XXXVi METEOROLOGY OF BEN NEVIS. 



of the earth's surface. In all such situations, the maximum velocity is during 

 the day and the minimum during the night. At Pike's Peak, the velocity of 

 the wind is about 6 miles per hour greater in the early morning than at noon. 



As regards the months, the maximum wind force occurs from November to 

 March, the mean of the five months being at the rate of 310, or 22 miles an hour, 

 and the minimum force 1*91 from June to August, or 11 miles an hour, the 

 difference being thus 11 miles. The maximum velocity of the wind on Ben 

 Nevis occurs therefore during the coldest season of the year, and the coldest 

 hours of the day, and the minimum velocity during the warmest season and the 

 warmest hours of the day approximately. 



The occurrence of the greatest velocity during winter is to a considerable 

 extent to be accounted for by this season being the stormiest period of the year, 

 when steep barometric gradients, and the strong winds so often the accompani- 

 ments of the cyclones that make their tracks over the British Islands are of most 

 frequent occurrence. But there is another cause which results in the prevalence 

 of stronger winds at High-Level Stations. At High-Level Stations in tropical 

 regions, there is little difference of temperature and atmospheric pressure 

 observed throughout the year. Thus, at Medellin, in Columbia, 6°12" lat. 

 N'., and 75° '45" long. W., and 4951 feet above the sea, the mean barometers for 

 January and July are 25158 inches and 25174 inches, and the temperatures are 

 709 and 70 o, 5. But in higher latitudes, where there is considerable difference 

 between the summer and winter temperatures, pressures also differ greatly. On 

 Pike's Peak, 14,134 feet above the sea, where the mean temperature of January 

 is 2 0- 5 and July 40° "0, the corresponding pressures are 17*493 inches, and 

 18*077 inches, being thus fully half an inch higher in summer than in winter, 

 even though pressure at comparatively low levels in that part of America are 

 much less in summer than in winter. Since, owing to the marked diminution of 

 pressure at great heights in the atmosphere during the cold months of the 

 year, barometric gradients, greatly steeper than in summer, are formed in 

 extra tropical regions in winter for the prevailing westerly upper currents of 

 the atmosphere. From this cause doubtless the wind force on Ben Nevis 

 is largely augmented during the winter months. 



It is quite otherwise with the manner of the distribution of the wind velo- 

 cities during the twenty-four hours of the clay which is so marked a feature of 

 the meteorology of Ben Nevis, as well as of all High-Level Stations, situated on 

 peaks, from which we have observations. In these situations, the maximum 

 occurs during the night, and the minimum during the day, in all months of the 

 year. The distinct, but not strongly accented secondary maximum that occurs 

 early in the afternoon of the summer months, may be here referred to as 

 evidence that the causes giving rise to the daily maximum wind velocity at 

 these hours of the day at lower levels are felt at the Observatory only, however, 



