1903.] C. Little — On two remarkable rain-hursts in Bengal. 27 



westward and southward. The southerly element in this progressive 

 movement was less marked in the second than in the first disturbance, 

 as shown by both pressure and temperature changes. 



The explanation of the weather changes for the periods represented for 

 the purpose of this paper by the 30th Jane and the 11th August appears 

 to be that just previous to these dates, depressions were crossing Thibet 

 towards the Himalayan range, the first moving in a south-westerly, and 

 the second in an almost due westerly direction. These depressions on 

 reaching the Himalayas became to a certain extent broken up, more 

 especially the former whose direction of motion had been almost per- 

 pendicular to the range of high hills. Owing to the comparatively 

 srnn 11 height of the hills to the north of Assam, a disturbance of some 

 intensity entered that province and moving south-westward caused the 

 rainfall in Assam and Bengal. The higher hills in Nepal, formed a 

 more serious obstacle to the progress of the general disturbance, and 

 that may be the reason why on both occasions the changes appear to 

 have been delayed in Bihar and the United Provinces. The fact that 

 the depressions had to pass over a range of hills extending in places to 

 between twenty and thirty thousand feet, adds greatly to the difficulty 

 of establishing continuity in the changes that occurred. What adds 

 still further to the difficulty is that when a cyclonic storm encounters a 

 range of hills of height sufficient to cause disintegration of the cyclonic 

 system of air motion, local storms with large irregular changes of 

 pressure and temperature and with irregular rainfall generally occur. 

 In almost every case where a cyclonic storm moves northwards from 

 the Bay of Bengal towards the Himalayas the storm breaks up very 

 suddenly on reaching the hills, and instead of a well defined depression 

 with cyclonic winds we find in a few hours a uniform distribution of 

 pressure with numerous thunderstorms, it may be along the whole line 

 of the Himalayas. Judging by what one observes of these storms, from 

 the southern side of the range of hills it is very improbable that weather 

 becomes disturbed in Thibet after a storm from the Bay of Bengal 

 disappears amongst the hills. But that is not a sufficient reason for 

 arguing that a cyclonic storm may not cross the Himalayas from Thibet 

 into India. In the first place the Thibetan storm is at a high altitude, 

 because of the Central Asian plateau, and a second reason is that the 

 obstacle which the hills present, to the progress of a storm, from the 

 Thibetan side is not nearly so serious as to storms from the south. 

 There would be more or less isolated peaks to pass, instead of the solid 

 wall, formed by the lower ranges up to 10,000 feet, surmounted by the 

 peaks. 



Among the general conclusions given in the Monthly Weather 

 J. ii. 5 



