292 



SOILS. 



the sea-breeze dies down, and toward and after sunset the 

 land-breeze takes its place. 



The principle of this local change of winds, together with 

 the rotation of the earth, the absorption of moisture by air, 

 and the fact that the latter becomes cooler when it expands 

 on rising and warmer when it is compressed by descending, 

 serves to explain all the major phenomena usually observed 

 in connection with winds. The air of the equatorial belt, 

 heated throughout the year, necessarily rises and creates an 

 indraught from both north and south; but since the air thus 

 flowing in has a lower rotary velocity than the earth's surface 

 at the Equator, it lags behind and so gives rise to northeast 

 and southeast winds, respectively, between the two tropics and 

 the equatorial belt. These regular winds, from the aid they 

 give to commerce in passing from continent to continent, are 

 known as the trade winds. On the other hand, the air that 

 has risen from the hot equatorial belt, cooling by expansion 

 as it rises and flowing northward and southward from the 

 Equator, on descending as it mainly does into the temperate 

 zones, has a higher rotary velocity than the land-surface and 

 so tends to give rise to southwest and northwest winds in the 

 northern and southern hemispheres respectively. At sea, on 

 coasts and in level inland regions to windward of mountain 

 chains, such winds are often quite regular during a portion of 

 the year. 



Cyclones. But local disturbances arising from heated land 

 areas or mountain slopes, as well as wide atmospheric changes 

 whose causes are not fully understood, give rise to waves of 

 alternating high and low barometric pressure, largely con- 

 verting rectilinear or slightly curved wind-motion into whirls 

 or " cyclones " 1 ranging from a thousand to over two thou- 

 sand miles in diameter. These in the case of low-pressure 

 waves or centers, toward which the air flows from the outside, 

 revolve in the direction contrary to the movement of the hands 



1 This designation is popularly and incorrectly applied to the comparatively 

 limited, but very violent and destructive rotary storms or whirlwinds which 

 originate locally on the heated plains of the Middle West of the United States, and 

 are almost always accompanied by violent electric phenomena. These should 

 properly be called tornadoes. At sea such whirlwinds give rise to waterspouts, in 

 deserts to sand storms. 



