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Large Differences of Temperature between the Ben Nevis and Fort-William 



Observatories. By R. T. Omond. 



The immediately preceding paper deals with the average difference of temperature 

 between Ben Nevis and Fort- William throughout the year, but the difference often departs 

 widely from these average values. The greatest departures from the average during 

 the time that hourly readings are available at both places, viz. from 1st August 1890 to 

 30th September 1904, were Ben Nevis 17°"6 warmer than Fort-William at 9 a.m. on 

 19th February 1895, and Ben Nevis 28°- 8 colder than Fort- William at 2 a.m. on 19th 

 December 1890, being respectively 33 0, 3 below and 13°"1 above the mean difference, 

 which is Ben Nevis 15 0, 7 colder than Fort- William. 



The cases of small differences of temperature, including those in which the tempera- 

 ture at the summit of Ben Nevis was higher than that of Fort- William, have already 

 been dealt with in several papers : * they generally occur during anticyclonic weather. 

 In this paper the times of occurrence and weather connected with large differences of 

 temperature between the Observatories are tabulated and examined. They are associated 

 with considerable dryness of the air at Fort- William, and an absence of rainfall at both 

 stations. 



In carrying on the general discussion of the Ben Nevis observations, sheets were 

 prepared showing, amongst other data, the difference of the Ben Nevis and Fort- 

 William temperatures at each hour of the day and night. These sheets have been 

 examined from 1 a.m. on 1st August 1890 to midnight on 31st July 1903, and all the 

 hours at which the difference of temperature was 20° or more noted. The results of this 

 examination are summarised in Table I., which shows the total number of hourly 

 observations in each month in which the difference of temperature between the two 

 Observatories amounted to 20° (including all cases from 20° to 20°'9), 21° (including 

 21°'0 to 21 0, 9), 22°, etc., up to the largest difference observed, 28°'8. The total number 

 of cases is 13,824 during this period of thirteen years, which contains 4741 days, or 

 1 13,928 hours. Thus the difference of temperature between Ben Nevis and Fort- William 

 was 20° or more during a fraction over 12 per cent, of the whole time. 



A glance at Table I. shows how rapidly the frequency pulls off as the difference 

 increases, and that the higher differences of 24° and upwards almost all occur during 

 the summer half of the year, from March to September. The adiabatic change of 

 temperature effected in dry air when raised or lowered 4400 feet is 24° ; that is to say, 



* Trans. Roy. Soc. Edin., vol. xlii. p. 496, " Relations of Pressure and Temperature at the Ben Nevis Observatories," 

 by Alexander Buchan. Trans. Roy. Soc. Edin., vol. xliii. p. 505, "The Inter-relations of Pressure, Temperature, 

 etc.," by Alexander Buchan. Journ. of Scot. Met. Soc, vol. x. p. 127, "The High Temperature of September 1895 

 and the Ben Nevis Observatories," by Alexander Buchan. Journ. of Scot. Met. Soc, vol. xi. p. 65, "Change of 

 Temperature with Height dining Anticyclones," by R. T. Omond, 



