Nov. 2 1, 1889] 



NATURE 



63 



particles of a fog ; a relatively small diffused quantity of 

 vaporous air in minute parcels could not produce by con- 

 densation any but extremely small and transitory water 

 particles, in the aggregate visible through long distances, 

 but probably individually beyond the power of the micro- 

 scope to discern. They may be compared to the blue 

 mist escaping from the safety-valve of a boiler under high 

 pressure : the invisible steam turns for a moment blue, 

 and then to the ordinary white of visible steam. The 

 haze may possibly be equally momentary in duration, dis- 

 solving long before reaching the white stage, but fresh fila- 

 ments are perpetually keeping up the process and giving 

 the appearance of a persistence like that of smoke or dust. 

 According to Espy, every cloud is either forming or dis- 

 solving (Buchan's " Handy Book of Meteorology," p. 175). 



The action of a north-east wind setting in over 

 England would be represented by a trough of water, say 

 2 feet square and 2 inches deep, containing warm water 

 flowing in one direction, while cold water enters from the 

 whole length of the opposite side. The cold water would 

 force its way under the warm, and the two opposite 

 currents would continue to flow ; but through friction and 

 diffusion there would be a great deal of mixture of 

 portions of the upper with the lower stream. 



A haze similar to that accompanying the east wind is 

 frequently seen where two currents of the same wind 

 meet at different temperatures, as at the junction of two 

 valleys, or at projecting headlands (Buchan's " Handy 

 Book of Meteorology," p. 171). It is also common with 

 a humid wind, otherwise clear, when it passes over ranges 

 of hill and valley of moderate elevation, owing probably 

 to the mixture of parcels of air of different temperatures 

 by alternate upward and downward thrusts. The thin 

 white mist which appears in gales from the south-west 

 on sunshiny days is probably due to the forcible and 

 rapid mixture of air warmed by the ground with colder 

 portions from a higher level, the deposition of minute 

 particles of dew being aided by the abnormal amount of 

 salt carried up from the sea in spray, and borne to great 

 distances inland. 



A very good instance of the powerful influence of the 

 mixture of two currents of air, not greatly differing in 

 temperature and other conditions, to produce haze oc- 

 curred on August 26, 1889, in southern Surrey. The wind 

 over a wide area, including the south of England, was 

 variable and gentle from west to north-west. At the 

 place of observation it had been about west-north-west 

 during the afternoon, and the views were fairly clear. 

 Cirro-cumulus, both at a moderate and at a great eleva- 

 tion, moved from north-west. At about 5.30 p.m. the 

 landscape was suddenly invested with haze, which, during 

 the following hour, was thick enough to obscure altoge- 

 ther hills about six miles off. Simultaneously the wind 

 dropped a good deal and shifted to north-west and north 

 for a short time, but soon backed, and the air again 

 became clear about 7.30. It would thus seem sufficient 

 that a reduction of temperature a little more than the 

 ordinary about the time of sunset should occur, in order 

 to precipitate visible moisture upon the dust-particles of 

 the air. Both the sensation and the appearance of the 

 sky resembled that during a disagreeable misty east wind, 

 and, just before the change, a very dark bank of cloud 

 appeared in the north, which, on passing over, was seen 

 to be more mist than a well-defined cloud stratum. It 

 seems not unlikely, judging from the experience of aero- 

 nauts, that in this case a current fVom north or north- 

 east was driven like a wedge into the general north-west 

 wind a few thousand feet or less above the ground. 



If the account of the formation of haze in an easterly 

 wind given in the foregoing pages be correct, there should 

 be a clearing of the atmosphere when either the east 

 wind extends itself to the upper regions or the westerly 

 wind succeeds in driving back its opponent out of the 

 lower space. In point of fact, the air does clear itself in 



either of these events. Moreover, a clearing away of 

 haze is a good indication of a strengthening of the polar 

 current or its expulsion by the equatorial ; other signs, 

 such as the motion of cirrus and the aspect of the clouds, 

 plainly informing us which of the two changes will occur. 

 (2) The second favourable state for the production of 

 haze was given as " fine settled weather, with variable 

 currents, a dry air, and little dew." This state prevails 

 often with anticyclones, and the movement of the air is 

 to a great extent vertical, an interchange taking place 

 between upper and lower strata. Consequently, there is 

 a great mixture of portions of air at different tempera- 

 tures, with a result like that already described. The 

 heterogeneous character of the lower atmosphere in a 

 horizontal direction declares itself by the poor transmis- 

 sion of sound. But a great deal remains to be explained 

 in the production of haze in these conditions. The cause 

 is probably the same as that which sometimes covers the 

 whole of the British Isles with a damp fog, extending 

 high into the atmosphere. This occurs when two winds 

 of a different character meet in such a manner as to 

 interdiffuse gradually over a wide area. But in the case 

 of haze, how can it endure when the general dryness 

 of the air is far above the point of saturation? Haze 

 sometimes continues in summer right through the day, 

 when the dry and wet bulbs show a difference of 12° to 15°. 

 It would seem as if our methods of estimating the dew- 

 point do not altogether hold for air in a certain condition 

 and for certain particles in it. Is it not possible that 

 condensation to a slight degree may occur upon some 

 minute crystalline particles, such as the salt-dust which 

 pervades our atmosphere, at temperatures above the dew- 

 point ? Such action would only be consistent with the 

 effect of crystals in hastening the boiling and congelation 

 of water. It is probable that, if means were available for 

 testing the temperature of successive minute portions or 

 strands of air passing over a thermometer, we should find 

 a great variation from one moment to another. A differ- 

 ence of 12° between the dry and wet bulbs may represent 

 a mean between much higher and much lower values ; 

 and on the driest days, when haze prevails, there may be 

 extremely minute portions with a temperature at the 

 dew-point — that is, containing more vapour than, at the 

 particular temperature to which it is a certain moment 

 exposed, can remain uncondensed. That volumes of 

 air at different temperatures take a long time to become 

 thoroughly incorporated, may be regarded as certain. 

 Threads of smoke in a still room often remain for many 

 minutes unbroken, and behave as if they were held toge- 

 ther by some cohesive force, and, generally, strains of air 

 or gas at widely differing temperatures, when mixed, tend 

 to hold together rather than to diffuse. Thus, small sur- 

 faces, of which the vapour-particles are at different tem- 

 peratures, are frequently in contact. When we consider 

 that different currents of air frequently prevail within a 

 a few thousand feet of the earth's surface, and that within 

 five miles a temperature of — 2° may exist early in Septem- 

 ber,^ it seems possible that, in so bad a conductor of heat as 

 air, temperature at different points on the same level may 

 vary greatly. On September i and 2, 1889, the condition 

 of the air was instructive with regard to the formation of 

 fog and haze. The night of August 31-September i was 

 fine, and radiation rapid, so that in the morning there 

 was a copious dew. From 6 to 8 a.m. there was thick 

 fog, which, as the sun's power increased, lightened and 

 lifted, but the sun did not finally break through till past 

 II. The wind was fresh from north-east. A thin blue 

 haze remained after the fog had dissipated, and did not 

 altogether disappear during the day. The air was not 

 damp, even before the fog had lifted, though there was a 

 very slight drizzle about 9 a.m. On September 2 the 

 night had been very fine and clear, but. in the morning 



» See " Travels in the Air," Glaisher's ascent of September s, 1862. 



