CHAPTER VI
CLOUDLAND
One of the things a tea kettle is good for is to provide, by means of the little cloud seen at its nozzle and erroneously called “steam,” an example of what happens when the invisible gas that is truly steam, or water vapor, is cooled below its dew point in the free air. This cloud has, however, been the starting point of a vast number of halfway explanations. A generation or so ago physicists were content to say that aqueous vapor turns to drops of water in the air merely on account of being cooled. The question of how the drops get their start, or why the moisture forms drops at all, does not seem to have troubled them.
One way in which air or any other gas is cooled is by expanding against pressure. Some of the energy in the gas, originally manifesting itself as heat, is applied to the work of expansion, and thus ceases to be heat. Hence the temperature of the gas falls. Conversely, if a mass of gas is compressed, the mere process of compression raises its temperature. The heat produced in pumping up a bicycle tire is the classic example of the latter fact. Heating by compression and cooling by expansion are called, respectively, “dynamic heating” and “dynamic cooling.” The processes thus described are of the utmost importance in meteorology.
If air of average humidity is admitted to the receiver of an air pump in the usual way and suddenly expanded by partial exhaustion, a cloud of moisture is seen to form in the receiver. This moisture is condensed and made visible by the dynamic cooling of the air. If, however, after the receiver is exhausted air of the same humidity as before is admitted through a filter of cotton wool, and is then similarly cooled by expansion, no cloud will form. Evidently the filter has removed from the air something that is essential to the process of condensation.
Perhaps it will occur to the reader that, in some obscure way, the filter has prevented water vapor itself from entering the receiver. There are several methods by which we can ascertain whether such is the case. One of the simplest is to admit a little smoke to the receiver before expanding the filtered air. In this case the cloud does form, showing that moisture is present, and also showing that smoke, though a perfectly dry substance, aids the formation of the water drops.
Such experiments have led to the conclusion, now universally admitted, that when water drops form in the atmosphere they always form around “nuclei” of something that is not water. These nuclei are often referred to as “dust particles,” but it is recognized that a vast proportion of them are very much more minute than the dust that worries housewives. They are largely beyond the power of the microscope, and some of them, indeed, appear to be of molecular size, consisting of molecules of hygroscopic gases, such as the oxides of sulphur and of nitrogen.
Another important fact about water drops in the atmosphere has come to light within the last half century. Since water is much heavier than air, meteorologists of an earlier generation were puzzled by the fact that the drops in clouds apparently float, instead of falling to the ground. In the attempt to account for this supposed floating, bygone authorities assumed that the drops were hollow “vesicles,” like little bubbles. This assumption was eventually disproved by the optical phenomena exhibited by the drops, as well as on other physical grounds. Moreover, it is now known that a cloud never really floats, though the rate at which its constituent particles fall with respect to the air is generally very small, on account of the resistance they encounter. Thus a very slight upward current usually suffices to maintain the altitude of a cloud, or even to increase it. The speed with which a drop falls increases with its size. Hence large drops may fall rapidly from great heights all the way to the ground, constituting rain; but in a great many cases such drops evaporate on falling into warmer air below the cloud level, and thus the lower surface of the visible cloud remains at about a constant height.
The drops in clouds and fog have often been measured, either by noting their optical effects or by microscopic examination. Many are found to be from 0.0006 to 0.0008 inch in diameter. The speed with which such drops fall through still air can be calculated. A drop 0.0008 inch in diameter falls at the rate of about half an inch a second, or 150 feet an hour. Even if a cloud consisting of such drops preserved its integrity for an hour or more while sinking, its descent at this slow rate would hardly be perceptible from the ground.
Some clouds consist of ice needles or tiny snowflakes. Apparently these icy particles are produced directly in solid form, without passing through the liquid stage. It is supposed that, like drops, they require nuclei on which to condense, but this matter has not been fully investigated. Another point that awaits elucidation is the fact that the clouds that form in air much below the freezing point sometimes consist of water and sometimes of ice. The common fleecy clouds of our winter skies are composed of water drops, and such clouds also occur in the polar regions. Dr. G. C. Simpson, when serving as meteorologist of Scott’s antarctic expedition, observed fog consisting of liquid water at a temperature of 29° below zero, Fahrenheit. Why such greatly “undercooled” drops should sometimes occur in the atmosphere when at other times, with higher temperatures, atmospheric moisture takes the form of ice is not at all clear.