230 J. HOPKixsoN — THE cli:mate of watfoed. 



Rainfall (Tables XIII-XV, p. 231).— The decade 1877-86 is a 

 very misleading one from which to form an idea of the mean 

 rainfall at Watford, this period having been an exceptionally- 

 wet one. We have now, from the observations commenced at 

 Nash Mills in 1833, a record of the rainfall in Hertfordshire 

 for 60 years, and during the whole of that period there is only 

 one decade at all approaching in wetness that of 1877-86, and 

 that is 1876-85, the year 1876 having been wetter than 1886. 

 Not only is this the case, but if we extend our comparison to 

 the whole period for which we have records of the rainfall in 

 the British Isles, the 167 years from 1726 to 1892, it will be 

 found that there has not been any period of ten consecutive 

 years with a mean annual rainfall approaching within several 

 inches that of 1876-85 or 1877-86.*' In every year of our 

 decade 1877-86 but one, 1884, the rainfall was above the 

 average for half a century or any longer period as far back as 

 our records extend, and in most of the years much above it ; 

 in seven years out of the ten it exceeded 30 inches, and in 

 four of these years it exceeded 33 inches per annum. 



In order, therefore, to arrive at an approximate determination 

 of the mean annual rainfall at Watford, it is necessaiy to com- 

 pare the fall during these ten years with that of a long period 

 at some not far-distant station having a sufficiently long record. 

 Nash Mills, Hemel Hempstead, is the only rainfall-station in 

 Hertfordshire with a record of at least half a century, and, as 

 it is only six miles from Watford and has about the same 

 rainfall, the ratio the rainfall of these ten years bears to that 

 of fifty years is likely to be about the same at Watford as there. 



The mean annual rainfall at Nash Mills for the fifty years 

 1810-89, was 27-18 ins.; for the ten years 1877-86, it was 

 31'07ins. ; the difference, 3*89 ins., is 12-52 per cent., or, say, 

 121^ per cent., or one-eighth, of 31 -07 ins. The value at Watford 

 of 30-96 ins. for the ten years 1877-86, should therefore be 

 reduced by 12^ per cent, to give an approximate value for the 

 fifty years 1840-89. Or we may ascertain this by proportion, 

 thus:— 31-07: 27-18 :: 30-96: 27-08. 



The mean annual rainfall at Watford, although 30-96 ins. in 

 1877-86, will therefore probably have been about 27 ins. in 

 1840-89. This is a sufficiently long period for our purpose, 

 but if we wished to arrive at the probable value for the last 

 century^ a further reduction would have to be made. 



Por the half- century the approximate mean monthly and 

 seasonal rainfall would be as follows : — 



See Symons' ' British Eaiufall, 1891,' Frontispiece. 



