xliv METEOROLOGY OF BEN NEVIS. 



When heavy rain is the result of moisture-laden air ascending from lower levels, 

 the values of the rainband are not affected by it. There is another state of weather 

 in which precipitation appears to be indicated by the spectroscope, but no rain 

 follows. This occurs when snow crystals are rapidly deposited from the fog on 

 the windward side of exposed objects, as already described. 



Occasionally, the observers are above the clouds, with clear sky and bright 

 sunshine overhead, while all other hills are covered with a continuous white sheet 

 of cloud, extending to the horizon in every direction. In such conditions a strong 

 rainband is always obtained in June, July, and August, in a horizontal line close 

 over the clouds, although the rainband may be absent on the blue sky. About 

 half an hour before sunrise on 18th December 1885, the upper-glow was very fine; 

 on it the rainband was 3 ; but when the sun appeared the rainband on his disc was 

 beyond the scale of the instrument. It was estimated at 12, being the highest yet 

 observed. 



The rainband observations of 1st July 1886 are specially valuable. The fol- 

 lowing were the readings : — 



At 6 a.m. the value was 6 



9 4 



,, noon ,, 2 



„ 6 p.m. „ 2 



At 6 a.m. temperature was 36°*7, at noon 40°"1, and at 6 p.m. 43 c, 5. Till 

 1 p.m. the Ben was enveloped in mist, accompanied with a slight drizzle ; at 2 p.m. 

 it cleared, at 3 p.m. the humidity had fallen to 62, and before sunset upwards 

 of six hours of sunshine was recorded. The decreasing rainband indicated that the 

 moist air aloft was slipping away or sinking below the level of the summit, and 

 that the air which was taking its place was comparatively dry. This increased 

 dryness and warmth of the air indicated that an anticyclone was forming and 

 settling over this part of Europe.* 



St Elmo's Fire. — During the five winters from 1883 to 1888, fifteen cases of 

 St Elmo's fire were recorded. These all occurred during the night time, and during 

 the winter months from September to February. It is difficult, if not impossible, 

 to observe this meteor in ordinary daylight, or in strong twilight, and this con- 

 sideration perhaps accounts for the absence of recorded cases during day and during 

 the summer months. On one occasion it was heard in the day time, being identi- 

 fied by the peculiar and unmistakable hissing sound which accompanies it. 



These fifteen cases have been discussed by Mr Rankin, in connection with the 

 observations of pressure, temperature, wind direction, and rainfall for 30 hours 

 before and 24 hours after the time of occurrence of St Elmo's fire. It is shown 

 that the weather which precedes, accompanies, and follows it has very definite 



* "Rainband Observations on Ben Nevis," by Angus Rankin, Jour. Scot. Met. Soc, Third Series, vol. i. p. 279. 



