Precipitation

“ The precipitation of the water from the air is primarily due to the cooling to which it is subjected as it rises in the atmosphere. Over the sea the ascent is accomplished by the simple diffusion of the vapour or by the uprise through the aërial shaft, such as that near the equator or over the centres of the whirling storms. ”

“ The amount of precipitation being sufficient, the production of glaciers would depend upon temperature, which itself would be dependent upon local conditions, which again might, or might not, be due to elevation of land surfaces. His idea, in brief, was that while during the glacial epoch there might be over the entire globe a period of sufficient warmth to produce the desired evaporation, the precipitation would fall as rain or snow, according to the local uplift or depression. ”

“ The work of evaporation is of course not done by the atmosphere, though the atmosphere determines the effect of the solar energy which does the work. [25]
The precipitation is distributed with great inequality, and this inequality affects both the rain and the snow. Some regions have heavy precipitation and some light; some regions have much rain and little snow; others have much snow and little rain; others have rain and no snow, and still others have snow and little or no rain. ”

“ Whenever these draw from the ocean currents of heated air, nearly saturated with moisture, into the upper region, the cooling effect consequent on rapid expansion must produce copious precipitation, and it is most probable that on the higher part of the range this, even in the hottest season, takes the form of snow. ”

“ The average precipitation equals the evaporation. The heat that is absorbed and evolved in the process of lifting up and letting down such a body of water has a powerful influence upon climates as well as upon winds; it is the chief source of that motive power which gives to the winds their force, to the storm its violence. Six hundred and twenty pounds of aqueous vapour occupy in the open air the space which it takes one thousand pounds of dry air at the same temperature to fill. ”

“ In some parts of the earth the precipitation is greater than the evaporation; thus the amount of water borne down by every river that runs into the sea may be considered as the excess of the precipitation over the evaporation that takes place in the valley drained by that river. ”

“ Influences favourable to heavy precipitation,—Mere altitude, with its consequent refrigeration, does not seem as favourable as mountain peaks and solid surfaces to the condensation and precipitation of vapour in the air. In the trade-wind regions out at sea it seldom rains; but let an island rise never so little above the water, and the precipitation upon it becomes copious. ”

“ Rain-water is seen to sink beneath the surface with every shower, and since this source seems altogether adequate for ground-water, and since no other source is known whence any considerable amount of ground-water might come, it is concluded that atmospheric precipitation is its chief source. ”

“ Any or all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. It is distinguished from cloud, fog, dew, rime, frost, etc., in that it must fall; and it is distinguished from cloud and virga in that it must reach the ground. Precipitation includes drizzle, rain, snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. 2. The amount usually expressed in inches of liquid water depth, of the water substance that has fallen at a given point over a specified period of time. ”

“ In the mean time, also, the vapours which the trade-winds bring from the north and the south, expanding and growing cooler as they ascend, are being condensed on the lower side of the cloud stratum, and their latent heat is set free, to check precipitation and prevent a flood. ”

“ The conditions which surround the liquefaction of ice and snow, the evaporation of water, and the condensation of the water vapor of the air are fundamental factors in the science of weather. The distribution of precipitation—that is, rain, snow, hail, and the floating forms of fog and cloud—affect the habitability of the earth and human activities to a very great degree. ”

“ Except in dry regions, where wind-work sometimes exceeds water-work, the movements of the atmosphere are of less importance directly than precipitation in determining the rate of erosion. But even in regions which are not arid the winds have much to do with the rate of evaporation and the distribution of rainfall, so that their indirect effect is great. ”

“ Again, we have an alternation of times during which evaporation predominates with times during which condensation predominates: shown in the tropics by strongly marked rainy seasons and seasons of drought, and in the temperate zones by corresponding changes of which the periodicity, though less definite, is still traceable. The diffusion and precipitation of water, besides the slow alternations answering to different parts of the year, furnish us with examples of rhythm of a more rapid kind. ”

“ Abundant precipitation, on the other hand, combined with such conditions as cause heavy snows to be melted suddenly, together with the absence of forests which tend to absorb moisture and to give it out but slowly, produce disastrous floods, such as have so frequently occurred. ”

“ The precipitation, however, is subject to great variations at different seasons—which fact, together with the sudden melting of the stored-up snow in the spring, causes considerable variations in the volume of the various tributaries. Fortunately, all are not at their highest at any one time ”

“ The normal cycle is conditioned by medium temperatures and ample precipitation in the form of rains. The glacial cycle exhibits low temperatures and abundant precipitation in the frozen condition. The arid cycle is marked by higher temperatures and low precipitation—so little rain that lakes can not rise to the rims of closed basins due to warping of the crust, for the simple reason that evaporation and soakage take care of the rainfall and no rivers can reach the ocean. ”

“ A greatly increased rainfall might produce lakes without forming glaciers, but we appeal in vain to the facts or the imagination for a probable cause of an increased oceanic evaporation, with a more abundant precipitation on the land. ”

“ After twenty or thirty minutes (the hotter the water, the quicker the precipitation) the precipitate has settled in large flocks at the bottom, and the clear water is drawn off into the pure water reservoir. The precipitating and settling of the impurities can also take place in cold water; it will require, however, a pretty long time. ”

