222 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Sept., 1905. 



regard to the month of Februarj', in 1821 the fall was 

 0.04 in., whilst in 1866 the fall was 4.03 ins. 



Owing to the great variability in the rainfall, it is 

 very desirable that the averages should be based upon 

 as long a period as possible; most of the recognised 

 authorities on the subject assert that the period should 

 not be less than 30 years. 



.Meteorologists in particular, and the people of the 

 IJritish Isles in general, owe a deep debt of gratitude 

 to the late Mr. G. J. Symons, F.R.S., for having 

 commenced the collection of rainfall statistics in i860, 

 and for publishing the results yearly in the volumes of 

 British Rainfall. The number of stations at the time 

 of his death, in 1900, was over 3,400. He not only 

 collected these statistics, but he also secured uniformity 

 in the measurement of the rainfall and in the exposure 

 of the gauges, and he thoroughly checked the accuracy 

 of the returns sent to him. It is satisfactory to know 

 that the work is still being carried on under the able 

 supervision of Dr. H. R. Mill. 



I-rom the rainfall maps of the British Isles compiled 

 by Mr. Symons, Dr. Buchan, and Ur. Mill, it is seen 

 that the average annual rainfall exceeds 40 inches along 

 the western coasts, and that in several districts it 

 exceeds 75 inches, chiefly the west Highlands of Scot- 

 land, the English Lake District, and the .Snowdonian 

 District of North Wales. Over the eastern part of 

 Ireland and of .Scotland, and the south of Kngland, the 

 rainfall is mostly between 30 and 40 inches, while over 

 the eastern counties of Kngland it is less than 25 inches. 



The average annual rainfall of Kngland is about 32 

 inches, of Wales 49 inches, of Scotland 47 inches, f)f 

 Ireland 42 inches, and of the British Isles as a whole 

 39.5 inches. 



At Seathwaite, in Borrowdale, Cumberland, I he- 

 average annual rainfall reaches the large amount of 

 '.■?5-49 inches; while about a mile further away on the 

 shoulder of the hill, near .Stye Head, the average rain- 

 fall is 175 inches. 



The average monthly rainfall at Seathwaite is as 

 follows : — 



The prevailing winds over the British Isles are mostly 

 from the south-west. These come off the Atlantic warm 

 and highly charged with moisture; and as they strike 

 against the hills in the west, the moisture is condensed 

 and falls as rain. Thus the heaviest rainfall occurs in 



the west, and the amount increases according to alti- 

 tude. 



A few years ago the author discussed the average 

 rainfall for the 10 years 1881-1890, at 309 stations in 

 England and Wales, grouping the stations according to 

 altitude above sea-level. The results for each hundred 

 feet were as follows : — 



Rainfall 2 /-eg ins. 

 30*50 .. 

 3I-49 .. 

 3249 .. 

 ,, 40-64*,, 



37-38 „ 

 39-01 ,, 



These results show clearly an increase of rainfall with 

 altitude. 



Wishing to confirm the statement already made that 

 the heaviest rainfall occurs on the west coast, Ike, the 

 author subdivided the above stations into western and 

 eastern — considering those as " western " which 

 drained towards the west, and those as " eastern " 

 which drained towards the east. The following inter- 

 esting results were obtained : — 



West. East. 



Altitude 100 ft. .. Rainfall 3315 ins. 24S2 ins. 

 200 ,. .. ,. 35 87 ,, 25 94 ,, 



300 .. •■ .. 3572 .. 26.89 .. 



400 „ 39-56 ,. 2845 ,, 



,, 500 .... ,, 46-08* ,, 29H7 ,, 

 600 .... ,. 38 08 ,, 35S4 ,. 

 700 41-25 .. 35-27 .. 



These values show in a very striking manner that 

 ihe rainfall is considerably greater in the west than in 

 the east, the excess being nearly a quarter. If the 

 stations had been more numerous, and if the observa- 

 tions had extended over a longer period, there is no 

 doubt that the results would have been more uniform. 



The place which has the heaviest known rainfall in 

 the world is Cherrapunji, an Indian station situated in 

 the south-west of Assam, on a small plateau forming 

 the summit of one of the spurs of the Khasia hills. 

 The hill on which Cherrapunji is situated rises pre- 

 cipitously about 4,000 feet from the lowlands of 

 Cachar and Sylhet, which are barely 100 feet above 

 sea-level. The south-west monsoon, advancing from 

 the Bay of Bengal, sweeps over these low lands, and, 

 meeting the hills, is suddenly deflected upward. Rapid 

 condensation takes place and heavy rain falls. The 

 average annual rainfall at Cherr.ipuiiji is about 500 

 inches, which f.'ill mostly between April and September. 

 In the month of August, 1841, the rainfall amounted 

 to 264 inches. The heaviest rainfall in one day was 

 40.8 inches on June 14th, 1876. 



The extremes in this coimtry appear very insignificant 

 compared with tile amount just named, nevertiiele.ss 

 thev are often considerable. For instance, on August 

 6th, 1857, the observer at Scarborough measured gj 

 inches, but the rainf.ill actually exceeded that amoimt, 

 as the gauge had overflowed. At Seathwaite 8.03 

 inches fell on November 12th, 1897. On July 14th, 

 1875, more than 5 inches fell over Monmouthshire. 

 On June 23rd, 1878, Mr. .Symons, at Camden .Square, 

 London, recorded a fall of 3J inches in an hour and a 

 half. 



It is these exceptionally heavy rainfalls whit-h are so 

 serious and which do such an amount of damage. It 

 is, therefore, necessary for engineers and surveyors to 

 know something of the rate at which rain may be 



•These values .ire l.-irKcly increased by the heavy rainfall at 

 I Seathwaite. 



