138 A YEAR IN SCIENCE 



they soon rise, being carried upward by the cur- 

 rents of heated air* 



We already know that when air is heated it expands 

 and becomes lighter. The surrounding colder air, 

 because it is heavier, is then forced into the place occu- 

 pied by the heated lighter air. The heated air is then 

 pushed up and produces an air current. This, on a 

 small scale, is a wind. If for any reason the air above 

 one place becomes heavier than that above another, 

 there will be a transfer of air from the place where the 

 pressure is greater to that where it is less. This move- 

 ment of the air we call wind. Winds are caused by 

 unequal pressure. The inequality of pressure is usually 

 the result of unequal heating. 



It is a well known fact that all parts of the earth 

 are not equally heated. It is also known that the 

 amount of heat received by the earth at any given 

 place varies from hour to hour, day to night, and 

 month to month. These variations in temperature are 

 largely the result of the shape of the earth, its move- 

 ments, and its position with reference to the sun. 



Just as the earth is unequally heated, so likewise is 

 the air immediately above it unequally heated. As a 

 result the pressure of the air varies in different places, 

 being lightest where the air is hottest, and heaviest 

 where its temperature is lowest. In those portions 

 where the earth is greatly heated, the warm air will be 

 pushed up by the cold air from the surrounding regions 

 which will come in to fill its place. 



