100 THE STUDY OF PLANT COMMUNITIES • Chapter V 



geographic areas, it has been variously used for comparing cli- 

 mates in studies of the distribution, migration, and success of pop- 

 ulations of man, birds, and insects. The system is subject to modi- 

 fication and has been used also as a graph of temperature-precipi- 

 tation (sometimes called a hythergraph). The latter method has 

 been used 243 for characterizing climates of widely differing climax 

 types in different parts of the world and for distinguishing grass- 

 land climates in North America. 52 The method probably has not 

 been given the use it deserves in plant studies. Because tempera- 

 ture-relative humidity diagrams have been used with some suc- 

 cess, it seems reasonable to suggest that similar graphs of tempera- 

 ture-vapor pressure deficit might give even more distinctive pat- 

 terns and might, therefore, be even more useful in detailed studies. 



WIND 



Air moves from a region of high pressure to one of low pressure, 

 and the differences in pressure are largely the result of unequal 

 heating of the atmosphere. The equatorial regions receive more 

 heat than regions to the north or south; consequently, low pres- 

 sures normally exist in the lower latitudes. The tendency, then, is 

 for air to move from the poles toward the Equator, there to rise 

 and return toward the poles. This pattern, although true in gen- 

 eral, is modified by the deflecting action of the earth's rotation and 

 by differences in temperature resulting from oceans and land 

 masses. 



Continents in temperate zones tend to become very hot in sum- 

 mer, and the resulting low pressures produce winds that blow 

 inland. The cold of winter reverses the pressure, and winds tend 

 to be outblowing. In mountainous areas or along sea coasts these 

 seasonal trends may have daily variations again produced by tem- 

 perature-pressure differences. Mountain valleys and slopes, which 

 are often warmed rapidly during the day, produce valley breezes 

 blowing upward. At night, the rapid cooling of bare high ridges 

 results in a flow of cold air down the valleys. The contrast between 

 day and night temperatures of land and water results in an off- 

 shore breeze at night as the land cools rapidly and higher pressures 

 result. During the day, the land again heats up rapidly above the 

 temperature of the sea, and an inshore breeze develops that may 



