The inquiry may arise, Why is the moisture condensed, almost always, in the upper regions of the air, where it is rare? Because the more rare and therefore expanded it is, the more moisture it will hold. This, taken with the fact that cold currents are encountered high up, sufficiently answers the question.
It is interesting to know that the processes of nature are interdependent. It is not enough that we have the evaporation of moisture that will ascend into the higher regions of the air and there be condensed into cloud and possibly rain, but we must have the means for distributing these conditions over a large area, and for this purpose we have the phenomenon of wind. Why the winds blow can be accounted for to a certain extent,—we might say to a large extent,—but there yet remain many unsolved problems relating to wind and weather. Of the phenomena of wind we will speak more fully in a future chapter.
CHAPTER IX.
CLOUD FORMATION—CONTINUED.
As water in its condensed state is 815 times heavier than air, the question naturally comes to one why it does not immediately fall to the earth when it condenses. There are at least two and probably more stages of condensation. Investigators into the phenomenon of cloud formation claim to have ascertained that the first effect of condensation is to form little globes of moisture that are hollow, like a bubble, with very thin walls. Everyone has recognized the ease with which a soap bubble will float in the air, and yet it is simply a film of moisture. These little balloons, so to speak, are called spherules. 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. There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. A cloud never stands still, although in some cases it may appear to do so. If we take a stone in our hand and allow it to drop without applying any force to it, it will fall directly to the ground. But if we give it an impetus in a horizontal direction it will travel some distance before striking the ground. If we could give the same impetus to a body as light as a globule of water-dust it would probably travel indefinitely without falling. Dust that would settle directly to the ground from an elevation in still air would travel thousands of miles without falling, before a wind having any considerable velocity.
Suppose the sun to be shining with intense heat upon a certain area of the earth's surface and the conditions to be right for very rapid evaporation of moisture. The air which is heated close to the ground, being expanded, will rise, together with the invisible particles of moisture, and there will be a column of moisture-laden air continually ascending until it reaches a point in the upper atmosphere where it is condensed into a cloud that takes on the billowy form which in summer time we call a thunder cloud, but which in the science of meteorology is called cumulus, or heap-cloud. If there were no air currents this billowy cloud would stand as the capping of an invisible pillar of ascending vapor, but as it is never the case that air is not moving at some velocity in the upper regions, it floats away as rapidly as it is formed. This peculiar kind of cloud is formed in the mid-regions of the atmosphere, and it is a summer cloud as well as a land cloud. Of course, it may float off over the ocean and maintain its peculiar shape for a certain distance, but it is rare that such a cloud would ever be seen in mid-ocean or in midwinter. As the warm season advances in summer, and evaporation from the earth is less than the rainfall, there is less and less moisture in the air, when, of course, the conditions for cloud formation, especially inland, are not so favorable as in the early spring or summer. Frequently there comes a time when we have a long season of dry, settled weather. Probably during most of the days clouds will form and we think it is going to rain, but before night they have vanished, and the same thing is repeated the next day and the next, perhaps for weeks at a time.
The explanation is this: We have already said that so long as the air remains in a uniform condition as to temperature it will absorb moisture in a transparent state until it is filled to the measure of its capacity at a given temperature. If there were no change of temperature, it would not condense into cloud. Clouds may be absorbed into the atmosphere—or evaporated—and become invisible; and this process is going on to a greater or less degree continually. If we watch the steam as it escapes from a steam boiler, the first effect is condensation into cloud, but as it floats away it gradually melts and is absorbed into the atmosphere as invisible vapor. This is especially true on a warm day; the same process takes place in the air that is going on at the level of a body of water or at the surface of moist earth.
As before stated, condensation always takes place when a body of moisture-laden air comes in contact with cold. When the steam escapes from a boiler, even on the hottest day, it is hotter than the surrounding air; the first effect is condensation, and then evaporation takes place the same as it would at the surface of the earth when the condensed particles of moisture are separated into the invisible atoms that accompany evaporation.