FOG DISPERSION

Evening Post, Volume CV, Issue 146, 22 June 1928, Page 3

FOG DISPERSION

SCIENTIFIC RESEARCHPROBLEMS OF THE AIR LABOKATOEY TESTS :A paper under what must surely be tho shortest title'on record, "Jog," was read by Captain IT. .- Entwistlc, B.Sc., before the R.Ae.S. anftLAe.E. oa Bth December, says "Flight:" Tho subject of fog dispersion"is'onb which»'■■ has received a considerable amount of attention, in'the past, and.'-'aiso one, about which three has been much speculation; In a laboratory it isa simple matter to manufacture- or- disperse, a fog artificially, but. the step ■-'from a laboratory, experiment '.'to .'a* .practical commercial process is a very,large one. One of the difficulties attending the •operation of any method,of fog dissipation on a large scale is.that in a fog the air is never still, and muchi more energy would be required to clear a given area' ' than would bo required if the'fog were • stationary and there .were/only a de-1 finite volume with'which'tb: deal. , From a consideration-, of the conditions governing the formation of fog, the possible methods of dispersion that suggest themselves are:— r- . (1) By artificially warming .'.the air over the area to; be cleared sa that the fog.is "evaporated,\ or ,".,..:: '.. (2) By forcing in-froin above the fog, either by propulsion or suction, sufficient dry air to' clear/the area offog. , -.. There is-also the possibility of using an, electrical'mothod. in which the air' is subjected to-a very, strong-electric field. As long ago as 1884 Sir Oliver Lodge attempted to dissipate; fogs at Liverpool by means of powerful high-, tension discharges, and successfully dissipated a fog throughout a^ space of. a few yards, from the discharge electrode. An attempt lias also been made in America, to dissipates cloud -by sprinkling on it from an airplane sand charged to > 14,000 volts. It is, claimed that by this method a. single airplane was able to. dissipate in a few minutes a cloud a mile or two long and 1000, feet thick, - using about 1001b of charged sand. No further details are available, so that it is difficult to form any definite opinion as to this method. , '■■■ ... All three methods—thermal, mechani- , cal, and electrical—have been considered in Great Britain, and the conclusion reached has been that while each method is capable of clearing a,limited volume of air, no one method is -applicable to the large scale ' problem. la connection with the thermal method, Sir Napier Shaw has calculated " that, for, an aerodrome 400 yards wide with . a thickness of 50ft of fog, and a cross ' drift of about two miles an hour, a consumption of 13 tons of coal aii hour would be necessary, theoretically, in. order to warm the air by 5 degrees Fahrenheit. ;In practice at least four or five times as much fuel would- probably .be required.' In connection with the: mechanical clearing of an aerodrome, it has been computed that the clearing of a fog '80ft ' thick" fronu a limited area, by drawing the" air .downwards through, suitable gratings into subterranean tunnels that would be drained mechanically, would involve a downward current at the surface of the aero? drome of approximately 15ft per. second. With regard to the electrical method of dispersing fog, it has been shown that air known methods for discharging large quantities of ■ electricity, into the air , fail on account of, the, smali volume of air affected. „ . „,,.;,.. „,';•;;' ■ ■ If it must be admitted that-the problem of the dissipation'of. fog does not appear capable of an early solution, the question'arises as to whether penetration "of a fog to provide-adequate illumination can be effected. This - question is of practical.importance from the point of view of enabling the pilot of an aircraft to land in fog. It has already received considerable attention in England/and»as the result of certain tests ,of different methods of illumination through a fog, a Neon bea- . con has been installed at Croydon .aerodrome for use in foggy weather. The problem is one of optics, and un- x fortunately there is very little experimental data at presenffrbm which a'reliable opinion can be formed as to tho probability-of an ultimate practical solution.' One point which, it seems certain, will have to be taken into account, is the fact that smoke fogs and water fogs behave differently in regard to the transmission of light of different wave-length's. This fact is simply illustrated by the well-khOwn phenomenon that the sun appears when seen through a London- fog, .but appears / white when seen through a-country fog.. V The explanation is to be found in the sizes' of,'the particles of which smoke fogs and* watpr fogs are bbmposed. The former are of the order of 0.5 microns, , which is comparable with-, tho wavelength of light, while-the latter are of the order of 10 microns. :., The smoke particles accordingly scatter only the ; shorter wave-lengths, the longer red waves being transmitted, while the larger water particles scatter all colours irrespective ■of their wave-length. Attention has ;beon drawn recently to the possibility of using infra-red light ; for fog penetration. Such .experimental, evidence as is available points to the fact that water fog is not' transparent to the wave-lengths of the near infrared portion of the spectrum. . Laboratory experiments on the transmission of light of various wave-lengths .through •-. , an artificial'fog;'carried out by Utterback, showed that.the transmission was granted for. wave-lengths .between 0.53 microns and 0.5 microns,.and.that it.decreased, rapidly towards the red but not so rapidly towards the violent end. Experiments have also been made in Germany in which a spectrum was projected through a box filled with steam condensed to fog by, a cold- water jacket, and the greatest absorption was-.- found at the red end of the spectrum. The available evidence would seem to point to the fact that it may be possible to use infra-red light for the successful penetration of certain- smoke fogs, but that it is doubtful whether it -Would be possible to use rays of this wave-length for penetrating water fogs. Before either question can be settled, however, more decisive experimental evidence is required, apart from, any question of evolving a suitable source of radiation and the necessary receiving apparatus for rendering visible tho < long wave-lengths that would be involved. '-. '•. .■'....