526 MR MOSSMAN ON SILVER THAW. 



An examination of the daily weather charts of the 198 days on which silver thaw 

 occurred, showed that on 137 days the distribution of pressure was cyclonic, and on 

 61 days anti- cyclonic ; but the mean duration of the phenomenon on cyclonic days 

 was only 3 "2 hours and on anti-cyclonic 7 '6 hours. Also, the longest continued case 

 under cyclonic conditions was 11 hours on 23rd December 1890, but under anti- 

 cyclonic conditions it lasted for 41 consecutive hours on 3rd and 4th January 1889. 

 The distribution of pressure is substantially the same in all cases. There is a cyclone 

 with its centre off the N.W. coast of Norway, and an anti-cyclone to the south of Ben 

 Nevis, with its centre in the British Isles or in Northern France, or even still 

 further to the south. Of the 198 cases only 3 took place in a type of pressure- 

 distribution differing from the above ; in them the cyclone was still to the west of 

 Norway, but the anti-cyclone lay over the Gulf of Bothnia. No cases occurred when 

 the barometer was higher to the north than to the south of Ben Nevis, nor when the 

 central part of an anti-cyclone covered Ben Nevis. 



The lowest temperature at which silver thaw occurred was 18°. It is of rare 

 occurrence below 27°, and at only 34 out of the 873 hours during which it has been 

 observed was the temperature below 27°. Most of the cases occur just before a thaw. 



As might be expected from the distribution of pressure, the winds were mostly 

 westerly. The average velocity of the wind was about 20 miles an hour, and rarely 

 rose to a gale ; in fully 90 per cent, of the cases the wind was below 30 miles an hour. 



The rainfall, which constitutes the phenomenon, is always heavy, being an average 

 of '032 inch per hour in cyclonic cases and 0*019 in anti-cyclonic. The record at 

 Fort-William on the same days shows that this heavy fall is confined to the hill-top ; 

 on the average of the year, the rainfall of Ben Nevis is not quite double that of Fort- 

 William, but on these days it was occasionally seventeen times greater. 



A comparison of the temperature observations at Fort-William with those on Ben 

 Nevis on the days of silver thaw showed nothing very definite. The least difference 

 was 2°*9 on the 20th December 1886, and the greatest 18°"2 on 14th August 1885, 

 but the average was about 17°, or close to the normal difference of temperature. On 

 the average of the 12 longest continued cases of cyclonic silver thaw, of which the 

 mean duration was 7 hours, the temperature on Ben Nevis began to rise 15 hours 

 before it commenced, continued rising while it lasted, and thereafter fell for six hours, 

 after which it rose again. The barometer fell steadily till the silver thaw began, rose 

 during its continuance and for 9 hours subsequently, when it began to fall again, this 

 latter fall coinciding with a distinct backing of the wind and rise of temperature, indi- 

 cating the approach of a new depression. In the 12 longest continued anti-cyclonic 

 cases, temperature rose for 18 hours before, continued rising during the silver thaw 

 and for 3 hours after, and then fell steadily. The barometer rose till the thaw began, 

 but fell steadily during and after it. 



An examination of the records of Gales kept at all the Scottish lighthouses, shows 

 that 73 per cent, of the cyclonic and 63 per cent, of the anti-cyclonic cases were 



