Meteorological Observations on Ben Nevis.-Report of the Committee, consisting of Lord MCLAREN (Chairman), Professor A. CRUM BROWN (Secretary), Dr. JOHN MURRAY, Dr. ALEXANDER BUCHAN, Hon. RALPH ABERCROMBIE, and Professor COPELAND. (Drawn up by Dr. BUCHAN.)

THE Committee were appointed as in former years for the purpose of cooperating with the Scottish Meteorological Society in making observations en Ben Nevis at the two observatories situated respectively at the top and bottom of the mountain.

During the year the hourly eye observations by night and by day have been uninterruptedly made by Mr. Omond and his assistants; and the continuous registrations and other observations have been carried on at the Low Level Observatory at Fort William with a like fulness of detail as in previous years.

Owing to frequent storms and heavy snowfalls, which lay long and deep, the climatic conditions at the top of the mountain were very severe; but the Directors have the greatest satisfaction in reporting that the health of the observers has notwithstanding been good. The Directors tender their best thanks to Messrs. Charles Stewart, B.Sc., Craig, Shand, Herbertson, and Rankin.

TABLE I.-Showing Monthly Mean and Extreme Pressures, Temperatures, Rainfall, Sunshine, and Clouds.

1893

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Year

Mean Pressure in Inches.

Ben Nevis Ob- | 25-336) 24.936) 25-373 25.580 25-493 25-491) 25-377 25-437) 25-225] 25·171| 25·360) 25-101| 25-324 servatory

Fort William

Differences

29-991 29-503 29-972 30-151 30-018 29.974 29-842 29-902 29-718 29-699 30-004 29-682 29-871 4-655 4567 4.599 4.571 4.525 4-483] 4-465 4:465 4-493 4.528 4:044 4581 4547,

Mean Temperatures.

Ben Nevis Ob-
servatory

Fort William
Differences

23.2

23.8 28.5

34.9

37.4 42.5 42.0 43.6 36.1 31.8 28.8 26-7 33-0

37.6 39.9 43.5 49.2 53.6 57.5 59.0 59.5 52.6 47-7 39-1 42.4 48.5 14:4 16.1 15.0 14:3 16.2 15.0 17.0 15.9 16.5 15.9 13.3 15.7 15.5

13.0

20-2 22.7 29.8 33.1 32.0

10-3 8.4 24.8 25.0 32.7 36.0 41.2 42:0 41.3 32.0 32.8 26.5 21.6 13.9 11.8 14.7 12.5 13.3 11:4 8.9 9.3 13.6 15.8 16.3 14.4 7.5

18.4 17.0 10-2 7.2

For the year 1893 Table I. gives the monthly mean and extreme pressures, temperatures, hours of sunshine, amounts of clouds and rainfall, and number of days of no rain on the one hand, and on the other of days when the rainfall was not less than one inch at the two observatories, the mean pressures at the top being reduced to 32° only, while those at Fort William are reduced to 32° and sea level.

The mean temperature of the year at Fort William was 48°.5, being 30-2 greater than that of the previous year, and 10.3 in excess of the mean annual temperature of the place. The mean at the top of the mountain was 33° 0, which is 3°3 in excess of the previous year and 2°.2 greater than the mean annual temperature deduced from all the observations made since 1881. The following show the deviations of the monthly results from their respective means :—

Thus the outstanding feature of the meteorology of the year was the abnormally high temperature which prevailed during the six months from March to August. The mean temperature at the top for the six months was then 5° above the mean, whilst at Fort William it was only 2°.8, or but little more than half the excess at the top of the mountain. The reason for this extraordinary difference is the remarkable and prolonged

continuance of anti-cyclonic weather during the time when, as has been often referred to in our Reports to the British Association, the tempe rature at the top is frequently much higher, absolutely, than it is at Fort William.

The lowest mean monthly temperature at Fort William was 37°-6, for January, and at the top 23°2 in the same month, these being respectively 10.7 and 1°.5 under the average of the month. The warmest month was August at both stations, where the means were 59°-5 and 43°6, or 2°.5 and 4°.1 in excess of their averages.

The maximum temperature at the top was 62° 6 on June 18, and at Fort William 83°2 on August 9. The minimum at the top was 6°.4 on January 2, and at Fort William 13°-9 on January 6 As compared with previous years the minima for the five months from April to August were relatively high reading at both stations, showing that the temperature was during these months not only high as regards the means, but was marked by a singular absence of such low temperatures as usually occur.

