THE WATER VAPOR OF THE AIR 221 



0.076 ft., or 0.076 X 12 in. in depth. 0.076 X 12 in. equals 0.91 in., 

 the depth of rainfall resulting. 



Only a part of the moisture in the air is ever precipitated 

 during a storm. In most storms the movements of the air 

 bring additional vapor. Other special conditions assist in fur- 

 nishing the heavy rains that sometimes fall. 



PROBLEMS 



Solve and fill out the additional problems in the following table in 

 the same manner as the solved problem. 



12 34 5 



Dry thermometer 50.0 70 60 50 30 



Wet thermometer 46.5 60 56 45 28 



Difference 3.5 10 



Relative humidity, 77 % 



Grains vapor per cubic feet, 3 . 14 

 Pounds vapor in 2 miles of air, 4 . 737 

 Inches, rain if all precipitated 0.91 



251. The Dew-point. Using the readings of the wet and 

 dry thermometers and Table XIII we can obtain the dew- 

 point. This is the temperature at which dew or frost would 

 begin to form. If the temperature is 70 and the wet bulb 

 reads 65, the 62 found in Table XIII at the crossing of the 

 70-line and the 5-column, means that the moisture then 

 present is all that could exist as vapor if the temperature fell 

 to 62, and dew would begin to form with any further cooling. 



252. The Dew-point and the Fall of Temperature at Night. 

 In clear weather with calm or light winds, the temperature 

 in late afternoon and night often falls at the rate of 3 (or 

 more) per hour until the dew-point is reached. Then the 

 condensing of water vapor into dew liberates the heat of 

 vaporization (Art. 150) that had been used in forming the 

 vapor. The freeing of that large amount of heat warms the 

 air and checks the rate of cooling, often to about 1 or 1% 

 per hour, after the dew-point is passed. 



