232 REPORT— 1871. 



On tlie Rainfall of Scotland. By Aiexakber Bxjchan-, M.A., F.R.S.E. 

 Secretary of the Scottish Meteorological Society. 



The paper ■was illustrated by a map of Scotland, showing the average annual 

 rainfall at 290 places, many of the averages heing from obscr-\-ations carried 

 on through long series of years. The map brought out the large rainfall in the 

 west ascomjiared with the east — a dift'erence which is strouglj- marked even in the 

 group of the Orkney Islands. The average rainfall in the west, at stations re- 

 moved from the intlnence of hills, is from about 36 to 40 inches; but in the cast 

 in similar situations the rainfall is as low as from 24 to 28 inches. In casting the 

 eye towards the watershed of the country running north and south, it is seen that 

 in ascending toward it from the west there occurs a rapid but by no means uni- 

 form increase, and in descending ii'oni it toward the east a rapid but by no means 

 uniform decrease. The largest rainfalls occur almost wholly among the hills 

 forming that part of the watershed of Scotland which is north of the Forth and 

 Clyde. The places characterized by the heaviest annual rainfidl are, so far as 

 observation has yet enabled us to determine, the following : — Glencroe, 128 

 inches; Ardlui, head of Loch Lomond, 115 inches; Bridge of Orchy, 110 inches; 

 Tyndrum, 104 inches ; Glen Quoich, 102 inches ; and Portree, 101 inches. At no 

 great distance from several of these places the rainfall is by no means excessive, 

 tlius pointing out an enormous difference of climate between places not fiir apart. 

 Along the watershed of that part of Scotland which lies south of the Forth and 

 Clyde, no such excessive rainfall occurs, — the highest being 71 inches at Ettrick 

 Pen Top 22G8 feet high. This diminished rainfall in the south, as compared with 

 that at places further north similarly situated, is due to the mountains of Ireland 

 draining the south-west winds of part of their moisture before they arrive at these 

 parts of Great Britain. 



The distribution of the rainfall is very instructive in many districts, as in the 

 valley of the Forth, from the head of Loch Katrine to Nortli Berwick, where the 

 amoimt varies from 91 to 24 inches; in Clydesdale, where the quantit}' is greatest 

 at the head and foot of the valley respectively, being considerably less at inter- 

 mediate places ; and along Loch Linnhe and through the Caledonian "N^alley, where 

 the variations of the rainfall are very great, and striljingly show the inilueuce of 

 purely physical causes, such as the configuration of the surface, in determining the 

 amomits. In all these districts, as well as elsewhere, many cases might be referred 

 to which conclusively prove that the amount of the rainfall is very far from being 

 detei-miued by mere height. In truth it is to local considerations we must chielly 

 look for an explanation of the mode in which rain is distributed over any district ; 

 and hence in estimating the rainfall, particularly of hilly districts, no dogmatic 

 rule can be laid down. 



From observations which have been made at fifty places for lengthened periods, 

 it appears that the deficiency of the three driest consecutive years' rainfall from 

 the average, is generally from one fourth to one seventh, but that in some cases it 

 is as gi-eat as one third and in others as small as one ninth. Since then the defi- 

 ciency pf the three years of greatest drought has varied from about 33 to 11 per 

 cent. ; itis evident, at least in so far as Scotland is concerned, that no dogmatic rule 

 can be given stating a rate of deficiency applicable to all cases. 



If those districts were shaded off in which the rainfall does not exceed 30 inches 

 annually, the great grain-producing district of Scotland would be indicated ; and 

 it is interesting to note that in those districts which produce the best wheat the rain- 

 fall is lower thau elsewhere, being in many places as low as 24 inches annually. 



On the Rainfall of the Nor/hern Hcmiqihere in July, as contrasted with that 

 of January, tvith RcmarJcs on Atmospheric Circulation. By Alexakdlr 

 Btjchan, M.A., F.R.S.E. 



On the Great Heat of August 2nd~4th, 1868. 

 By Alexander Buchan, M.A., F.R.S.E. 



