i8o ELEMENTARY SCIENCE 



us we are just as conscious of its reality. Why is it, by 

 the way, that winds nearly always feel cool, even though 

 the moving air may be warmer or just as warm as the air 

 it replaces ? 



On a weather-map (see Fig. 66) you will note that cer- 

 tain areas are marked "low" and others "high." This 

 refers to readings of the barometer. You remember that 

 a low barometer indicates the coming of a change of 

 weather, and, usually, a storm. This is very easy to 

 understand when you remember that air or any other 

 gas tends to diffuse equally in every direction. Evidently, 

 this tendency of air to diffuse equally will cause it to 

 move from areas of high pressure to areas of low pressure. 

 For an area of low pressure is simply an area over which 

 there is less air than there is over an area of high pressure. 



But what is it, you ask, that causes air to thin out in 

 certain places (low pressure) and to pile up (high pressure) 

 in others ? Heat is the answer. Variations in the amount 

 of heat. How often in studying phenomena we come 

 back to heat changes as the cause! In fact, nearly all 

 changes in the atmosphere are due to heat; heat is the 

 great explainer. 



Now we might have said that variations in atmospheric 

 pressure are caused by convection currents. But what 

 causes convection currents? Heat, of course. These cur- 

 rents, whether in water or in air, are simply expressions, 

 as it were, of heat, tools in the hands of the sun's energy. 

 This you learned when you studied these currents (see 

 Fig. 2). You learned that the greater amount of heat 

 received in the equatorial belt makes the atmospheric 

 pressure there less than it is to north and south; that is, 

 the greater heat near the equator causes the air to expand 



