RAIN AND ITS OEIGIN IN THUNDERSTORMS. 385 



The degree of accuracy to which this measurement could be made during heavy rain 

 or highly charged light rain was 01 electrostatic unit per cubic centimetre of 

 rain-water. A charge of O'l electrostatic unit of electricity on a cubic centimetre of 

 rain was taken as the lowest charge with which the research was concerned. All 

 charges less than this amount were written as nil. 



Before discussing the results of the electrical measurements, it is desirable to give 

 a short description of the character of the rain in Simla during the period when the 

 measurements were made. The months of April and May are the hot months of the 

 year. But although Simla is at an elevation of about 7000 feet above sea level, 

 and the temperature does not therefore rise very high, yet the almost uninterrupted 

 sunshine during these two mouths is very favourable to the formation of thunder- 

 storms. In 1908, the first thunderstorm occurred on April 5, and afterwards storms 

 of greater or less intensity occurred at intervals of seldom longer than a fortnight. 

 The monsoon normally arrives in the Simla hills during the second half of June, and 

 previous to its arrival the thunderstorms become more and more frequent, culminating * 

 in storms of some violence which accompany the actual arrival. In the year under 

 review it was difficult to fix a definite day for the setting in of the monsoon, because 

 of the gradual manner in which the changes occurred. 



The greatest storm of the year occurred 011 July 1, and afterwards the thunder- 

 storms became less frequent and the monsoon rainfall more steady and continuous. 

 From this date the monsoon gave more or less heavy rain each day and continued 

 until the beginning of September ; as usual it ended with one or two sharp 

 th understorms. 



From the beginning of the thunderstorms in April to the end of the rains in 

 September the part of the apparatus for measuring the rainfall and the electricity 

 associated with it was in constant action, so that the electrical state of nearly every 

 shower was recorded. The potential gradient and coherer records commenced on 

 June 18 and the latter were entirely free from failure. The data used in the 

 following discussion have been obtained from every storm during which any 

 electricity was recorded, however light the rain, and also from those storms during 

 which no electricity was measured, but rain fell at a greater rate than 0'007 cm. in 

 each two minutes' interval. The latter limitation has been chosen because with 

 a rainfall of this amount charged to O'l els. unit per cubic centimetre of rain the 

 electrometer would have shown the smallest measurable deflection. Thus when the 

 rainfall was heavier than this amount and no electricity was recorded it could be said 

 with certainty that the rain was not charged to O'l els. unit per centimetre, while 

 with lighter rainfall which showed no electricity the same conclusion could not be 

 drawn ; it was therefore considered best to neglect such lighter rainfall. 



It has been found impracticable to divide the storms observed into thunderstorms 

 and storms which were not thunderstorms. For three months rain fell nearly every 

 day and it was quite impossible to take notes during the whole time that the rain 



VOL. ccix. A. 3 D 



