FORECASTING THE WEATHER 



BY watching the clouds, the direc- 

 tion of the wind, the amount 

 of moisture perceptible in the 

 air, and other local signs, any person 

 may become a tolerably good weather 

 prophet. Some of the principal phe- 

 nomena that must be noted and in what 

 manner are described in a recent report 

 by Alfred J. Henry, Professor of Me- 

 teorology in the United States Weather 

 Bureau. 



CLOUDS 



Clouds are formed from the moisture 

 that is always in the air in varying 

 quantities, even over the desert. Like 

 the air itself, the moisture that is within 

 it is invisible so long as it remains in 

 the form of a gas. When a mass of air 

 is cooled by any means whatsoever, a 

 portion of its water vapor is condensed 

 and becomes visible — a mist or cloud is 

 formed. A familiar illustration of cloud 

 formation in nature is afforded when a 

 current of warm, moist air strikes a 

 cold mountain. The colder surface of 

 the mountain condenses some of the 

 moisture that is in the air, forming a 

 cloud which frequently obscures the 

 top of the mountain and floats away in 

 the prevailing winds. This simple phe- 

 nomenon indicates to an observer on 

 the leeward side of the mountain that 

 a warm, moist current of air, with 

 probably rain or snow, is approaching. 

 In some parts of the world the forma- 

 tion of a cloud cap on a mountain top 

 is not an indication of precipitation ; 

 yet in the majority of cases it is be- 

 lieved to be a reliable prognostic of 

 falling weather. In general, the for- 

 mation of cloud after a clear spell is the 

 first sign of coming rain. Unfortu- 

 nately there is no definite interval be- 

 tween the time of the first appearance 

 of clouds and the occurrence of rain. 

 Rain may not fall for several days after 

 the first appearance of clouds, and, on 



the other hand, it may begin within two 

 or three hours after the first cloud makes 

 its appearance. 



The various cloud forms generally 

 observed in the United States, with 

 their especial significance, are given in 

 figures 1-6. 



THE TEMPERATURE AND MOISTURE 

 OF THE AIR 



An increase in the amount of moist- 

 ure in the air is indicated in various 

 ways. It is especially noticeable to the 

 senses when coupled with a high tem- 

 perature. Man does not need a ther- 

 mometer to tell him that the air is 

 oppressively warm, nor a hygrometer 

 to tell him that there is an unusual 

 amount of moisture present. A pitcher 

 of ice water on a hot summer day is not 

 a bad sort of hygrometer. The pitcher 

 is naturally cooler than the surrounding 

 air, and consequently some of the water 

 vapor in the air is condensed and collects 

 on the outside of the pitcher. It will 

 be remembered that water vapor changes 

 to the liquid state when the air is cooled 

 below a certain point. The principle 

 illustrated by the pitcher of ice water is 

 repeated on a grand scale in nature 

 every time rain or snow falls. First, 

 there is the cooling of the air and the 

 formation of cloud, the latter being com- 

 posed of minute particles of water ; 

 second, there is the further cooling of 

 the cloud mass, so that its particles join 

 to form small raindrops, which fall to 

 the earth by their own weight. When 

 a rain cloud strikes the top of a moun- 

 tain, rain does not necessarily fall, but 

 small mist-like particles of water are 

 deposited on the relatively colder sur- 

 faces of the rocks and other objects on 

 the mountain sides and top. These par- 

 ticles coalesce and run down the sides of 

 the objects on which they are deposited 

 precisely as frequently happens on a 

 pitcher of ice water on a warm, humid 



