Forecasting of Weather Changes. 581 



the country, are often very marked. The general rule is 

 that with the approach of a storm the temperature rises 

 ahove the normal for the place and season, if it is the cold 

 part of the year, but after the storm passes the temperature 

 falls below the average. 



The rise in temperature is due to three causes: (1) The 

 warming of the air by the heat due to the condensation of 

 moisture; (2) the checking of radiation by the moisture 

 in the air ; (3) the importation of warmer air from farther 

 south under the influence of the storm center. 



It was shown in (41) and (42) that the formation of a 

 pound of water at 212 from a pound of steam at 212 is 

 associated with the development of 9G6 heat units, and the 

 freezing of a pound of water is also associated with the ap- 

 pearance of 142 heat units. When, therefore, a pound of 

 snow forms in' the air from a pound of water vapor there 

 is imparted to the air in which this occurs 



966 + 142 = 1108 heat units 



and if snow enough falls to represent an inch of rain the 

 heat produced in the air is at the rate of about 



H9 4. 



1,108 X ^gp = 5761.6 heat units 



per square foot of the surface upon which the snow falls. 

 The warming of the atmosphere when it snows heavily 

 must he very considerable and this is why it is seldom more 

 than a few degrees below freezing when a heavy snow is in 

 progress. 



The low temperature following a storm is due to three 

 chief causes: (1) The rapid loss of heat by radiation from 

 the ground under the clear sky ; (2) the descent of cold air 

 from high altitudes; and (3) the importation of colder air 

 from farther north under the influence of the storm center. 



If reference is made to Fig. 268, it will be seen that the 

 southeast quadrant has ^ mean temperature of 59 F., 

 while the northwest quadrant has a mean temperature of 

 37 



