xxxn 



METEOROLOGY OF BEN NEVIS. 



being carried on, and for this purpose it is not the means, but the individual 

 observations which are so valuable in their bearings on weather changes. 



Diurnal Oscillations of the Barometer. — Table III. gives the simple arith- 

 metical means of the hours for the several months as observed, and the 

 difference of the mean of each hour of the day from the mean of the month. If 

 the month began and ended with the same height of the barometer, the differ- 

 ences of the hourly means from the monthly mean would represent the true 

 hourly variation. But such a coincidence very seldom occurs. 



In January 1884 the barometer at 1 a.m. of the 1st was 25*805 inches, and 

 at midnight of the 31st 24-837 inches. On the assumption, then, that the day 

 began at midnight, the mean for this hour was 25*196 inches ; but if the day 

 be considered as beginning at 1 a.m., the mean for the same hour was 25164 

 inches, or 0032 inch less than in the former case. If now the hourly differences 

 were taken directly from the figures of the upper part of the Table, each 

 mean from 1 a.m. to 1 p.m. would be above the monthly mean and the means 

 of the other hours below it. Hence, to show the true hourly variation, correc- 

 tions determined from the difference of the two midnight means have been 

 applied to the hourly means of the Table ; and from the new figures thus 

 obtained, the lower part of the Table has been constructed, where the heavy 

 type indicates that pressure is above the mean, and the light type, in italics, 

 that it is below it. 



From the four years' observations, the following are the four diurnal 

 phases, compared with the daily means, of the barometric curves for the 

 seasons, with the approximate hours of their occurrence : — 





1 .First Minimum. 



First Maximum. 



Second Minimum. 



Second Maximum. 





inch. 



hour. 



inch. 



hour. 



inch. 



hour. 



inch. 



hour. 



Winter, 



-0-012 



6 A.M. 



+ 0-003 



11 A.M. 



-0-005 



3 P. M. 



+ 0-010 



9 P. M. 



Spring, 



-0-015 



5 „ 



+ 0-010 



2 P.M. 



+ 0-001 



5 „ 



+ 0-008 



9 ,, 



Summer, 



-0-019 



5 „ 



+ 0-012 



2 „ 



+ 0-005 



7 „ 



+ 0-010 



9 „ 



Autumn, 



-0-019 



5 „ 



+ 0-005 



noon 



+ 0-001 



5 ,, 



+ 0-011 



8 ,, 



Year, . 



-0-016 



5 „ 



+ 0-005 



1 P.M. 



+ 0-001 



5 „ 



+ 0-009 



9 „ 



In all seasons the curves show the double maximum and minimum pressure. 

 The morning minimum is much greater than the afternoon minimum, as 

 happens on all mountain peaks which rise considerably above the plains 

 surrounding them. The partial exception of January is occasioned by the 

 lowest pressures, accompanying several storms of January 1884, occurring 

 about that time of the day, particularly the low pressure of the great storm of 

 the 25th, when sea-level pressure fell to 27 333 inches at Ochtertyre, and at 

 Fort- William to 27 '4 inches. The relatively larger minimum in the early 

 morning is due to the cooling of the atmosphere during the night, by which the 

 air contracting sinks to a lower level, thus lowering pressure at high levels. 



