152 FIRST YEAR SCIENCE 



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 doldrums, are regions of calms. But unlike the 

 doldrums, they are dry belts, since the temperature of the 

 descending air is increasing, owing to adiabatic heating 

 ( 59), and thus its power to hold moisture is increasing. 

 Therefore the tendency in these belts is to take up mois- 

 ture rather than to deposit it. 



In the middle latitudes there is a belt of irregular winds 

 which have a prevailing tendency to move from west to east 

 or northeast. This general eastward drift of the air is con- 

 stantly being interrupted by great rotary air movements hav- 

 ing a diameter of- from 500 to 1000 miles. These are called 

 cyclones and anti-cyclones. In this region of the "wester- 

 lies," since the air tends to move from lower to higher lat- 

 itudes, an abundance of moisture is usually supplied. 



In the anti-cyclone the air movement is slowly down- 

 ward and outward from the center and in the cyclone it 

 is inward toward the center, and upward. The center of 

 the anti-cyclone is a place of clear sky and high pressure, 

 while that of the cyclone is a place of cloudy sky and low 

 pressure. The anti-cyclones, or high-pressure areas, have 

 dry, cold, light winds, while those of the cyclones, or low- 

 pressure areas, are usually strong and wet. 



75. Land and Water Winds. As the land is much more 

 rapidly heated by the rays of the sun than is the water, 

 the land during the daytime becomes hotter than the 

 water near it. On this account the cool air over the 

 water flows in over the land and displaces the lighter warm 

 air. Therefore near large bodies of water when the 

 temperature is high there is often in the daytime a. wind 

 blowing from the water to the land. At night, as the land 

 loses its heat more rapidly than the water, the wind blows 



