64 



NA TURE 



[Nov. 2 1, 1889 



a thick wet fog, with fresh north-east wind, prevailed. 

 This fog cleared, and the sun shone through, about 9 a.m. 

 A mist, however, remained much later. Now, in these 

 cases, the fog was due to the cooling of the earth by 

 radiation (for it did not appear till after midnight) and to 

 the cool north-east wind co-existing with higher currents 

 from a different quarter.^ The persistence of the haze 

 much beyond the fog reveals the difference between a 

 general saturation and what might be termed molecular 

 saturation. The fog breaks, decreases rapidly, and has 

 gone when the last few shreds of clouds lifted from the 

 earth vanish in the blue, but the haze looks unchanging 

 and uniform over the country. When we see volumes of 

 vaporous air separated, without any apparent reason, into 

 dense clouds and clear intervals, e.g. cumulus in a blue 

 sky, it becomes easy to understand that very small micro- 

 scopic clouds, in which condensation is only mom^entary, 

 may permeate air otherwise far from saturation. 



It would hardly be reasonable to exclude electricity as 

 a possible agent in the otherwise not wholly accountable 

 phenomena of mist and cloud. It may be that the dust- 

 particles of two currents of air differing in electric quality 

 or quantity may be attracted to each other, or that the 

 mixture of currents of different temperature may in some 

 way set up molecular aggregations. 



Whatever the cause, we should bear in mind the small 

 quantity of non-transparent matter required to produce 

 the dimming effect of haze. If the eye can observe the 

 colour produced in a drop of water by the fifty-millionth 

 of a gramme of fuchsine, possibly a weight of water or 

 dust not much greater would suffice for visibility in a 

 column of air 1000 feet long. The atmosphere is at all 

 times charged with dust-particles to a degree which it is 

 difficult to realize. The purest air tested by Mr. Aitken 

 previous to his measurements on the top of Ben Nevis, 

 contained about 34,000 dust-particles to the cubic inch — 

 this was on the Ayrshire coast. In every cubic foot there 

 would be 35,232,000 particles, and, in a horizontal column 

 of 1000 feet, 35,232,000,000 particles. It is manifest that 

 a condensation upon a small proportion of these, or an 

 agglomeration of a small proportion into larger groups, or 

 a momentary adhesion by electric attraction, would suffice 

 to produce optical effects. 



The evidence concerning the appearance of haze by 

 irregular transmission of light due to unequally heated 

 currents of transparent air seems to be quite insufficient, 

 and however great the heat near the surface of the 

 ground, say in the desert, with consequent distortion 

 of images, it does not, as a rule, bring about the haze so 

 common in temperate climates. 



Haze of an abnormal kind need barely be mentioned 

 here — namely, that due to smoke, palpable dust, and the 

 products of volcanoes. It may, however, be very widely 

 spread and very dense. In 1783 Europe was for months 

 covered by the dust ejected by an Icelandic volcano, and 

 the Atlantic for 900 miles west of the north-west coast of 

 Africa is every year subject to a haze composed of fine 

 particles of sand from the Great Desert. 



(3) Opposition of currents, such as takes place when 

 several shallow barometric depressions pass over the 

 country, results in mixture of differing air, partial con- 

 densation, sultriness, haziness, and frequently thunder- 

 storms. Not at all improbably, the differing electric 

 conditions of two winds, the rapid condensation of 

 vapour, and the projection of highly vaporous air to a 

 great height, accelerate the growth of water-particles, 

 until they fall to the earth in large drops. The saying 

 that thunderstorms advance against the wind is merely a 

 way of asserting that two winds are adjacent, one above 

 the other, and that the clouds move in the upper current. 

 The haze preceding thunderstorms announces beforehand 



• "On Saturday evening, August 3', a balloon, a-; it ascended, (-rossed 

 and recrossed Lute n stveral times." — Daily News, S«p'ember 2, 1S89. 



the contention which is going on, and the conglomeration 

 of dust or water particles by electric attraction or rapid 

 cooling. 



(4) Damp weather with light winds and varying tem- 

 perature, as thaw after frost, with snow on the ground. 

 The cause of haze in this condition is obviously the con- 

 tact of warm moist air with air cooled by contact with, 

 and by radiation towards, the ground. In this case, 

 again, it is mixture of portions of air of different tem- 

 peratures which produces partial condensation and haze. 

 It must be remembered that the air is always charged 

 with an immense quantity of fine dust, such as particles 

 of salt,^ that these are capable of radiating, and that 

 when they fall i° or 2^ below the temperature of the air, 

 moisture may be deposited upon them sufficiently to 

 become visible. In the case supposed, of an equatorial 

 current supervening after frost and snow, the mist pro- 

 duced by mixture of parcels of air at different temperatures 

 will be thin and blue if the filaments in which saturation 

 and deposition occur are very small in proportion to the 

 surrounding unsaturated air, and white if the proportion 

 of saturated air is large. For the blue mist or haze 

 indicates deposition in very minute clusters of water- 

 molecules, and instant reversion to the invisible state by 

 the contact of unsaturated air, while the white mist is the 

 result of condensation in much larger quantities in air 

 on the whole very near or at the point of saturation. 



Consider next the conditions of weather in which the 

 air is most transparent. 



(i) A state of great humidity, such as that which occurs 

 often before bad weather, the wind being between south 

 and west. What does this clearness signify, according to 

 the views of the causation of haze above detailed ? Chiefly 

 that the air up to a great height is fairly homogeneous — 

 that is, of the same kind and quality as regards moisture, 

 electricity, and temperature, with due allowance for the 

 normal changes depending on altitude. The humidity is 

 not owing to this homogeneity, but often accompanies it, 

 simply because the south-west and westerly winds have 

 passed over a large extent of ocean. In fact the air 

 throughout has been subjected to the same influences, 

 and nothing has occurred to disturb its uniformity, so 

 that It can for some considerable time carry a large 

 amount of aqueous vapour without precipitation. When 

 precipitation does occur, it is usually by the thrusting up- 

 wards of the warmer strata into cold upper strata, and 

 then condensation proceeds without check and rapidly 

 from invisible particles to rain-drops. Thus, on reaching 

 the first mountainous region, or in passing over land 

 heated to a temperature much above that of the sea 

 surface, the ascent of the most humid strata into the cold 

 upper air is often followed by rain. The remarkable 

 transparency before rain signifies a correspondence in 

 direction as well as in qualities between the upper and 

 lower strata. If the wind be between west and south, as 

 it usually is in these cases, we are informed of a similar 

 wind at a high level— that is, that the upper current, as 

 well as the lower, is more than commonly humid, and its 

 vapour tending to condense by passing towards higher 

 latitudes. It only requires slight disturbances in a 

 vertical direction to precipitate the abundant vapour, and 

 hence the frequency of showers, especially where large 

 columns of heated air rise from the land, at a distance 

 from the south coast, and in hilly country. The south- 

 westerly wind being a warm one, is more likely to ascend 

 and to have its vapour condensed to rain than a colder 

 current. The clear lower air indeed owes its clearness 

 partly to its ascending movement. 



(2) Strong winds and showery weather. Strong winds 

 usually prevail when the air up to a great height 

 partakes more or less of the same movement. There is 



' Salt is shown to bs present everywhere in the atmosphere by the 

 spectrum of a flame. 



