METEOROLOGY OF BEN NEVIS. XXXlll 



As this nocturnal cooling below the daily mean temperature is relatively greater 

 in summer than in winter, or as the diurnal range of temperature is greatest in 

 summer, the morning minimum of pressure falls lowest during the warmer 

 months of the year. 



On the other hand, as the atmosphere is heated by the sun's rays during the 

 day, the air thereby expanding rises to a higher level, and hence at great 

 heights pressure is increased in the afternoon. One of the remarkable results of 

 this state of things is, that only in winter does the afternoon minimum fall below 

 the mean daily pressure. In summer this afternoon minimum is O005 inch 

 above the daily mean. So decided are the temperature effects of solar and terres- 

 trial radiation on the diurnal distribution of the pressure on Ben Nevis, that it 

 is for the year above the daily mean sixteen hours of the day, and below it only 

 eight hours. The largest excess of any hour above the mean annual pressure 

 is 0*009 inch at 9 and 10 p.m. ; whereas at 5 a.m. pressure falls 016 inch 

 below the mean — the defect in the early morning being thus double the excess 

 in the evening. 



In June, when the direct solar heat is strongest, this temperature effect on 

 the pressure is most decidedly marked, and as the afternoon minimum falls only 

 003 inch below the preceding maximum, a close approach is made during 

 this month to a diurnal curve of pressure, with only one maximum and mini- 

 mum. It is interesting to note that even a closer approach to a single diurnal 

 maximum and minimum of pressure takes place over the open sea in the higher 

 latitudes at substantially the same hours, and a single diurnal maximum and 

 minimum is also approximated to, and over many regions reached, in the 

 interior of continents in the higher latitudes, the maximum, however, occurring 

 in such cases about 8 a.m. and the minimum about 5 p.m., being the reverse of 

 what occurs at such high-level observatories as Ben Nevis. 



Another remarkable feature in the meteorology of Ben Nevis is the great 

 regularity, from month to month, of the times of occurrence of the four phases of 

 the pressure, and the comparatively small number of clays' observations required 

 to show them. In winter, however, there are exceptions to this regularity, it 

 being at this season that the strength of the sun's rays is least, and the disturb- 

 ance arising from the passage of cyclones, with their low pressures, greatest. 

 Thus in December 1883 the monthly means showed only a single maximum at 

 11 a.m. and minimum at 10 p.m. ; and again, in December 1884, a single 

 maximum at 9 p.m. and minimum at 7 a.m. ; but the means of the four years 

 give a curve with both maxima and minima. 



The diurnal phases of the barometric curve are widely different from the 

 above, in places among the mountains situated in valleys, particularly deep 

 valleys. Thus at Gries, in the Tyrol, which is situated in a deep valley, the 

 following is the diurnal variation in June : — 



vol. xxxiv. e 