At the top the registrations of the sunshine recorder show 680 hours out of a possible 4,470 hours, being 122 hours fewer than during the previous year, and 228 fewer than during 1891. The following months exceeded the averages: March by 20 hours, April by 60 hours, November by 23 hours, and January by 6 hours. The maximum was 151 hours in April, and this is also longer than any previous recorded April. All other months fell short of the averages, and during the whole of December only one hour's sunshine was recorded, and on the following month, viz., January 1894, only three hours' sunshine occurred. At Fort William the number of hours of sunshine were 1,065, which is respectively 114 and 155 hours fewer than during the previous two years. The maximum was 186 hours in June, and the minimum 6 hours in December. At these stations, in common with a large surrounding district, 1894 was characterised by a singular deficiency of sunshine, which is remarkable in view of the high temperature of the year. At the top of the mountain the proportion of the actual to the possible sunshine was only 15 per cent., and at Fort William the percentage was 30, or double that at the top.

At the top the percentage of cloud covering the sky was 84, being the average of previous years. It varied greatly in the different months, being above 90 per cent. in February, July, August, October, and December, which were characterised by a marked deficiency of sunshine, and reached 98 per cent. in December, when, as already stated, only one hour of sunshine was recorded. On the other hand, the minimum 61 per cent. occurred in April, the month of the absolute maximum sunshine, being 66 per cent. above the maximum of the month.

The following table shows the lowest humidities of each month :—

Dry Bulb.

TABLE II.- Lowest Hygrometric Readings each Month.

Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.

о

Wet Bulb

Dew-point

Elastic Force

024

Relative Humidity 18

47

31

33

40

36

24.6 22.8 14.8 35.2 32-8 48.1 52.3 47.1 51.6 17-0 19:1 20.6 12:0 25.4 26-2 35.9 42.6 33.6 37.2 29.0 22-2 14.6 -12-2 6.5 -9.8 9.7 12.3 22.5 32.8 18.7 22.8 16.8 -1.3 058 *026 067 075 •120 •186 101 •122 *093 47 32 41

37-727-8

-3-6

041

*037

31

As compared with the similar table in last year's Report the following

results are interesting for 1892 and 1893: Lowest dew-point, -278 and — 12°-2, highest dew-point 21°-2 and 32°-8 ; lowest elastic force 0.010 inch and 0.024 inch; highest elastic force 0.114 inch and 0.122 inch; and lowest relative humidity 7 and 18, and highest 47 and 47. These results point to a large excess of aqueous vapour in the air at the Ben Nevis Observatory during 1893.

The rainfall for the year at the top was 165.77 inches, being 18.00 inches above the mean annual rainfall. At Fort William the amount was 83.54 inches, which is 10-37 inches above the mean. These amounts are respectively 12 and 14 per cent. above their averages. The maximum monthly rainfall at the Ben Nevis Observatory was 25-66 inches, in December, and the minimum 5:48 inches, in May. The minimum monthly fall for 1892 was 5.42 inches in March, and these two years show the largest minimum falls of any of the years since the Observatory was opened. Hence, at this high level situation the rainfall was not only considerably above the mean, but it continued to be relatively large through all the months of this year, which will be long remarkable for an unprecedented drought over a large portion of the British Islands. On the other hand, at the Low Level Observatory the amount of the rainfall was short of the average for each of the five months from March to July, the deficiency amounting to 5-12 inches.

At Fort William the rain fell on 235 days, and at the top on 260 days, being respectively 3 days under and 26 days above their averages. The maximum number of days on which rain fell was 31 days at the top and 29 at Fort William in December, and the minimum number 16 days in March and 12 days in June respectively.

The maximum daily rainfall at the top was 4.29 inches on November 28, and at Fort William 3.25 inches on October 24. At the former station instances of one inch a day or upwards occurred during each of the twelve months, whereas at Fort William, during the five months from March to July, the rainfall on none of the days reached an inch. During the year the rainfall amounted to an inch or upwards on 51 days, but at Fort William the number of days was only 12, being a lower proportion .at Fort William, as compared with the top, than has previously been recorded. Thus, while during the spring and early summer of 1893 Fort William participated in some degree in the prevailing drought, the rainfall and moisture at the Ben Nevis Observatory were above their average, a result probably occasioned by the stronger ascending currents from the superheated surface of the earth carrying to higher levels than usual the moisture of lower levels.

