Observations on Rainfall. 95 



22. — Observations on Rainfall. 



By Baldwin Latham, C.E., F.G.S., F.M.S. 



[Read 2.-^rd March, 1881.] 



As all supplies of water are dependent upon rainfall, an 

 accurate determination of the amount of rain falling in any 

 district is a matter of paramount importance. 



The most casual observer must have noticed that there are 

 great variations in the amount of rain falling in different years, 

 and also in different parts of the country. The greatest rain- 

 falls, as a rule, occur on our western shores, but rainfall also 

 increases with the elevation of the ground above sea level. 

 To some extent however, the physical outline of a district 

 modifies the distribution of rainfall. 



Five years ago, when the author commenced to make 

 observations upon the underground waters in this neighbour- 

 hood, he soon found that there were some marked peculiarities 

 with regard to the rise of the water in the ground which 

 needed close investigation in order to account for the pheno- 

 mena observed. For example, it was clearly demonstrated 

 that in the deep wells in the North Downs, the water line of 

 which was removed over 160 feet from the surface, the water 

 commenced to rise, after a low period, before it did in the 

 comparatively shallow wells in some of the low grounds. In 

 seeking an explanation of this phenomena, it was essential to 

 establish a number of rainfall stations in the district, in order 

 to determine with accuracy the quantity of rain falling in our 

 immediate neighbourhood. It is a well established fact that 

 rain increases, with elevation to a certain altitude, after which 

 it again diminishes. Dr. Miller, of Whitehaven, found from 

 observations made in that neighbourhood, that the maximum 

 quantity of rain occurred at an elevation of about 2,000 feet 

 above the sea level. After this altitude was attained, the rain 

 diminished, as has been demonstrated in certain mountainous 

 districts. The increase of rainfall with the altitude of the 

 ground is due to the degree of coldness and the diminution of 

 the capacity of the air to retain moisture. Dr. Miller has 

 shown that with elevations of from 1,000 to 3,000 feet, the 

 atmosphere has but small absorptive capacity. Warm air 

 from a lower region, which is either driven upwards by the 

 sloping nature of the ground, or is ascending as in a cyclonal 

 depression, or is moving from a warm to a cold quarter with 

 the gyrations of the wind, when it reaches a certain degree of 

 altitude, or has been cooled down by any of the movements of 

 the air referred to, will part with a portion of its moisture, 

 which will be precipitated as rainfall. 



As temperature diminishes about i degree for every degree 



