252 THE WEATHER 



The air at the ground is always crowded or pushed away from 

 a region of higher pressure toward a region of lower pressure. 

 Every wind that blows is caused chiefly in that way. 



275. Pressure Areas Determine the Direction of Winds. 

 It follows that the direction of all our winds is controlled 

 chiefly by the positions of the nearest areas of high pressure or 

 low pressure. A HIGH to the north or a LOW to the south makes 

 our winds blow from the north. A HIGH to the west or a LOW 

 to the east causes our west winds. What two pressure condi- 

 tions will cause easterly winds? Southerly winds? The wind 

 directions due to pressure areas are modified by the rotation of 

 the earth, as explained in Arts. 298 to 301. 



276. The Velocity of the Wind. The velocity of the wind is 

 controlled principally by the amount of difference in pressure 

 in a given distance. The greater the difference in pressure in, 

 say, 500 miles, the stronger will be the winds in that region. 

 For example, between Valentine, Nebraska, and Miles City, 

 Montana, the pressure difference is 0.76 in. and the distance 

 about 375 miles. The wind at Rapid City, S. Dak., is 34 

 miles per hour. Compare that with the similar distance from 

 Valentine to Kansas City, having a pressure difference of only 

 0.34 in., where the wind between, at Lincoln, Nebr., is only 14 

 miles per hour. The difference in pressure is called the BARO- 

 METRIC GRADIENT. Greater differences are " steeper " gra- 

 dients, and cause stronger winds, other things being equal. 



277. Winds ; Summary ; Effect on Temperature. Looking 

 again at your map, it is clear that all the apparently conflicting 

 winds of that morning were merely obeying this universal 

 law. They blow toward Nebraska from all sides because the 

 lowest pressure is over Nebraska; and they blow away from 

 western Canada and away from the Virginia coast in all direc- 

 tions because the highest pressure is at those places. To a 

 considerable extent the winds carry their temperatures with 

 them; thus warm air' is carried northward by the south 

 winds, and cold air is swept southward in the north winds. 

 The distribution of temperature is thus partly controlled by 



