers the whole of the Gulf of Mexico. In general, the lands of the Torrid Zone receive an average of about ioo inches of rain per year—rather more in the northern than in the southern part.
In the temperate zones, on the coasts facing westerly sea winds, the rainfall for the greater part is seasonal. Along the Pacific Coast of the United States, it increases with the latitude. Thus at San Diego, California, the annual fall is 10 inches; at Los Angeles, 16 inches; at San Francisco, 23 inches; at Portland, Oregon, 45 inches; at the coast stations of Alaska from 80 inches to more than 100 inches. At San Diego practically all the rain falls between October 15 and April 15; in the seventy-two years from 1850 to 1912, a shower amounting to more than a trace of rain fell only twenty times in July. In San Francisco there is an average of about one rainy day in July; in Portland, Oregon, the July rainfall averages 0.54 inch; in Seattle it is 0.69 inch.
On the west coast of Europe, owing chiefly to higher latitude, the seasonal character of the annual rainfall is not so marked as in California. In Portugal and southern Spain, most of the rain falls in the winter months. At Bordeaux, a little higher in latitude than Seattle, the July rainfall is in excess of 2 inches every month in the year. But while the average of the winter months in Seattle is above 5 inches, in Bordeaux it is about 3 inches.
The thirtieth parallel crosses the northern part of Mexico and also the northern part of Africa. A zone several hundred miles in width along this parallel—in sailors’ vernacular, the belt of “horse latitudes”—is the region of high barometric
Tufted wind clouds. A typical squall cloud indicating the onset of furious gusts of wind. Note the eddies on the lower side and the direction in which they point, Mount Weather, Va.
Southern coasts in general have an abundance of rain. Along the Indian Ocean and the Guinea coast the rainfall is seasonal. On the Gulf Coast of the United States rain falls pretty evenly throughout the year.
The Precipitation of Cyclonic Storms.—Cyclonic storms, or lows, are rather more frequent in the United States and Canada than in Europe. They are much more frequent in occurrence east of the Western Highlands, and they also are much more energetic.
Rather more than half of the storm-whirls of this sort are noted first between the Columbia River and the Strait of Juan de Fuca — not because they do not occur elsewhere, but for the reason that there are fewer weather stations north of Vancouver. When a low is crossing the mountains it is in a region of dry air; therefore it does not possess much energy. When it has crossed the Rocky Mountains it is in a region where both the absolute and the relative humidity are greater. Therefore it is apt to develop a much greater energy; for the latent heat set free by the condensation of water vapor is the fuel of a cyclone. The northerly cyclone usually traverses the Great Lakes, where the increased humidity imparts greater precipitation, finally moving out into the Atlantic. As a rule, the rainfall of such storms is not very heavy. It may drop a little more than 1 inch of rain over the track of its passage, but usually the precipitation is not much greater.
The more southerly cyclones frequently bend towards the Gulf of Mexico, and begin to curve towards the northeast after passing the Mississippi River. They are much more energetic than the northerly storms and drop perhaps as much as 2 inches of rain along their tracks. In various instances a storm first discovered in the plains of Texas finally travels a course between the St. Lawrence River and the coast. 
A summer shower. Cumulo-nimbus belt, the front of an advancing wedge of cold air, overlying warm air next the ground, showing thunder-head (right); strato-cumulus rolls (center), Mount Weather, Va.
The severest cyclonic storms are the West Indian hurricanes and the typhoons of the China coast. Throughout their courses they move through regions of very moist air. In these storms, the velocity of the wind results from a very rapid up-draught. The precipitation, therefore, is excessive. In the vicinity of the Gulf Coast of the United States, from 8 to 10
Snow crystals, magnified about 50 diameters, Jericho, Vt.
inches of rain may fall during the passage of a hurricane storm, and a downpour of 4 or 5 inches is usual.
In the United States, cyclonic storms are more characteristic of winter than of summer weather; they are therefore usually described as winter storms. The precipitation may consist of rain, sleet or snow—rarely, if ever, of hail.
Snow.—When condensation below 32° F (0° C) occurs, the precipitation takes the form of the ice crystals popularly known as snowflakes. They form in almost infinite variety, but they may usually be classified as tabular (disk-shaped) or columnar 
Annual rainfall.
Occasionally the snowflakes take the forms of soft pellets—the graupeln of the German meteorologist. At other times they are half-melted, but retain traces of crystallization. The presence of slowly falling snow crystals during fairly clear weather is common in many localities. It is the greatly-dreaded poguenib of the far-western Indian, who associates it with pneumonia.
Snowfalls have been recorded in every state in the Union. They occur occasionally along the Gulf Coast between Pensacola and Brownsville. Snow has fallen in Florida as far south as Fort Myers.[3] A line drawn from Savannah through San Antonio, El Paso, and Yuma to San Francisco marks roughly the limit south of which snow seldom falls. South of the thirty-fifth parallel, snow rarely lies on the ground more than a day or two. At New Orleans a measureable snowfall occurs about once in fifteen years. It is about as frequent in the city of Los Angeles, although the mountain summits in the vicinity occasionally are snow-clad.