“ Clouds produce rain. In the formation of a cloud the precipitation of moisture probably forms a fine watery powder, which remains suspended in the air in consequence of the minuteness of its particles: but if from any cause the precipitation is collected in larger portions, and becomes drops, these descend by their weight and produce a shower.
However rain is formed, it is one of the consequences of the capacity of evaporation and condensation which belongs to water, and its uses are the result of the laws of those processes. Its uses to plants are too obvious and too numerous to be described. ”

“ A different seasonal distribution of precipitation would banish agriculture from thousands of acres of land which are to-day giving our farmers good crop returns. The fact did not escape the watchful eye of Captain Lewis that the rainfall of the warmer months over the region which he crossed is essentially spasmodic and "patchy" in character, i. e., is of the shower or thunderstorm type, as contrasted with the more general and widespread rains and clouds of the large storms which characterize the winter months over the country as a whole. ”

“ Whatever may be the effect of these forests in producing precipitation, they certainly prevent evaporation, retaining the moisture in the ground and keeping the rivers and springs constantly supplied. ”

“ Precipitation and evaporation are the converse of each other; and the same air can not precipitate and evaporate, take up and let down water, at one and the same temperature. As the temperature of the air is raised, its capacity for receiving and retaining water in the state of vapor is increased; as the temperature of the air is lessened, its capacity for retaining that moisture
PAGE 179 THE EQUATORIAL CLOUD-RING.
is diminished. ”

“ When the air contains all the moisture it can hold, it is said to be saturated, and if a slight fall in temperature occurs when the air is saturated, condensation immediately begins in the form of rain, snow, or fog. If, however, the air is not saturated, a fall in temperature may occur without producing precipitation. ”

“ If the seas and evaporating surfaces were changed, and removed from the places they occupy to other places, the principal places of precipitation probably would also be changed: whole families of plants would wither and die for want of cloud and sunshine, dry and wet, in proper proportions and in due season: and, with the blight of plants, whole tribes of animals would also perish. Under such a chance arrangement, man would no longer be able to rely upon the early and the latter rain, or to count with certainty upon the rains being sent in due season for seed-time and harvest. ”

“ We thus have an insight into the operations which are going on in the equatorial belt of precipitation, and this insight is sufficient to enable us to perceive that exquisite indeed are the arrangements which Nature has provided for supplying this calm belt with heat, and for pushing the snow-line there high up above the clouds, in order that the atmosphere may have room to expand, to rise up, overflow, and course back into its channels of healthful circulation. ”

“ Very high mountain ranges usually consist of many ridges, among which rain-clouds are entangled in their ascent, and in such cases precipitation towards the windward side of the main range, though on the leeward sides of the minor ridges of which it is formed, may occur to so large an extent that before the summit is reached the clouds are exhausted or nearly so, and in this case the total precipitation is less on the leeward than on the windward side of the main range ”

“ The average amount of annual precipitation on the land is variously estimated at from forty to sixty inches, the lesser figure being probably more nearly correct. Since much of this water falls at high altitudes, the work which it accomplishes in getting back to the sea is great. The water which falls on the land, if withdrawn wholly from the ocean, would exhaust that body of water in 10,000 to 15,000 years if none of it returned. ”

“ The quantity of rain that falls depends upon the configuration of the land. Any cause which cools the winds from the sea and leads to the condensation of the vapor they carry--any obstacle which blocks their course--causes precipitation. ”

“ The conditions favorable to glaciation are, therefore, heavy precipitation and low annual temperature. If the precipitation is scanty, the small snowfall is soon melted; and if the temperature be too high, the moisture is precipitated not in the form of snow but as rain. It is important here to keep in mind that snow is a poor heat conductor and itself protects its deeper layers from melting. ”

“ With elevation gain, average daily temperature drops at the rate of 5° per 900 meters. Precipitation, wind velocity, and evaporation loss increase. Soil thins. These factors, along with others such as fire frequency, north or south exposure, and availability of moisture, combine to determine the range of each community. ”

“ The increase of humidity—favorable to precipitation—is noted by the gathering of moisture on cold objects, the collection of perspiration on our own skins owing to diminished evaporation, and the dampness of many hygroscopic substances. The last effect is illustrated by the packing of salt, the tightening of cordage and of strings of musical instruments, the dull or damp appearance of stone walls and columns, the settling of smoke, and by a number of other similar phenomena, all of which have been appealed to, with more or less justification, as evidence of a gathering storm. ”

“ But, though surcharged with moisture, rain does not fall to any great extent in the open sea, nor until the atmospheric current impinges on land, when it seems to be squeezed, like a sponge by the hand, with resultant precipitation. ”

“ The amount of precipitation is greater in the north-west and on the mountains, because in the one case the mountains of lower elevation are a less obstruction to the moisture-bearing winds from the west, and in the other the mountains condense the moisture; the mountains which stand in isolated groups upon the plains are frequently in summer the focus of local thunder showers. ”

“ It is undoubtedly the case that mingled with these little bubbles of moisture there are fine particles of solid water hanging on and carried along with them. Undoubtedly this is true; at least just before the final act of condensation takes place; and when the little hollow spherules collapse they are gathered together in drops of water larger or smaller according to the rapidity of condensation. [Pg 70] There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. ”

“ 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 ”