Weather Study 



869 



say that the pressure is ' 'low. ' ' So high pressure means a heavy atmosphere 

 and low pressure a light atmosphere. 



At the beginning we assumed that the atmosphere over the broad, level 

 plain was quiet and that it pressed down equally on every part of the sur- 

 face. We will now assume that the pressure was normal, or 30.00 inches, 

 and note the changes in pressure that result from the interchange of air 

 between the warm center and the cool margins. So long as none of the air 

 raised by the expanding layer at the surface, moved away toward the cool 

 margins, no change in pressure occurred; but the instant the air began to 

 glide down the slopes away from the warm center, then the pressure at the 

 surface decreased, because, some air having moved away, there was less to 

 press down than before. The pressure at the warm center, therefore, 

 became less than 30.00 inches, or in other words, low. Likewise, the air as 

 it moved away from the warm center, having lost much of its heat during 

 its ascent, was gradually pulled down by gravity because of its greater 

 density, thus increasing the pressure over the cool margins. We, therefore, 

 have low pressure at the warm center, 29.90 inches and high pressure, 30.10 

 inches, at the cool margins. From this illustration we obtain the six prin- 

 ciples of convectional circulation, viz. : 



1 . Low pressure at warm center. 



2. High pressure at cool margins. 



3 . Ascending currents at warm center. 



4. Descending currents at cool margins. 



5. Surface winds from high pressure to low pressure. 



6. Upper currents from low pressure to high pressure. 



Now, we all know that the temperature of air is much higher at the 

 equator than at the poles and we may, therefore, let Fig. i represent a sec- 



Fig. 2. Isobars of the world. 



