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Report on Hygrometric Researches mainly at the Ben Nevis Observatories* 

 By A. J. Herbertson, M.A., Ph.D. (With Plate.) 



Introductory. 



Since the beginning of meteorological work on the summit of Ben Nevis, occasional 

 spells of wonderfully fine weather have been experienced, in which the exceeding 

 dryness of the air has been very remarkable. It was observed by Mr Wragge in 1882, 

 and has been recorded nearly every year since then. The most notable of these dry 

 periods occurred in March 1886, t and at that time the relative humidity, as calculated 

 by Apjohn's formula, was as low as 6. These low humidities are associated with a high 

 pressure area, and a marked departure from the mean temperature difference of the top 

 and bottom stations ; indeed, the normal condition of temperature descending with 

 increasing height may be reversed, and the air on the summit of Ben Nevis be warmer 

 than that of Fort- William. An example of this reversal of temperature conditions 

 occurred in December 1892, when the observers on Ben Nevis spent Christmas Day in a 

 warm, sunny, and very dry atmosphere ; while those in Fort-William were shivering in 

 raw, dull, cold air, at times over 8°F. lower than that on the summit. 



These remarkable drynesses are not found at any low-level station in our islands, 

 and, naturally, Glaisher's hygrometric tables, used in this country, do not provide 

 for them, although the Greenwich observations on which the tables are based were 

 supplemented by results obtained in India and in Canada. The students of Ben Nevis 

 observations, who wished to deduce from the readings of the dry and wet bulb ther- 

 mometers what was the temperature of the dew-point, the pressure of the water vapour 

 in the air, and the relative humidity during these dry spells, were in a difficulty. If 

 they employed Glaisher's factors for calculating the dew-point to extend his tables, 

 they were using data which had never been tested for such extreme conditions ; and if 

 they took advantage of the formulas published by Apjohn, or August, or Regnault, 

 they were making the reductions of these remarkable readings in quite a different way 

 from that employed for ordinary observations. 



To solve some of these difficulties, Dr H. N. Dickson spent part of the autumn of 

 1885 on the summit of Ben Nevis. He made an extended series of direct determina- 

 tions of the dew-point, using Professor Chrystal's modified form of Mr Dine's hygro- 

 meter, and read the temperatures of the dry and wet bulb thermometers at the same 



* Summary of communications to the Scottish Meteorological Society on 13th March 1893 and 

 20th July 1894, :ind to the Royal Society of Edinburgh, 6th February 1893 and 20th May 1895, with some 

 supplementary notes. 



t ' Meteorology of Ben Nevis,' Dr A. Buchau, Trans. R.S.E., vol xxxiv* pp. xxiii., xxiv. 

 TRANS. EOY. SOC. BDIN. — VOL. XLIII. 3 X 



