These watery particles pass into the air by evaporation, and there are several ways by which the reduction in temperature necessary to render them visible can be brought about. It may take place through contact with a colder body of air, by expansion, or by a reduction of pressure owing to a rise in altitude. Clouds are said to be formed by this last method, for a volume of hot air rises higher and higher until it presently reaches a point when its contained vapour condenses, and becomes visible as a cloud. Meteorologists repeat one of these processes in the laboratory, by releasing from pressure damp air placed in a convenient glass globe, and are able to see something of the methods of cloud formation. It has been customary to speak of a cloud as being composed of watery particles floating motionless in the upper air; but although it may appear unchanged in form, it is all movement. So soon as ever a cloud is formed, its particles of moisture commence to fall slowly, the rate of fall being in proportion to the diameter of the particles, and this is due to the slight resistance the air makes to such very small atoms. In passing, it may be said that one observer estimates the diameter of these particles as from .00033 inch to .00025 inch. The component parts of a cloud are always in motion and recognizing this fact it becomes possible to take the first step in considering the formation of a raindrop.
An easy way out of the difficulty of explaining the formation of a raindrop, is to say that, since clouds are so often of two opposite electric potentials, there is always a continuous bombardment of watery particles taking place, and some of these must unite and fall as rain. The meteorologist is always tempted to call in electricity as an agency whenever he is anxious to discover a cause for some particular phenomenon. This often explains one mystery by another. The production of rain, snow, and hail has for many years been explained by vaguely ascribing them to the action of electricity, without any information being forthcoming as to the precise way in which this action takes place. Meteorologists are at present attempting to find a more satisfactory explanation. Another theory is that the particles of moisture in a cloud, like all other objects, radiate heat, and, growing cold, condense moisture upon their surfaces, thereby increasing in weight until they assume the proportions of a drop. This seemed a reasonable explanation of the formation of a rain drop until modern research decided that whenever moisture is condensed, latent heat is set free, so that all moisture deposited on a watery particle only serves to raise its temperature, and cause evaporation of the moisture thus acquired. The particles of water could not by this means grow to the full estate of a rain drop, and the theory is being gradually abandoned.
A rain drop is, according to modern meteorologists, explained in a very simple way. It has been seen how the hot, damp air is formed into a cloud, and also how the minute particles of water at once commence to fall slightly earthwards. Now these little particles as they pass into a warm layer of air would soon be evaporated, and would never reach the earth at all. Their downward journey, however, is often through a cloud many miles thick, and the most modern and simple theory is that in this journey they overtake some of their fellows, and the joined particles increase their rate of travel, overtake more and more particles until they presently become heavy enough to take the final plunge to earth. Were it possible to be just beneath a cloud, an observer would see rain drops coming from it of all sizes. The same process goes on in drops, which trickle down a window pane, or in the effervescing globules in a bottle of seltzer water. In the latter instance, the process is reversed, for the globules are seen overtaking one another in an upward direction. There are many points in favour of this theory of the formation of rain drops, and at least it gets rid of those elaborate complications, electricity and condensation. With respect to the formation of rain by the impinging of clouds upon the tops of cold mountains in the northwest, one authority argues that moisture is in these circumstances not condensed solely because of the contact with the cold hills; that rain there is due to a mechanical cause, the watery particles being squeezed together by the grinding effect of the clouds on the sides of the mountains in such a way that they coalesce, and fall as drops.
A rain drop's roundness is due to the action of capillarity. Just as a circle made by dropping a stone into water owes its shape to the fact that the force is able to act equally in all directions, so a rain drop is spherical, owing to similar untrammelled action on the part of capillarity. These are some of the explanations of the formation of a rain drop, but meteorologists still have the subject under consideration.
The periods of rainfall are divided broadly into times of drought and times of flood, and it is in these matters that meteorology is seen in its practical aspect. Some people ask, "Where does all the rain come from?" Others are surprised that rainfall totals up to such large quantities.
A fall of rain to a depth of one inch over a very limited area, represents millions of gallons, but in spite of this vast quantity of falling water, many times multiplied if the annual rainfall be taken into account, there still are water famines. The question has often been debated whether man can modify climate or effectively tamper with the processes which produce rain. Rain making has not, so far, been a success, though the firing off of heavy guns has been tried, along with the legitimate avocations of the meteorologist. The afforesting or deforesting of a district has, however, a marked effect upon rainfall. Three notable instances are Ascension Island, Malta, and the neighbourhood of the Suez Canal, where the planting of trees seems to have had the result of increasing the rainfall. The effect of trees is felt more in the storage of rain water, while leaves and roots serve to retain moisture that would otherwise quickly drain away. A hill may be converted into a sponge by the judicious planting of trees. The question of the storage of rain water becomes more pressing each year, and the longer the settlement is put off, the more difficult will decision become. Engineers called upon to prevent floods and to conserve rain water reply, "Save our forests, cover the land with trees."
The fact that such problems arise, serve to show how great is the amount of water formed by the continual falling of the tiny raindrops. As long as this beneficent downpouring is allowed to drain away unused or uncontrolled, so long will droughts annoy and water famines bring distress.
In recording weather conditions, symbols are sometimes used in order to shorten reports and, while not universal, most nations adopt these: The symbol for rain is ●, a small circle filled in; for lightning
; for thunder T, while the two latter combined make T