XXII. 



THE CLIMATE OF WATFORD, 



DEDUCED FRO:\I METEOROLOGICAL OBSERVATIOXS TAKEX 



DURIXG THE TEX YEARS 1877-188G. 



By JoBra HoPKUfsoN, F.L.S., F.G.S., F.R.Met.Soc. 



Eead at IFatford, I8th April, 1893. 



From meteorological observations extending over a period of ten 

 years it should be possible to arrive at some idea of the climate of 

 Watford. For all elements of climate but the rainfall it is not 

 likely that the mean annual value deduced from any ten years' 

 observations -will diifer greatly from the mean for a much longer 

 period, nor that the extremes will be greatly exceeded. Taking, 

 for example, the temperature, which is the most important element 

 of climate, the mean annual temperature deduced from observations 

 extending over any ten years probably will not differ more than 

 half a degree from the average for half a century, or even a 

 century. At the Royal Observatory, Greenwich, during the half- 

 century 1841-90, there appears to have been only one decennial 

 period when this difference was exceeded, and then but slightly. 

 The mean annual temperature at Greenwich in our decade 1877-86 

 was 0°-3 below the half-centmy's average, the mean for this decade 

 being 49°- 16, and for the half-century 49°-46.* At Watford in 

 this decade it was 48°-3 (or more precisely 48°-33), and therefore 

 the true value for the half -century would probably be about 48°- 6, 

 or a little less than a degree lower than the Greenwich mean 

 temperature for the same period. 



Again, it is a well-founded assumption that the mean of the 

 extremes in a long series of years gives a fairly accurate idea of 

 the mean temperature of a place. If I had left unthsturbed the 

 minimum and maximum thermometers from the beginning of 1877 

 to the end of 1886, from a single reading of these thermometers 

 the mean temperature of Watford would have been inferred to 

 be 49°-0, which is a little more than half a degree above its true 

 value for the period. On the above assumption, if 48°-3 were the 

 true mean for a long period, a lower minimum temperature than any 

 I have recorded would be more likely to occur, or to have occurred, 

 duiing such period, than a higher maximum. This inference also 

 follows if we assume that the mean temperature at Watford during 

 the half-century 1841-90 was 48°-6 ; and if duiing that period the 

 maximum attained during 1877-86 has not been exceeded, the 

 minimum is not likely to have fallen more than a degree lower 

 than the minimum I have actually recorded. 



After temperature the most important element of climate is 

 rainfall, but the mean rainfall of any place cannot be directly 

 ascertained, with any probability of accuracy, from observations 

 extending over a shorter period of time than 40 or 50 years. It 



* See ' Quart. Joura. R. Met. Soc.,' voL xvii, p. 236, and vol. xviii, p. 239. 



