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Abstract of Paper on Silver Thaw at the Ben Nevis Observatory. 



By R. C. Mossman.* 



The phenomenon of silver thaw, — that is, rain falling when the temperature is below 

 the freezing-point and congealing when it falls, — is of somewhat common occurrence at 

 the Ben Nevis Observatory. A prolonged fall of silver thaw occasions considerable 

 inconvenience to the observers ; the rain freezes on their coats, gloves, and even on their 

 faces, while everything outside becomes covered several inches thick with solid ice. 

 But the most serious effect is the choking of the louvres of the Stevenson screen where 

 the thermometers are, necessitating frequent changing of the screens. 



In the six years, 1885-1890 inclusive, 198 cases of silver thaw were observed, 

 lasting in the aggregate 873 hours, or a mean duration of 4|- hours for each case. In 

 December, January, and February the average duration of each case was six hours, 

 whereas during the other months of the year it was not more than three hours. For 

 the latter half of 1890 Mr Aitken's dust-counter was in use at the Observatory, and 

 during these six months the observations show that the number of dust particles is 

 very small when silver thaw occurs. 



The subjoined table gives the number of cases of silver thaw in each month, the hours 

 they lasted, and their average duration for the six years, 1885 to 1890 inclusive : — 



Mouth. 



Cases. 



Hours. 



Mean duration. 

 Hours. 



January, 

 February, . 

 March, 



April, 



May, 



June, 



July, 



August, 



September, 



October, 



November, 



December, 



Year, 



34 



28- 

 27 



11 

 11 



8 





 3 



4 



12 



29 

 30 



198 



199 



149 



93 



53 



20 

 27 







7 



10 



38 



96 



181 



873 



5-9 

 5-3 

 3-4 



4-8 

 1-8 

 3-4 



o-o 



2-3 



2-5 



3-2 

 3-3 



6-0 



4-4 



Three-fourths of the whole cases occur in the five months, November to March ; none 

 have been observed in July. 



* See Journal of Scottish Meteorological Society, vol. ix. p. 115. 



