ON METEOROLOGICAL OBSERVATIONS ON BEN NEVIS. 



285 



Table IV. — Showing the Hourly Variation of the Rainfall on Ben Nevis, 

 expressed in Percentages above or helow the dailij means. 



Hence, only those days were entered as foggy or misty when fog or mist 

 was recorded for each of the twenty-four hours of the day. The hourly 

 temperature and pressure of such days were extracted from the daily 

 sheets, and the averages for each month calculated. Those days were 

 regarded as clear days when the sun shone at least several hours, and 

 when fog or mist was virtually absent. Means were similarly calcu- 

 lated for these clear days. Thereafter the monthly means for the three 

 years were ascertained, and the hourly results ' bloxamed ' as explained 

 above. 



The results show two sets of curves, essentially different the one 

 from the other, the monthly curves for foggy and misty days revealing 

 a diurnal variation of pressure quite distinct from that of the curves 

 for clear days. Table V. shows the differences between the two sets of 

 curves, the plus sign ( + ) indicating a higher pressure for foggy days, 

 and the minus sign ( — ) a lower pressure for those days as compared with 

 the pressure for clear days at the same hours. With clear skies the 

 daily maximum pressure occurs at 11 a.m. in winter, but at 2.30 p.m. in 

 summer ; whereas with fog or mist it occurs at all seasons between 

 10 and 11 P.M. With clear skies the minimum occurs at 4.30 A.M., but 

 with fog at 6 A.M. 



From Table V. it is seen that, with fog, pressure is higher than with 

 clear skies from 7 p.m. to 4 A.M., attaining the absolute maximum at 

 midnight, but lower from 5 a.m. to 6 p.m., the absolute minimum being 

 about noon. The important bearing of these results on solar and terres- 

 trial radiation and other physical inquiries is obvious. No small part 

 of the large excess of pressure during the night hours in fog is probably 

 occasioned by the latent heat set free in the condensation of the aqueous 

 vapour into fog or mist. This necessarily, in the circumstances, increases 

 the barometric readings where it occurs, viz., on the top of the mountain,, 

 and particularly at night when the surface temperature of the mountain 



