CHAPTER XIV. 



EXAMPLES OF ATMOSPHERIC MOTIONS. 



200. Indian Southwest Monsoon in July. In giving a few examples of the 

 kinematic diagnosis of atmospheric motions, we shall begin with a case of great 

 simplicity, namely, the Indian Southwest Monsoon in July, at the time of its highest 

 development. 



Plate XXXI gives the discontinuous representation of this air-motion taken 

 from plate 1 7 of Sir John Eliot's Climatological Atlas of India. The arrows repre- 

 sent the average wind-directions for the month, and the numbers the corresponding 

 average intensities, changed from miles per hour to meters per second. In this 

 case the distribution of arrows is a regular one, and it causes no difficulty to draw 

 the lines of flow from them. The moderately idealized contour-lines of the blank 

 map on which the construction is performed are a good help for the understanding 

 and correct drawing of these lines. The chart representing the lines of flow and 

 curves of equal wind-intensity is given on plate XXLXII. 



On the peninsula of India the motion represented by these two systems of 

 curves is of great regularity. A striking effect of the topography of the land is seen, 

 inasmuch as the lines of flow make a bend around the southern projection of the 

 peninsula in order to avoid going across the mountains of the west coast. In the 

 places where the wind must still travel directly toward the shore and the slope of 

 the mountains, decided minima of velocity are seen to exist. 



In the northern part of the chart the most marked peculiarity is the long line 

 of convergence which goes up the whole length of the Ganges valley, in order to 

 end in a constellation of a point of convergence and a neutral point situated above 

 the Punjab plains. This long line of convergence is evidently an effect of the 

 Himalaya chain. The observations do not go to a sufficient height in the mountains 

 to let us see the complete character of the motion. But in all probability a correlated 

 line of divergence must exist higher up on the slope of the chain. These two parallel 

 lines of convergence and of divergence will then give the limits in horizontal pro- 

 jection of a rolling mass of air, which is kept in rotatory motion by the Monsoon- 

 current passing across the mountain in greater height (cf. fig. 52 b, p. 59).* 



Plate XXXIII shows the forced vertical velocity at the ground. This chart has 

 been derived from the preceding one by the method described in section 181. The 

 shaded parts are the areas of ascending and the unshaded ones those of descending 

 motion, the shaded ones on the windward and the unshaded ones on the leeward 

 sides of the mountains. The ascending motion reaches its greatest values on the 

 west coast, where it has a maximum amounting to 15 cm. per second. 



Plate XXXIV gives the free vertical motion derived by use of the solenoidal 

 condition as described in section 188 (C). Properly adjusting the units, we can 



'Mr. E. Gold has arrived at a similar conclusion. Nature, Feb. 1908, p. 355. 



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