448 



DR BUCHAN AND MR OMOND ON THE 



Diurnal Range of the Barometer in Fine and in Cloudy Weather. 

 By Alex. Buchan, LL.D., F.R.S., and R. T. Omond. 



This investigation was undertaken with the view of ascertaining how the mean 

 hourly values of the barometer differed on days in which radiation, solar and terrestrial, 

 was great from days in which it was interfered with by clouds. A preliminary exam- 

 ination of the barometric readings at the Ben Nevis and Fort-William Observatories 

 showed that there was a well marked difference between the diurnal fluctuations in 

 these two kinds of weather ; and the investigation was thereafter extended to several 

 other places in the northern hemisphere. The materials for this work are only available 

 from comparatively few places, it being necessary to have before us the hourly readings 

 of the barometer for every day, and also an hourly record either of sunshine or of cloud. 

 These two separate data have not been published for any place in the British Islands 

 except Ben Nevis and Fort- William. But several of the Continental meteorological 

 observatories give in their reports the readings of the barometer and the amount of bright 

 sunshine at each hour of each day. Certain tropical stations also give these data, while 

 at the special Arctic stations where observations were taken in 1882 and 1883, hourly 

 readings of the barometer and of the amount of cloud are given. The records of the 

 following nine stations have been utilised in this discussion. 



San Jose, Costa Rica . 



Lat. N. 

 9° 56' 



Long. 

 84° O'W. 



Height 

 Feet 



3756 



Hong Kong, China 



22 18 



114 11 E. 



110 



Trieste, Austria 



. 45 39 



13 46 E. 



85 



Magdeburg, Germany . 

 Ben Nevis, Scotland 



52 8 

 . 56 48 



11 38 E. 

 5 1 W. 



177 

 4407 



Fort- William, ,, 



. 56 49 



5 7 W. 



42 



Sodankylii, Finland 



. 67 24 



26 36 E. 



594 



Bossekop, Norway 



Jan Mayen, North Atlantic 



. 69 57 

 . 70 59 



23 15 E. 



8 28 W. 



35 



98 



These stations extend from the tropics to the Arctic regions, and include 

 inland, coast, and island stations. At each of the first six stations a period of three 

 years was taken, but at the three Arctic stations only one year was available. At each 

 place days were selected on which the sky was clear or only slightly clouded during the 

 whole day, and the hourly barometric readings for these days taken, grouping them 

 according to months. Then similarly readings were taken of those days in which the 

 sky was overcast or nearly so all day, the only exception being at Ben Nevis, where, 

 instead of overcast, days of continuous fog were taken for the cloudy period. No 