In the vicinity of the Great Lakes the ground is covered most of the winter. In the New England and Middle Atlantic states the annual snowfall is 7 to 8 feet. It decreases toward the west, being about 2 feet in North Dakota. In the basin region of the Rocky Mountain States snowfalls occur at long intervals only; in the plateau region they may be expected yearly on the range summits. The heaviest snowfalls occur along the northern Rocky Mountain and the Sierra Nevada Range summits; from 10 to 30 feet may be estimated as the annual fall. The amount varies greatly from year to year; at Summit, California, 60 feet of snow fell during the winter of 1879-80.
On the Pacific Coast slope the yearly snowfall in the mountains is a matter of great importance. Since the construction of the various irrigation projects in the arid region, humanity is realizing more and more the dependence of productive lands, not only on the yearly amount of snow-fall, but on the conservation of the melting snow, as well. In the arid regions of the United States, the winter snowfall is the moisture of the summer crops.
Except at great altitudes, practically all the snow falls between the first of December and the middle of April in the zone of latitude that includes the New England States and New York.[4] Flurries of snow occur in May as far west as the Rocky Mountains; and at elevations of 2000 feet or more they occur in June.
Sleet; Ice Storms; White Storms.—In Weather Bureau nomenclature, sleet consists of small pellets of ice, apparently formed when rain-drops are frozen in passing through a stratum of cold air next the ground. Usually the pellets are not larger than duck shot; occasionally they are the size of peas. Sleet has been reported as hail so frequently that the Weather Bureau has issued an explanatory pamphlet calling attention to the fact that the ice pellets of sleet differ materially in structure from hailstones. Ice pellets may contain enough air to give them a whitish opaque appearance; therefore they are likely to deceive observers. Sleet storms are very apt to occur in the morning, when the temperature is at its daily minimum, but this is by no means always the case. Sleet may occur when a cold wave flows under warm, moist air; it is likely to result when a warm southerly wind flows over the top of very cold surface air.
Sleet is often mixed with rain; at such times it forms an ice-coating on the ground, making a surface that is more or less pebbly. Frequently it happens that the rain-drops are not frozen in their fall, but congeal as they strike. In this way, ground, sidewalks, trees and other surfaces become covered with a coating of ice. Weather Bureau practise and popular consent join in designating this form of precipitation as an ice storm. The ice storm is apt to be followed by the destruction of tree branches snapped off by the wind, and by an unusual number of accidents in city street traffic.
Several conditions of temperature and precipitation may result in an ice storm. If the rain-drops fall through a stratum of air below freezing temperature and strike an object whose surface is also below freezing temperature a varnish of ice will be formed, and it is likely to increase in thickness so long as precipitation continues. When the temperature is very slightly above the freezing point, and the surface air is dry, it is possible that rapid evaporation may chill the varnish of water below the freezing point and change it to ice. Rain-drops in the air may be chilled to a temperature several degrees below the freezing point; they change to ice instantly as they strike.[5]
Damp snow and snow falling on tree limbs, poles and wires whose temperature is slightly above freezing, is very apt to cling to them. The weight of the accumulated snow may be sufficient to break tree limbs and line wires. Not only is there a considerable material damage; there is also a troublesome and expensive interruption of communication. Popularly, the condition is known as a white storm.
A temperature materially below 32° F following a white storm is apt to result in much damage to shade trees and orchards. Branches will bend freely, as a rule, when the temperature is above freezing; but they become brittle under intense cold if coated with ice. The distinction between an ice storm and a white storm is chiefly one of appearance. The Weather Bureau makes no distinction between them.
Hail.—Hail is a product of thunder-storms. The hailstone consists of alternate concentric layers of snow and ice. The manner of the formation of the hailstone is conjectural. About the only thing of which one may be certain is that the hailstone is alternately in layers of moist air below the freezing point and layers of warmer air—that is, it is whirled through alternate layers of snowy air and of misty air. The updraught that occurs during thunder-storms shows that such a movement takes place in cumulo-nimbus clouds; and when the hailstones become too heavy to be carried by the updraught they fall to the ground.
Hailstones usually vary in size from a quarter of an inch
Hailstone; sectional view.
They occur usually in the southeast quadrant of a cyclonic storm, having the same relation to the area of low barometer as does the tornado. The path of the hailstorm is rarely more than 3 or 4 miles wide—sometimes not more than half a mile—and it may traverse a distance of 25 or 30 miles, or more.
Sometimes the hail is scattered in windrows; and many cases in the United States have been reported where the wind-rows were several rods in width and more than 2 feet deep. Near St. Quentin, France, a windrow more than a mile long left a mass of ice which did not disappear for several days.
