30 



WEATHER AND CLIMATE 



difference between the wet- and dry-bulb readings is 10 

 degrees, find 10 in the difference column and read down 

 until you come opposite 71 in the extreme left row. Read 

 56 per cent as the relative humidity. 



Record the notes and results of this experiment in your 

 notebook. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



Where does the moisture in the air come from? 

 The blanket of air that surrounds the earth is like a 

 great sponge. Sometimes it contains only a little mois- 

 ture and can soak up large amounts of water. At other 

 times it is wet and can take up little or none. This 

 vapor comes from many sources. You have all 

 breathed on a cold mirror or window pane and noticed 

 that water vapor is deposited. Many living things, 

 both plant and animal, give moisture to the air. Al- 

 most every chimney in the world pours water vapor 

 into the air, because most fuels contain hydrogen, 

 which forms water vapor as it unites with oxygen and 

 burns. By far the greatest amount of moisture, how- 

 ever, is furnished the air from the bodies of water, 

 such as oceans, lakes, and streams, which are spread 

 over the earth. 



What is evaporation and what are the factors which 

 affect it? When a liquid turns into a gas or vapor it 



is said to evaporate, and the process is called evapora- 



tion. Water, a colorless liquid, evaporates into steam, 



an invisible vapor. What we commonly speak of as 



steam is not steam at all, but condensed water par- 



ticles. From the great bodies of water over the earth, 



water gets into the air-sponge by evaporation, which 



may take place slowly or rapidly. If the air-blanket 



contains much water vapor, evaporation is slower, 



while if it is drier the 



process goes on much 



more rapidly. Since 



warm air can hold more 



water vapor than cold 



air, when the air is 



warm evaporation us- 



ually goes on more 



rapidly than when it is 



cold. 



The energy of the sun 

 controls evaporation of 



water into the air. Everyone knows that wet clothing 

 will dry most quickly on a warm, dry, windy day. 

 This is true because all of the factors which control 

 evaporation are favorable to moisture's getting into 

 the air. Since warm air can hold more moisture than 

 cold air, and since the wind circulates the air so that 



no. 49 



RELATIVE HUMIDITY TABLES FAHRENHEIT 

 Temperature readings in degrees Fahrenheit. Relative humidity readings in per cent. Barometric pressure 29.0 in. 



80 96 91 87 83 79 76 72 68 64 61 57 54 51 47 44 41 38 



Copyrighted 1926, Taylor Instrument Companies, Rochester, N.Y., U.S.A. 



35 32 29 27 



