METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 467 



specification of the spell of clear weather of minimum duration. It was not possible to 

 enforce the limitation that the twenty-four hours should all belong to the same day. 

 It will be observed that a clear day only means twenty-four hours free of fog, and 

 implies nothing with regard to the presence or absence of cloud overhead. Tables I. 

 and II. give complete information as to the dates used. 



Summit and Base. — Although observations have been made at the base of the 

 mountain since the observatory on the summit was established, and for the last ten 

 years a first-class observatory has existed at Fort- William, in this paper no account is 

 taken of the observations made at the base. After the meteorology of the summit 

 has been thoroughly studied by itself, and that of the base by itself, there will be 

 greater light for the study of the combined observations and more assurance of the 

 validity of the conclusions arrived at. 



Details of Method of Selection. — A list of dates selected from the years 1885-1897 

 was made. The principle of the selection was, in the case of foggy weather, to pick 

 out every block of at least three days of continuous fog ; in the case of clear weather, 

 to pick out every spell of at least twenty-four hours continuous clear weather. By a 

 spell of clear weather is meant one in which no fog was logged. The blocks of foggy 

 weather vary in length from three days to eleven days ; the spells of clear weather 

 vary in length from twenty-four hours to two hundred and eighty-three hours. 



The following short table shows a summary of the cpmntit}^ of material which was 

 available on the principle of selection adopted. 



Summary of Material Used. 





Foggy Weather. 



Clear Weather. 











No. of Spells. 



No. of Days. 



No. of Spells. 



No. of Hours. 



January, 



22 



86 



13 



606 



February, 









12 



54 



26 



1,158 



March, . 









20 



78 



27 



1,335 



April, 









16 



59 



30 



1,728 



May, 









7 



27 



37 



2,228 



June, 









12 



42 



46 



2,895 



July, 









11 



46 



30 



1,502 



August, . 









17 



58 



19 



971 



September, 









16 



79 



31 



1,296 



October, 









17 



64 



21 



951 



November, 









20 



88 



19 



983 



December, 







' 



15 



66 



14 



697 



Totals, . 









185 747 



313 



16,350 



From it, it will be seen that 70 per cent, of the clear weather occurs in the months 

 of April, May, and June, and generally there is much more prolonged clear weather in 

 the spring half of the year than in the autumnal half. There is also less continuous 