In western Europe hailstorms are very destructive to vineyards and growing crops. For many years the practise of “bombarding the air” was followed. Long-barreled mortars with bell-shaped bores were charged heavily with powder and aimed vertically. At times when storms were expected, thousands of charges were fired into the air with the expectation that the resulting convection of the air might prevent the formation of hail. There is no evidence to show that the practise prevents hailstorms.
In the present state of human knowledge, forecasts of hail-storms cannot be made. The Weather Bureau is making efforts to gain all possible information concerning date, time, duration, extent of area and path of hailstorms. It is pretty well established, however, that hailstorms are more frequent in certain regions than in others; and that in certain limited areas in these regions of greatest frequency they are more destructive than in other areas.
Cloudbursts.—The cloudburst is an excessive downpour of rain, in which the water seems to fall in masses rather than in drops. Cloudbursts are rare; the area covered is small; the duration is a matter of a few moments only.[6] Only in a few cases have trustworthy measurements of the amount of precipitation been made. The ordinary barrel gauge would probably give a result at least 80 per cent true. The majority of recording gauges are of but little use in such storms. Moreover, the cloudburst does not always select for its performance a locality where Weather Bureau stations are in evidence.
The origin of the cloudburst is not certainly known. To call it an exaggerated thunder-storm may express a truth in some cases; certainly not in every case. All the water in an overhead saturated air at a temperature of 70° F over the area covered by the downpour would not make a rainfall sufficient to account for the water dropped by a cloudburst.
Thunderstorm: A typical cumulo-nimbus thunder-head. Note the heavy shower falling from the base of the cloud. Strato-cumulus clouds in lower left corner, and tufts of false cirri in right corner.
Summer Precipitation.—The rainfall of summer months within the United States is rarely a result of cyclonic movements of the air. For the greater part, it is due to the updraughts that result from surface heating; and this also is the cause of most of the tropical rains. Summer showers are apt to be sporadic in character, and the area covered may be small. It is not uncommon to find a rainfall of 2 or 3 inches at one locality while scarcely more than a trace falls at another locality only a few miles distant.[7] Occasionally the daily weather map shows half a dozen areas scattered over the eastern half of the United States in which rain is falling; less frequently, a belt 200 miles or more in width extending from the Gulf Coast to the Canadian border, sweeps eastward from the Mississippi Valley.
In summer the updraught, though strong, is more or less local, occurring over comparatively small areas. In winter the updraught, though weak, may involve an area more than 500 miles in diameter.
- ↑ 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.
- ↑ A remarkable collection of photographs of snow-flakes has been made by W. A. Bentley, and another by J. C. Shedd. The latter is published in the Monthly Weather Review, October, 1919. Professor Shedd classifies snow-flakes as first-, second-, and third-growth crystals. They have been classified also as columnar, doublets, and pyramidal crystals.
- ↑ On March 6, 1843, fifteen inches of snow fell at Augusta, Georgia.
- ↑ On the 8th of June, 1816, snow fell in all parts of Vermont; on the uplands it was 5 or 6 inches deep. It was accompanied by a hard frost.—Thompson’s History of Vermont.
- ↑ The Blue Hill Observatory reports rain falling when the temperature of the air was about 15° F—a very unusual phenomenon. It is not likely, however, that the rain-drops had reached a temperature much below freezing.
- ↑ A mining engineer in Arizona relates the following: “The day, up to 3 o’clock, had been moist—to the extent that distant objects possessed atmosphere—that is, there was not the illusion of nearness which a very dry air gives. In mid-afternoon there came a sudden darkening of the sky, a light patter of rain, and then a downpour so torrential that further progress along the trail was out of question. The loose rockwaste seemed to be washed out from under the horse’s hoofs, and a boss of rock near by seemed to be the only safe place. It was impossible to see anything more than a few rods ahead. The downpour lasted for not more than fifteen minutes. To say that the amount was 6 inches is merely a guess. Much of the trail was washed away and badly gullied. Pinal Creek was a roaring torrent; to have attempted crossing it would have been instant death. Across the summit, on the other side of the range, not a drop of rain fell.”
Another traveler wrote: “A heavy cloud had been hovering over Pilot Range for several hours, and we were not surprised to hear a low moan which soon became a roar. So we climbed out of the arroya in quick time. In a very few moments, there came a torrent that would have carried a ton boulder down the course. The cloud over Pilot Knob had dropped its shower and the sink below was full of water—the first time, perhaps, in fifty years.” - ↑ In the past few years insurance against rain has become very common during summer months. Clubs and outing associations thus protect them- selves against the losses of revenue which a rainfall might cause. In many instances the serious error of determining the rainfall by the record of a rain-gauge a dozen miles distant has led to expensive litigation. In various instances, heavy showers have occurred at the locality covered by the policy, while merely a trace fell at the station where the precipitation was recorded. Granted that insurance against rain is a legitimate business, it is evident that the installation of a gauge at the locality covered is the only way by which the amount of rain can be determined.