132 Our Surroundings 



Secondary Causes of Wind. The revolution of the earth 

 around the sun causes a great variation in the amount of heat 

 received in the different seasons on certain portions of the surface 

 of the earth. This necessarily affects the movements of air cur- 

 rents. The boundaries of wind belts vary with the seasons. The 

 daily rotation of the earth on its axis, although it does not cause 

 the winds to blow, does modify the direction of their paths. The 

 unequal distribution of land and water on the earth's surface 

 also affects the winds, since water gives off its heat into the atmos- 

 phere less rapidly than land does. 



The Belt of Calms. There is an irregular section of the 

 earth in the torrid zone, known as the heat equator or belt of 

 equatorial calms. This belt does not always cover exactly the 

 same area but shifts several degrees north or south with the 

 change of seasons, due to the inclination of the earth's axis in 

 its motion around the sun. 



This belt of calms receives a vast amount of heat from the 

 sun. The air over its heated land and water surfaces becomes 

 hot, grows light, and rises, its place being taken by air flowing 

 in from both north and south. The air pressure is low because 

 the air is warmer and hence lighter than either north or south 

 of this region, due to the direct and hottest rays of the sun. The 

 air appears generally ^alm because of its movements upward in 

 immense masses. Rains are frequent, since the rising air is cooled 

 on reaching higher levels and cannot hold its moisture. 



The Anti-Trade Winds. The immense masses of air ris- 

 ing from the belt of equatorial calms to higher levels flow north 

 or south, gradually shifting to an easterly direction and form- 

 ing what are called the anti-trade winds. A part of these air 

 masses settles to the earth in the neighborhood of the tropics 

 of Cancer and Capricorn, causing the horse latitude calms, or 

 belts of tropical calms. The portion of the anti-trade winds 

 which does not settle at the horse latitudes continues toward the 

 poles. 



Horse latitude calms are high pressure areas of cool, dry air 

 which has settled to the earth. In taking up heat again from the 

 earth's surface this air absorbs considerable moisture, drying up 



