METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 795 



If we consider the variations of vapour tension with reference to the vapour tension 

 itself, and compare the two kinds of weather in this respect, we find a marked difference. 

 Looked at from this point of view, we find some analogy between the foggy weather on 

 Ben Nevis and the rainy season in the Tropics, and between the clear weather and the 

 dry season. 



The following figures illustrate this : — 



In India the months June, July, August, and September are the chief Monsoon 

 months. The station Poona lies with respect to the west coast of the peninsula and to 

 the Indian Ocean beyond, in a position not unlike that of Ben Nevis to the west coast 

 of Scotland and to the Atlantic. Also, at Poona in the rainy season, and on Ben Nevis 

 during foggy weather, the prevailing wind is from the westerly quarter. On the other 

 hand, Poona is 60 miles inland, and only 2000 feet above the sea, whereas Ben 

 Nevis is on the coast and 4400 feet above the sea. If we compare the vapour tensions 

 in the months June to September at Poona, and in foggy weather on Ben Nevis, and 

 the same elements in the opposite months, December, January, February, and March, 

 the dry season at Poona with June to September on Ben Nevis in clear weather, we 

 have at Poona* in the rainy season for the mean vapour tension 0708 in., and 

 the mean range 0'037 in., or 5*2 per cent, of the whole vapour tension. On Ben Nevis 

 during foggy weather in the same months the mean vapour tension is 0'224 in., and 

 the range - 0153 in., or 6'8 per cent, of the whole vapour tension. In the months Decem- 

 ber to March at Poona the mean vapour tension is 0"465 in., and the range 0087 in., or 

 18 "7 per cent, of the whole. In the months June to September on Ben Nevis in clear 

 weather the mean vapour tension is 0*220, and the mean range 0"049 in., or 22'2 per cent, 

 of the whole. These figures show that the conditions in the two cases run in parallel lines. 



The temperatures are given in Tables XXVII. to XXXIII., and in Plates IV., V., 

 VI., VII., and VIII. Tables XXVII. and XXVIII. contain the absolute maximum and 

 minimum temperatures at every hour during the thirteen years. Tables XXIX. and XXX. 

 contain the monthly mean hourly temperatures in clear and in foggy weather. Tables 

 XXXI. and XXXII. contain the corresponding diurnal variation of temperature, and 

 Table XXXIII. contains the hourly difference between the temperature in clear and in 

 foggy weather. 



The mean temperature of the year is 3*57° F. higher in clear weather than in foggy 

 weather. Amongst the monthly values this excess is greatest in June, when it reaches 

 10*1 1° F. In the first three months of the year the difference is in the opposite sense. 

 The temperature is then higher in foggy than in clear weather, the excess being 2 - 92° F. 

 in February. The mean temperature of the months October to March is almost the 

 same for both kinds of weather, being 28 - 70° for clear and 28 "51° for foggy weather. 

 Amongst the hourly values the greatest excess of temperature in clear over that in 

 foggy weather is 13 "4° F. at fifteen hours in June, and the greatest difference in the 



* The Meteorology of the Bombay Presidency. By Charles Chambers, F.R.S., Official Publication of the India 

 Office, p. 148. 



VOL. XXXIX. PART III. (NO. 31). 6 H 



