WEATHER; WINDS AND STORMS; CLIMATE 113 



121. The Belt of Calms. The heated air within the 

 equatorial belt rises slowly and its vapor condenses in the 

 cooler, upper layers of air. Consequently there are frequent 

 rains, but not much wind, as there is very little horizontal 

 motion of the air. Because of this condition, the region is 

 named the belt of calms. 



During the year the belt of calms shifts its position north 

 and south across the torrid zone, according as the positions 

 of the sun and of the region of greatest heat change. The 

 rainy season in the torrid zone changes according to the 

 movements of the belt of calms. It comes in the northern 

 half of this zone during the northern summer and in the 

 southern half during our winter. 



122. The Trade Winds. If the earth were stationary, 

 the air moving into the equatorial belt would give rise to 

 constant north winds blowing north of the equator, and 

 south winds on the other side. The great velocity of rota- 

 tion at the equator, however, gives the constant winds a 

 northeast and southeast direction. This is explained by the 

 fact that, owing to the greater circumference at the equa- 

 tor, the rotation of any equatorial point is faster than that 

 of any point north or south of the equator. Hence the wind 

 that moves toward the equator does not reach the point 

 toward which it started, but a point farther west. As a 

 result, the wind in the northern half of the torrid zone seems 

 to come from the northeast, and in the southern half from the 

 southeast. 



These constant northeast and southeast winds are called 

 trade winds because of their help to navigation and to 

 trade. 



123. Storms. Though the term storm is usually applied 

 only to rain or snowfall, any marked disturbance of the at- 

 mosphere is, properly, a storm. A storm occurs when there 

 is a great difference of pressure over different regions, and 

 the air moves rapidly from the high-pressure area to the re- 



