WIND BELTS OF THE EARTH 221 



north and south, modifies the conditions in these wind 

 belts during the year. The planetary winds thus modified 

 are sometimes called terrestrial winds. 



Wind Belts of the Earth. Near the heat equator where 

 the air is rising there is a belt of calms and light breezes called 

 the doldrums. As the air here is rising and cooling (page 

 125), thus losing capacity to hold moisture, this is a cloudy, 

 rainy belt of high temperature in which much of the land 

 is marshy and the vegetation so rank and luxuriant that 

 agriculture is exceedingly difficult. 



Extending north and south of the doldrums to about 28 

 of latitude are belts in which constant winds blow toward 

 the doldrum belt and supply the air for the upward current 

 there. In the northern hemisphere these winds have a 

 northeast to southwest direction and in the southern hemi- 

 sphere a southeast to northwest direction. They are the 

 most constant winds on the globe in their intensity and direc- 

 tion, and are called trade winds. Since they blow from a 

 cold region to a warmer region, their power to hold mois- 

 ture is constantly increasing and clouds and rains are not 

 usual. The places where they blow are dry belts and in 

 them are found the great deserts of the world. 



On the poleward sides of the trade-wind belts lie the areas 

 of high pressure already referred to. These are called 

 the horse latitudes or belts of tropical calms and are rather 

 ill-defined. The air is here descending and the surface 

 movements are light and irregular. These, like the dol- 

 drums, are regions of calms. But unlike the doldrums, 

 they are dry belts; since the descending air is increasing 

 in temperature, owing to adiabatic heating (page 125), and 

 thus its power to hold moisture is increasing. Therefore 



