THE GENERAL STORM 251 



line for 30.0 in. ; and other lines for each other tenth of an inch that 

 appears on the map. North of Montana and on the Virginia coast 

 the pressure is 30.3 in. Write the word High in each region. Write 

 Low in Nebraska where the barometer reading was lowest (29.4 in.). 



274. Winds Caused by Unequal Pressure. In western Can- 

 ada and on the Virginia coast the barometer stands nearly an 

 inch higher than in Nebraska. Since the total weight or 

 pressure of air at sea level is about 15 Ib. per sq. in., and the 

 barometer measuring that pressure reads about 30 in., it 

 follows that each inch on the barometer scale stands for about 

 % Ib. of air pressure on each square inch of the earth's surface. 

 Therefore on the morning of March 2, 1904, the weight or 

 downward pressure of the air over western Canada was nearly 

 J/ Ib. greater per sq. in. than it was over Nebraska. Now J^ Ib. 

 per sq. in. equals 72 Ib. per sq. ft., 648 Ib. per sq. yd., nearly 10 

 tons per sq. rod, or more than 1,000,000 tons per sq. mile. 

 Wherever there is a difference of 1 in. in the corrected barom- 

 eter readings, there is a difference of 1,000,000 tons per sq. 

 mile in the pressure of the atmosphere. For J^ in. difference 

 in barometer readings there would be about 500,000 tons per sq. 

 mile of difference in air pressure, and so on. This immensely 

 greater weight of air over the Virginia-New Jersey coast 

 and over western Canada, crowds the bottom air outward in all 

 directions away from the center of the greatest pressure (high- 

 est barometer). It is this greater pressure which causes the 

 wind to blow, at the same moment, from the south at Albany, 

 N. Y., from the southeast at Philadelphia, and from the 

 northeast in the Carolinas. It causes north winds in Mon- 

 tana and the Dakotas, where the air is being crowded outward 

 away from the high pressure in Canada. 



In the same manner the air over the region surrounding a 

 LOW presses down with greater weight than the air over the 

 center of the LOW. This crowds the bottom air in from all 

 sides toward the center of lowest barometer. That is why the 

 winds on all sides are blowing toward Nebraska. 



Such air movements always occur around HIGHS and LOWS. 



