WEATHER PROVERBS 435 



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 forma- 

 tion 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 evapo- 

 rated from lakes, from moist earth and growing vegetation, and by this 

 vapor rendered still lighter, is buoyed up by cooler and heavier adja- 

 cent 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, sus- 

 tains 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 arid 

 thereby do work. Xow 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 gen- 

 eral 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 eve- 

 ning 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 pro- 

 duce 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. 



