on each dust mote, and the countless droplets thus formed will appear as a fog or cloud of greater or less density.
The most efficient method of producing the cooling necessary to cloud formation is to move the moist air to a place of lower pressure, that is, lift it to a greater elevation, where it will expand and thereby do work against the surrounding decreased pressure at the expense of the heat energy it contains. This effect is well illustrated by the formation of cumuli, or thunderhead clouds, in the summer time; the process of which, in general, is as follows: The earth is heated by sunshine and it in turn heats and expands the adjacent atmosphere and thereby renders it lighter, volume for volume, than the surrounding cooler air. The light, warm atmosphere often nearly saturated with water evaporated from lakes, from moist earth and growing vegetation, and by this vapor rendered still lighter, is buoyed up by cooler and heavier adjacent air, very much as a cork is made to bob up when let go beneath a water surface. The lifted, or, as we commonly say, the rising air, sustains at any particular time only the weight of the atmosphere that is at that moment above it. But, clearly, so long as the air is rising this weight is growing less, and therefore as it passes from a region of greater to one of less pressure it expands just as a compressed spring does when its load is decreased. However, as the spring expands it must do the work of lifting the remaining weight, and so it is with the atmosphere; in expanding it has to lift the air that is above it and thereby do work. Now this work is possible only because of the heat of the active air itself, and consequently as it expands it correspondingly gets cooler. But, as has already been explained, the amount of water vapor that any given volume can hold in the form of a transparent gas, rapidly decreases as the temperature falls.
A rising mass of air, therefore, cools by virtue of its own work in expanding against pressure, and soon reaches a temperature below which it can not contain, as a gas, all its water-vapor. Hence any further rise and consequent cooling leads to precipitation—a collection of the excess water vapor in droplets about dust particles—and the formation of clouds.
With the foregoing facts in mind it is easy to understand, in a general way, those actions of nature that give meaning to the sky colors of morning and evening, and, in large measure justify the proverbs that for ages have been associated with them. Thus we see that a red evening sky means that nothing more than incipient condensation exists even at the tops of the strongly cooled convection currents that obtained during the heated portion of the afternoon (more than this would produce a gray or even cloudy sky), and that therefore the air contains so little moisture that rain, within the coming twenty-four hours, is improbable.
