50 



PRINCIPLES OF STRATIGRAPHY 



back country is very cold as compared with the coast. The mistral, 

 a stormy northwest wind blowing from the Cevennes in southern 

 France, has a similar origin, descending from the cold central plateau 

 to the warm JMediterranean coast. 



Mountain and Valley Winds. Where the general air movements 

 are not too strong, a regular daily alternation of winds is observ- 

 able in mountain regions, blowing up the valleys by day and down 

 by night. Though not confined to valleys on the mountain sides, 

 they attain their best development there. In the Himalayas, the 

 winds blow up the valleys from 9 A. M. to 9 P. M. and down the 

 remainder of the night. Where the valleys open out into the plains, 

 the winds often blow with great violence, particularly in winter. 

 The upward flow during the day is explained by the increase of 

 pressure at points distant from the slope, owing to the warming and 

 rising of the air beneath these points and the expansion of the air 

 at the same level on the mountain slope. This causes a movement 

 of the air toward the mountain which must result in an upward 

 flow of the current on the mountain side. At night the cool wind 

 naturally flows down the slope, attaining its greatest velocity where 

 it leaves narrow, cool valleys and enters broader, well-warmed ones. 



Velocities of Wind. The velocity of the wind current varies 

 enormously, from the frightful rate of 400 or 500 miles per hour 

 obtaining in some tornadoes, to the gentle zephyr breeze of a frac- 

 tion of a mile per hour. The average velocity for the winds* of the 

 United States has been estimated at about 9.5 miles per hour (4.25 

 meters per second), and for Europe 10.3 miles per hour (4.60 

 meters per second). It is greater over the sea than over the land, 

 where friction retards it, and greater in the upper air than at the 

 earth's surface. The average velocity of the wind is also greatest 

 in about latitude 50°. The following wind scale gives the velocities 

 of the various types of wind according to Hann (40) : 



For comparison see velocities of streams given on p. 245. 



