SUMMARY 255 



the centers of the " highs." The "lows" represent cy- 

 clonic storms, and the "highs" the sharp, bright weather 

 that usually follows them. 



Figure 224, A, B, and C, are complete weather maps, as 

 issued by a local weather office. They show the isobars, 

 and also the isotherm for freezing; that is, the line joining 

 the places at which the temperature is 32 F. Note 

 carefully the progress of the "low" from March 13 to 

 March 15, and also the effect of the northwestern "high" 

 upon the isotherm of freezing. These maps show also the 

 temperature of individual stations, the direction of the 

 wind, and the amount of rain or snow. As they are 

 issued, they contain an official forecast for different parts 

 of the country. 



282. Summary. The " conditions of the weather" are due to the 

 properties of the air and water of the atmosphere. 



Ascending currents are pushed up by heavier air, which is pulled 

 downward by gravity. 



A given volume of moist air is lighter than an equal volume of 

 dry air. 



An ascending air column is an area of low pressure. 



About 70 per cent of the heat received from the sun is temporarily 

 absorbed by the earth. The lower atmosphere does most of the ab- 

 sorbing. 



Air is cooled by its expansion as it rises, and is heated by its com- 

 pression as it descends. 



When a given volume of air or of space deposits its moisture as 

 rapidly as it takes up new moisture, it is said to be saturated witL 

 moisture. 



The humidity of air is the weight of water present in a given volume 

 of air. 



Relative humidity is the amount present as compared with what 

 the space could hold if it were saturated. 



