THE WEATHER 45 



43. Temperature in a cyclone. When warm winds blow to 

 a cooler part of the country, they cause it to become warmer, 

 as when a south wind blows over the Northern states and 

 produces higher temperature. Likewise a wind from the 

 cold plains of the Northwest in winter may carry a freezing 

 temperature all the way to the Gulf coast. Since the winds 

 come into a cyclone from all directions, it follows that there 

 must be parts of it in which cool air is going southward 

 and cooling warmer places, and other parts of the storm in 

 which warm air from the south is carrying warm weather 

 northward. Let us consider as illustration the case of the 

 warm air from the south and its effects on temperatures in 

 the cyclone. If we refer to the diagrams of cyclones or to the 

 weather maps, we see that the spiral course of the air which 

 comes into the "low" from the south carries it a little to the 

 east of the center of the " low." This means, of course, that 

 the eastern part of the storm will have southerly and warm 

 winds. The western part, on the contrary, will consist of air 

 which is coming from the north or northwest and is com- 

 paratively 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 

 temperature are connected by dotted lines, just as places of 

 equal pressure are connected by continuous lines. The lines 

 representing equal temperature are called isotherms, 



44. Humidity in a cyclone. We are probably as much 

 concerned about the humidity of the air and about clouds 

 and rain as we are about the wind or temperature. In a 

 previous chapter we learned that the relative humidity varies 

 when the temperature varies. Air which appears moderately 

 dry when it is hot may appear to be very moist when cooled, 

 and some of the moisture may condense. When air from 

 the south is moving 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 



