METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 799 



those of temperature. It naturally occurs to see if there is any connection between the 

 two. 



Taking clear weather and the winter months during which the curves, both of tem- 

 perature and of pressure, show the greatest irregularity, and considering first the mean 

 curves of temperature and pressure for the six winter months, October to March, on 

 Plate VII., we see from 6 a.m. onwards a very considerable agreement in the irregu- 

 larities of the two curves. The alternations of flatness and steepness in the two curves 

 are very remarkable even during the hours when the sun's increasing influence is most 

 felt. Even where a rise of the barometer is not accompanied by a rise of temperature 

 we see a sharp rise or fall of the barometer, accompanied by an increased or diminished 

 steepness of the temperature curve. The nocturnal rise of temperature between 6 p.m. 

 and midnight, which is so marked a feature in the midwinter months, affects the mean 

 of the whole six winter months, and the nocturnal maximum of pressure is still more 

 pronounced in the six winter months. Turning now to the curves for the individual 

 months, we find in October the parallelism in the irregularities strongly marked. In 

 this month the sun's power is still considerable, and during the morning rise and after- 

 noon fall of temperature the rise of barometer between 5 and 7 p.m. may be put in con- 

 nection with the flattening of the temperature curve at the same time, when, with the 

 disappearance of the sun, the rate of cooling ought to be the greatest. The nocturnal 

 heating is characteristic of all the winter months, and is sensible even in April and May. 

 A nocturnal maximum of pressure is common to all the months of the year, but it is not 

 accompanied by a temperature maximum in the warmer months. On Ben Nevis, as in 

 most places, the barometer is subject normally to more than one rise and fall during 

 the twenty-four hours, and it is meant here only to point out parallelisms without 

 expressing any opinion as to cause or effect. The parallelisms in the changes of curva- 

 ture from hour to hour are more particularly apparent in the case of January. In 

 February and March, and in a less degree in other months, in clear weather there is 

 agreement in the irregularities in both curves in the early afternoon when the tempera- 

 ture is about its maximum. 



In foggy weather the march of both temperature and pressure is more uniform than 

 in clear weather. The most striking departure from uniformity occurs about midnight 

 and the early morning hours. In the months of March, April, June, July, August, 

 September, October, November, and December there is a sharp rise of temperature 

 between midnight and 1 or 2 a.m. This is greatest in summer, in July the difference 

 in the mean temperature at midnight and at 1 a.m. is 1*3° F. A similar feature is 

 observed in the barometric pressure in the months November, January, February, March, 

 and April. The maximum excess of the pressure at 1 a.m. over that at midnight is 

 0*034 in. in January. Hence, in foggy weather the most striking feature of parallelism, 

 although common to some months, reaches its maximum values at midsummer in the 

 case of temperature, and at midwinter in the case of pressure. 



The Table on page 803 gives a summary for the year of the movements of tempera- 



