Theory of the Winds. 



23 



circulation of the atmosphere. If we take the surface wind 

 directions and the surface barometric gradients for some 

 particular instant the agreement is not so good. 



Although even on the earth's surface there is a very fair 

 agreement between the directions of the winds and the baro- 

 metric gradients, it must be borne in mind that the direction 

 of the surface winds is varied greatly by local features, such 

 as hills and surface friction, whereas the barometer measures 

 the weight of an atmospheric column, and gives the sum of 

 the effects resulting from temperature, humidity, &c. of the 

 whole column. This subject has been dealt with by Shaw 

 and Dines *. 



At a height of about 1500 metres the velocity and direction 

 of the wind and the barometric gradient have been found to 

 be in fair agreement. 



Fior. 8. 





Latitude 





30-5 

 300 

 29-5 

 290 













_A=^ 1 

















I 









f 



° 1 



1 









1 

 i 







_y 



/" 









/ 



L 



ongitude 



100° E 











S. 80° 60° 40° 20 ' 0* 20° 40 ' 60' 80* N. 





Fig. 8 shows the variations of atmospheric pressure along 

 the meridian 100° E. for January and July. The pressure 

 variations are here shown to be much greater between 

 summer and winter in the Northern Hemisphere than they 

 are in the Southern Hemisphere, 

 over Asia and the Indian Ocean, 

 during the winter over the high 

 plateaus gives rise to an area of high pressure from which 

 the air flows north and south, the southern current being the 

 Dry Monsoon. In the Indian Ocean there is a high pressure 

 ridge about 30° S. Int., beyond which the pressure falls off 

 rapidly towards the Antarctic Circle. Here the wind moves 



This meridian passes 

 The cooling of the air 

 mountain ranges and 



* The Free Atmosphere id the Region of the British Isles, 

 logical Office Memoir No. 210 b. 



Meteoro- 



