36 ELEMENTS OF GENERAL SCIENCE 



is comparatively cool. We may therefore expect to find that 

 the temperature rises as a " low " approaches any given region, 

 and falls as it passes. On weather maps places of equal tem- 

 perature are connected by dotted lines, just as places of equal 

 pressure are connected by continuous lines. The lines repre- 

 senting equal temperature are called isotherms. Examination 

 of the maps will show that the isotherms are usually bent 

 northward in front of a storm and southward in its rear. 



34. Humidity in a cyclone. We are probably as much con- 

 cerned about the humidity of the air and about clouds and 

 rain as we are about the wind or temperature. It will be re- 

 called that in a previous chapter we learned that the relative 

 humidity varies when the temperature varies. Air which ap- 

 pears moderately dry (has a low relative humidity) when it 

 is hot may appear to be very moist when cooled, and some of 

 the moisture may condense. When ah- from the south is mov- 

 ing northward and warming the country over which it passes, 

 it is itself losing heat and becoming cooler. As it becomes 

 cooler the relative humidity becomes greater, until finally, at 

 some distance above the earth, saturation may be reached and 

 moisture may be condensed in the form of clouds. If conden- 

 sation continues, rain will fall. As pointed out before, the 

 warm southern winds are blowing northward in the front part 

 of the storm (the eastern half), and it is here that clouds and 

 rain are found. 



In the western half of the storm the winds are coming from 

 northern and cooler places and are being warmed. Capacity for 

 moisture is increasing, and there is no condensation in this part 

 of the storm area. The air in the western part of a cyclone is 

 usually clear, dry, and relatively cool. 



It should be noted in this connection, however, that large 

 bodies of water and great irregularities of the earth's surface, 

 such as mountain ranges, may cause marked local variations 

 in the character of the weather which accompanies the passage 

 of cyclones. 



