112 FIRST YEAR COURSE IN GENERAL SCIENCE 



spring." Our memories of the weather often mislead us, 

 for the impression made upon our minds by one hot week 

 or many days of rain is likely to be recalled as describing a 

 whole season. 



118. Wind. Wind is air in motion. Its strength is 

 designated by different expressions, such as light or stiff 

 breeze, if the velocity is from two to ten miles an hour; 

 strong wind, twenty to thirty miles; gale, forty to sixty 

 miles; and tornado, one hundred to two hundred miles. 



All winds result from the same condition, though the con- 

 dition may have different causes. This one condition is the 

 difference in pressure of the atmosphere over adjacent regions. 



Just as water always moves down hill in obedience to 

 gravity, so air moves out from a region of high pressure to 

 one of lower pressure. If over New York State and the 

 Province of Ontario the barometer indicates a pressure of 

 30.2 inches (or 768 mm.), while over New England the pres- 

 sure is 29.7 inches (or 755 mm.), the air moves toward New 

 England and it is called a west wind. The greater the 

 difference in pressure, the stronger the wind. 



119. Cyclonic Movements. If the low-pressure area is 

 surrounded by areas of higher pressure, the air moves in 

 from all directions, and the currents, as they meet, unite 

 in an upward whirling motion. This is called a cyclone. 

 It may vary in violence from a dust whirl in the street to a 

 destructive gale. 



120. Constant Winds. There are certain parts of the 

 world where winds blow all the time in the same direction. 

 The air in the equatorial belt is warmer than the air north 

 or south of it and contains much vapor, on account of the 

 great extent of ocean there. For these reasons, air in this 

 belt is lighter than the air outside, and the heavier air pushes 

 its way into the region of lower pressure. This causes 

 constant winds to blow toward the equatorial belt from both 

 north and south. 



