December 



D» 



1923] 



NA TURE 



86 



plotted, the distribution over the 24 hours is similar 

 but more uniform, and it can even be seen that the 

 maximum is reached later on Sundays than on week- 

 days (Eighth Report Advis. Comm. on Atmos. Pollu- 

 tion, p. 30). 



Records taken in Westminster by my impact appar- 

 atus at the height of the recent fogs showed about 

 50,000 solid particles per cubic centimetre, and very 

 little indication of water drops. These particles 

 averaged a little more than 0-5 micron in diameter, and 

 there was a tendency to a rounded, kidney shape, 

 with a fair number of small transparent spheres of 

 ;ibout the same diameter. 



With reference to the cause of the " London Par- 

 ticular," the view is generally held that a London fog 

 consists of condensed water particles dirtied by smoke 

 and oily hydrocarbons (Carpenter, " London Fog 

 Inquiry," 1901-2), and this view follows naturally from 

 the fact that during London smoke fogs there is very 

 often a dense water fog in the surrounding country. 

 There are, however, certain indications which point to 

 a different conclusion and suggest that a London 

 smoke fog, such as we have experienced recently, 

 'ccurs at the same time as the surrounding water fog, 

 not because it is a result of the latter, but because of the 

 conditions which favour water fog. I have, therefore, 

 been forced towards the view that the smoke fog of 

 London does not consist of water particles dirtied by 

 smoke, but almost entirely of smoke particles alone. 

 In support of this are the following facts : 



(a) During the recent fogs when the sun was 

 visible at all it appeared as a red ball, thus pointing 

 towards the presence of finely divided suspended 

 matter, and not towards large water globules. 



(6) Records taken by my impact method during 

 London smoke fogs show little evidence of water drops, 

 which, if present, would be obvious ; but there are 

 always immense numbers of small smoke particles. 



(c) It is not unusual in coming up to. London from 

 the coimtry during foggy weather to find a dense 

 white fog in the country, with a limit of visibihty of 

 perhaps 50 yards, giving place to a yellow fog in 

 London with a greater hmit of visibility ; thus, 

 while a large amount of smoke can always be detected, 

 there must be a great reduction in the quantity of 

 water in the London fog, since in spite of the smoke 

 addition visibility improves. 



{d) The air over London is warmer than in the 

 country surrounding, and although the combustion of 

 large quantities of fuel supplies a certain amount of 

 water to the air, it seems probable that condensation 

 of water in London to form fog would normally be 

 much less than in the surrounding country. There 

 are about 17,000,000 tons of coal burnt per annum in 

 London, and assuming a wind of 2 miles per hour and 

 an inversion of the lapse rate of temperature at about 

 400 feet, a condition likely to be met with during 

 foggy weather, this amount of coal would keep the 

 air immediately over London about 13° F. warmer 

 than in the surrounding country. 



(e) The hourly incidence of suspended impurity, as 

 shown in the curves, varied exactly with the darkness 

 and apparent density of the fogs as judged by the 

 eye. 



Doubtless in the early morning the conditions which 

 cause a water fog in the country also cause a similar 

 fog in London, if not so dense ; but as the day ad- 

 vances the smoky fires add their soot and heat to the 

 air, the latter evaporating the water and the former 

 replacing it by soot. 



Fires are notoriously Smoky shortly after lighting 

 until they get well heated up, and we find, as a rule, 

 that the smoky London fog commences in tlie morn- 

 ing about the time of fire lighting and dies away 



NO. 2824, VOL. r 12] 



gradually as the fires become well established ; while 

 it is usually at a minimum between midnight and 

 early morning. If the natural ventilation over the 

 city fails to carry away the smoke produced, there is 

 sufficient evolved in the morning in three or four 

 hours to provide Londoners with the densest smoke 

 fog they have ever experienced. Such a fog contains 

 5 or 6 milligrams of soot per cubic metre, and this 

 can be easily supplied by the 40 or 50 tons of soot 

 evolved per hour by the chimneys of London. 



Assuming the correctness of the above, we may 

 draw the following inferences : 



(i) The air over London being warmer than its 

 surroundings, water fogs will be fewer or less 

 dense during the day than in the country. 

 (2) The " London Particular " can be entirely 

 prevented bv abolishing smoke. 



J. S. Owens. 

 47 Victoria Street, Westminster, S.W.i, 

 December 4. 



Upper Air Conditions after a Line- Squall. 



Meteorologists are indebted to Wing-Commander 

 L. W. B. Rees for some very valuable observa- 

 tions during two aeroplane ascents at Cranwell (Lines.) 

 on October 19, showing the change which took place 

 in upper-air conditions during the passage of a " line- 

 squall " or, in the phraseology of Bjerknes.-a " cold 

 front." Observations of this kind are sufficiently 

 rare, and the features exhibited by the present ones 

 are so specially interesting that it seems very desir- 

 able to lay some emphasis on them. 



The accompanying diagram (Fig. i) shows the 

 details of the records. The first ascent (dotted line) 

 commenced at 9.15 a.m., and was made in front of 

 the line-squall in the " equatorial air," the origin of 

 which has now been traced back, on the charts in 

 the Meteorological Office, to a low latitude. The 

 figures against the curve show the relative humidity 

 at various levels, the corresponding number of 

 grams of water vapour per kilogram of air being 

 given in brackets. It is necessary to remark that 

 the report of cloud and rain encountered is not 

 inconsistent with the apparently unsaturated con- 

 dition of the air, for the cloud was not necessarily 

 continuous throughout the thickness indicated, and, 

 further, the instruments were read on the way up 

 when the clouds were only just commencing to 

 thicken and form rain. No rain reached the surface 

 until 10 A.M. The line-squall occurred at 10.5 a.m., 

 marking the arrival at the surface at Cranwell of the 

 " polar air " which has been traced back to the 

 neighbourhood of Greenland. The usual line-squall 

 features were recorded — heavy rain, sudden veer of 

 wind from S.S.W. to W., and sudden fall of tempera- 

 ture of 9° F. All the rain, 2-1 mm., occurred between 

 10 A.M. and noon, and presumably fell through the 

 undercutting wedge of colder air from the " equatorial 

 air " above. The second ascent (full line) commenced 

 at 2.15 P.M. in response to a request by wireless 

 telegraphy from the Forecast Division of the ]Meteoro- 

 logical Office, Air Ministry, where the first record had 

 been received and the weather charts showed how 

 valuable another would be. 



The second record shows that the aeroplane left 

 the ground in the " polar air " and penetrated the 

 " equatorial air " above, but what is particularly 

 striking is the extreme dryness of the intermediate 

 layer extending from about 5000 ft. to 9000 ft. When 

 a sharp discontinuity is absent between two different 

 air currents the effect is usually attributed to mixing 

 at the interface, but the present intermediate layer 

 some 4000 ft. thick cannot possibly have been a 



