BEN NEVIS OBSERVATORY. 



483 



centre of the sun is above the level horizon, but for Fort- William the " possible " is the 

 total computed time that the sun is above the visible horizon there. Two percentage 

 columns are given for Fort- William : the first is computed from the Fort- William 

 "possible" sunshine, and the second from what would be the possible for a level 

 horizon. 



Table XVIII. Wind-Force. — The mean hourly values at Ben Nevis for the 20 

 years 1884 to 1903, and the departures at each hour from the monthly mean values. 

 There are no corresponding hourly observations at Fort- William. The wind-force is 

 estimated on a scale of = calm, to 12 = hurricane. The following are the average 

 velocities in miles per hour corresponding to each number of the scale as deduced from 

 comparisons made with a Robinson hemispherical cup anemometer (see page 472). 



Ben Nevis Observatory Scale 

 Miles per hour 



1 

 6 



2 

 12 



3 



21 



4 

 30 



5 

 39 



6 

 49 



7 

 60 



8 



72 



9 

 (84) 



10 

 (97) 



11 

 (112) 



12 

 (130) 



The values for Force 9 and upwards must be regarded as rough approximations only. 



Table XIX. Cloud. — The mean hourly values of the amount of cloud at Ben 

 Nevis for the 20 years 1884 to 1903, and the departures at each hour from the monthly 

 average values. There are no corresponding hourly values at Fort- William. The amount 

 of cloud ranges from = clear sky to 100 = overcast ; fog or mist on the summit, thick 

 enough to prevent an observation of cloud, is counted as overcast. 



Table XX. Absence of Fog. — The mean hourly values of the number of times 

 that the summit was clear of fog or mist at each hour in the 20 years 1884 to 1903 ; the 

 same expressed in percentages, and the departures at each hour of these percentage 

 values from the mean percentage for each month. There are no corresponding hourly 

 values at Fort-William. 



This table was constructed by counting for each month the number of observations 

 at each hour in which the summit of Ben Nevis was not enveloped in fog or mist. No 

 account is taken of the presence or absence of clouds in the sky, the occurrence of 

 fog or mist on the summit alone being considered. 



Table XXI. Temperature. — The difference of the temperature at Ben Nevis and 

 Fort- William at each hour of each month on the average of the 13 years 1891 to 1903, 

 and the departures at each hour from the monthly mean values. The first section of 

 this table was computed by subtracting the average Ben Nevis temperature at each 

 hour of each month from the corresponding Fort- William temperature ; in other words, 

 by subtracting the second part from the third part of Table XI. The second section of 

 this table contains the hourly departures computed in the usual way, but with the 

 values carried out to hundredths of a degree. 



WIND DIRECTIONS AT BEN NEVIS OBSERVATORY. 



Table XXII. Wind. — The number of times each direction of wind was noted at 

 Ben Nevis at the hourly observations in each month on the average of 20 years 1884 to 

 1903 ; the same reduced to 8 points and expressed in percentages, and the departures 

 of these percentage values in each month from the annual percentage values. 