Auroras are reported to have been observed on the following dates :January 5, 9, 10, and 11; February 15; March 26 and 29; April 3, 11, 12, 26, and 27; May 9; August 12; September 11; October 4 and 17; and November 8, 9, 10, 11, 12, 13, and 14.

St. Elmo's Fire was seen on February 9; April 6 and 20; August 15; October 25; and December 8.

Thunderstorms occurred on April 6; May 19 and 21; June 8, 12, and 13; July 7, 8, 10, and 11; August 15; September 8; and December 11. On May 21 the thunderstorm passed below the level of the summit, being very severe, with much lightning and exceptionally heavy rain and hail at Fort William, while on the summit there was no lightning and only a slight shower of rain. On June 13 the thundercloud enveloped the summit for some time, and the lightning, entering the Observatory, damaged the telegraph cable, and greatly interrupted communication

during the rest of the month, so that on several nights the usual daily weather report could not be wired to the newspapers. The thunderstorm of July 7-8, though very severe, was fortunately unaccompanied by any damage.

At Fort William the mean atmospheric pressure at 32° and sea level was 29.871 inches, and at the top 25.324 inches, the difference being thus 4.547 inches. The lowest pressure at the top for the year was 23-888 inches in December and the highest 26-003 inches in April, the difference being 2.115 inches, being considerably above the average difference. This large difference was due to the low reading in December, which was an altogether exceptional month as regards the almost continuous saturated state of the atmosphere, and to the high readings which accompanied the anticyclonic weather of the spring and early summer. In truth, the monthly means were uninterruptedly above the average for the six months from March to August, the mean excess for the half-year being so much as 0-101 inch above the average, an excess only exceeded in 1887, the Jubilee year, when the mean monthly pressure was uninterruptedly above the average from February to July, the mean excess being 0.144 inch. This period was also strongly anticyclonic.

The important hygrometric research carried on at the High and Low Level Observatories and described in the Committee's last two Reports to the British Association has been continued. During the past year Mr. Herbertson has conducted the observations with the assistance of Mr. Angus Rankin, First Assistant at the Observatories, and of Mr. F. J. Hambly, F.C.S., F.I.C., Assistant Lecturer on Chemistry at University College, Dundee; and of Mr. Marr, Demonstrator of Botany in the same College.

An Assmann aspiration psychrometer was read for dry and wet bulb temperatures, in addition to the thermometers in the Stevenson screen. The dust particles in the air were counted, and the general weather conditions of each experiment were noted. Nearly 100 experiments were made at both observatories, of which 57 were synchronous.

A comparison of the readings of the ventilated thermometers with those of the screen only shows that in calm, or virtually calm, weather the wet-bulb in the Stevenson box is much nearer the dry-bulb reading than in the Assmann aspiration psychrometer. When no measures are taken for causing an air current to pass the thermometer bulbs, all readings made in calm or light airs require to be neglected in hygrometric work. Under ordinary conditions, the total amount of water vapour in the air does not vary much in a fine day.

A discussion of the simultaneous observations at high- and low-levels brings out some very interesting results. On September 11, 1893, with a normal temperature gradient between the two observatories, the watervapour remained fairly constant at both places all day, there being an excess of about 1.5 gramme per cubic metre at the lower station. On September 4 the summit temperature was only from 2° to 7° lower than at Fort William, instead of 16°0 the normal difference; and on this occasion the difference between the quantities of water-vapour was as great as from 6 7 to 4.60 grammes per cubic metre. This great variation was almost entirely due to changes in the amount of water-vapour in the upper air, since there was a steady increase of vapour from 9.15 grammes per cubic metre at 9 A.M. to 10.56 at 2 P.M., and 11·40 at 7 P.M. at the lowlevel station; whereas the vapour at the summit was 2.72 at 9 A.M., 5.96 at 2 P.M., 3.92 at 5 P.M., 5.55 at 7 P.M., and 5.89 at 9 P.M.; the maximum at the summit at 2 P.M. being evidently caused by an uprush of moister