April 2 r, 1892] 



NATURE 



583 



a system of three-hourly observations of the dust par- 

 ticles was started, and this has been kept up with but few 

 interruptions since. The dust observation is made 

 immediately after the usual hourly set is completed, and 

 it can thus be studied along with all the other hourly 

 records in their relation to the prevailing weather. 



A great many observations have in this way been 

 accumulated during the past two years, but we have not 

 had time for studying them in detail yet. A mere in- 

 spection, however, brings out some interesting points. One 

 of these is the enormous variation that is observed in the 

 number of dust particles, not only in the course of the 

 year, but often in the course of a few hours. At sea-level 

 the number of dust particles in the air at any time depends 

 very much on the locality and on the wind, whether 

 blowing from a polluted district or not. The mean of a 

 number of observations made by Mr. Aitken at Kingair- 

 loch, in the west of Scotland, is 1600 particles per cubic 

 centimetre. In London, on the other hand, he found 

 100,000 per cubic centimetre, and in Paris rather more. 

 On Ben Nevis the mean is 696 per cubic centimetre, the 

 maximum being 14,400 per cubic centimetre, and on 

 several occasions the minimum fell to o. A general 

 mean does not convey a fair idea of the dustiness of the 

 air at the mountain-top, although it may do so for places 

 at sea-level, because there is at the former place a great 

 daily range in the number of dust particles, depending on 

 the rise and fall of the air past the place of observation. 

 If there is any marked variation at sea-level it is entirely 

 of a different character. Below are the means, as well as 

 the maxima and mimima, of all the months that have a 

 fairly representative number of observations. 

 Number of Dust Particles per cubic centimetre on Ben Nevis. 



The above table shows that the Ben Nevis air contains 

 most dust in spring, and it is probable that sea-level air 

 is in this respect similar ; the cause of this greater amount 

 of dust in spring than at any other time of the year being 

 the great annual westward motion of the whole atmo- 

 sphere, or at least of a considerable depth of it, at that time 

 of the year. In a recent paper on " The Winds of Ben 

 Nevis" (Trans. R.S.E., vol. xxxvi. p. 537), it has been 

 shown that this is one of the very few points in which the 

 high- and low-level winds agree, viz. in the excess of 

 easterly winds in spring. The above means for summer 

 are probably too low, as that summer was exceptionally 

 cold, and the general circulation was very abnormal, and 

 that in the direction which would tend to give low dust 

 values. The maximum, 14,400, was observed at i p.m. on 

 April II, 1891 ; and, as an instance of how very much 

 the values change in a short time, at 8 a.m. that morning 

 the number was only 350 per cubic centimetre, and by 

 midnight it had again fallen to 600 per cubic centimetre. 



The daily variation is fairly well indicated from the 

 three-hourly observations. For the months of March, 

 April, and May, 1891, the following are the means for the 

 eight hours of observation : — 



' Bracketed values are for 1890 only. 

 NO. I 1 73, VOL. 45] 



Here there is a minimum indicated (526) at 4 a.m., and 

 a maximum (1438) at 4 p.m. All the forenoon values are 

 below the mean, and the evening values above it. It 

 would appear that during the forenoon the summit of 

 Ben Nevis is above the first or lowest cloud or dust 

 stratum. About noon this stratum rises to the level of 

 the summit, and during the afternoon hovers above it, 

 and falls again late at night. From this it might be 

 inferred that the summit is oftener clear of cloud in the 

 early morning, and oftener enveloped in the afternoon. 

 A table showing the number of times the top was clear 

 during the last seven years shows that only about 30 per 

 cent, is clear weather in which the summit is free from 

 fog ; but it does not show a daily variation as indicated 

 by the dust values, what little it does show being quite 

 the reverse— namely, a maximum of clear weather in the 

 middle of the day and a minimum at night. This clearly 

 indicates that when the dust layer falls below the summit 

 at night, radiation at once forms an independent cap on 

 the hill-top ; and again in the afternoon, although the dust 

 stratum envelopes the summit, the opposite radiation 

 warms it up and prevents condensation, or rather evapo- 

 rates the watery particles of the cloud. So that, contrary 

 to public opinion, the best time to visit the summit for the 

 sake of the " view " is the middle of the day, and not the 

 early morning. During fine settled weather the rise and 

 fall of this cloud stratum can be followed, but in average 

 weather the effect of radiation completely masks it. The 

 effect of solar radiation and nocturnal radiation on dust, 

 as particles and as strata, is a problem that has to be 

 studied and worked out. Very little is definitely known 

 about it at present. 



In the study of the nature and motions of clouds the 

 dust observations will be of great value. When a fog 

 envelopes the summit, the number of dust particles 

 observed may vary greatly without any apparent change 

 in the thickness of the fog, but as a rule dry thick fog 

 contains a great amount of dust, while thin wet mist con- 

 tains very little. It is when a thin drizzling mist en- 

 velopes the summit that the lowest values are always 

 obtained, and then there is a distinct difference between 

 the conditions at sea-level and those at the summit, the 

 winds at the latter place differing in direction by 90° or 

 more from the winds below, and sometimes the upper 

 winds are blowing straight out from the centre of a shallow 

 low-pressure area, and the dust that rises with the slight 

 ascending currents of the lower strata is almost entirely 

 filtered out before reaching a height of 4400 feet. Une of 

 Mr. Aitken's conclusions may briefly be put as follows : 

 Much wind, little dust ; much dust, little wind. That 

 dust seems to accumulate in the quietest places is fully 

 borne out by the Ben Nevis observations. This is true 

 not only horizontally, but also vertically, and it seems 

 probable that this is what chiefly determines the position 

 of cloud strata at all heights. And from this we may 

 infer that the motion of clouds is much slower than that 

 of the general aerial currents ; and again, since clouds 

 tend to form between currents, and may have as direction 

 of motion the resultant of the directions of these currents, 

 it follows that as indices to the motions of the upper air 

 the velocity and motion of clouds are very unsatisfactory. 



Observations of the apparent haziness of the atmo- 

 sphere are made whenever it is possible, and the relations 

 between the haziness of the air, the humidity, and the 

 number of dust particles, have been found to be the same 

 as what Mr. Aitken pointed out. Briefly, he found that 

 with a constant humidity the haziness increased or 

 diminished with the number of dust particles, and with a 

 constant number of dust particles the haziness depended 

 on the humidity (at least when the air was within 10 or 

 15 per cent, of saturation) ; for with increase of humidity 

 the air became thicker, because apparently condensation 

 begins on the dust particles before the air reaches its 

 point of saturation. 



