146 Our Surroundings 



conditions are ideal. Hence it becomes necessary for persons 

 whose health requires the best conditions to live in one section 

 for a part of the year and in another section for another part of 

 the year. Accordingly, many live in Florida or California or in 

 some other warm region in the winter and in a cooler region 

 in summer. 



SUMMARY 



Winds are movements of the air caused by differences in the 

 temperature of the earth. 



Winds are affected by the revolution of the earth around the 

 sun. The daily rotation of the earth also modifies the direction of 

 their flow. 



The belt of calms, known as the heat equator, is an almost 

 windless section of the earth in the torrid zone. It varies with the 

 seasons. 



The trade winds are constant movements of the air blowing 

 southwest in the northern hemisphere and northwest in the south- 

 ern hemisphere. Anti-trade winds flow in an opposite direction 

 to the trade winds and above them. 



Westerlies are air currents that flow from west to east over 

 large portions of the temperate zones. 



Cyclones are winds that have a whirling motion. They come 

 from regions of low pressure. 



Hurricanes and tornadoes are violent forms of cyclones. 



Important weather instruments are the barometer, thermom- 

 eter, aneroid barometer, barograph, and anemometer. 



The paths of storms across the United States follow certain 

 fairly well defined courses. 



The United States Weather Bureau foretells the nature of 

 weather and issues bulletins concerning it. 



There are marked differences in the climate of various parts 

 of the temperate zone in the United States due to local causes. 



FACT AND THOUGHT QUESTIONS 



1. Mention several helpful effects of wind you have personally noticed; 

 several destructive effects. 



2. Give the primary cause of wind. 



