Nov. 2 1, 1889] 



NATURE 



61 



continued, and in a few minutes the tops of the hills, 

 which rise precipitately from Strathearn to a height of 

 about 2000 feet, were obscured with cloud-banners grow- 

 ing continuously and descending till in about two hours 

 not only the hills above a level of about 1000 feet, but the 

 whole sky, were covered with gray cloud. The duration 

 of the neutral calm, from two to four minutes, seems to 

 be about the usual time occupied by a moderate east wind 

 in driving back the opposing current, according to my 

 observations in the neighbourhood of London. In the 

 suburbs south-west of London such a change is signalized 

 in the neutral band of calm by a dense yellow haze, pro- 

 ducing great darkness, the result of a banking up of 

 smoke to some altitude, together with the condensation 

 of aqueous vapour by the mixture of currents differing in 

 temperature. With lighter winds about equal to each 

 other in momentum, such a band often lasts much longer, 

 and I have known a west wind prevail at Richmond 

 simultaneously with an east wind m London, both with- 

 out fog, while at Wandsworth, between the two, a calm 

 continued for many minutes, with dense, almost noc- 

 turnally-black, smoke-fog, the pressure in each direction 

 being apparently equal. Generally speaking, the mist 

 thus produced at the junction of the two winds is exceed- 

 ingly dense in winter, moderately dense in spring and 

 autumn, and thinnest in summer, varying, in fact, from a 

 black fog in the cold season to a mere haze in the warmest 

 weather. Hence we have an ascertained condition for 

 the production of haze— the mixture of two opposite 

 winds. It may be here remarked that a very sudden 

 squall of wind from the north, displacing an equatorial 

 or south-westerly current, produces a somewhat similar 

 dense wall of mist, which it soon drives away before it. 



Haze very frequently prevails during a north-east or 

 east wind in all parts of Great Britain ; in the east of 

 Scotland it is, perhaps, more marked than in other locali- 

 ties, and attends both wet and dry weather. A dense 

 blue mist or haze brought by the east wind sometimes 

 invests the landscape for days before a continuous down- 

 pour from that quarter. This haze extends far out to 

 sea eastwards. The southern parts of England are less 

 troubled than the northern by this disagreeable infliction, 

 and the northern parts of France less still. In the east- 

 ern counties, and probably in other parts of England, 

 the density of the haze seems to increase in some pro- 

 portion to the dryness of the air, when only a slight wind 

 blows. On thoroughly rainy days, such as the north-east 

 wind sometimes brings to the London district, the amount 

 of haze is below the average ; and when the north-east 

 wind is accompanied by snow-showers, as it often is in 

 February and March, or by ram-showers later in the 

 year, it is remarkably and conspicuously clear. I cannot 

 remember any showery days with a steady north-east 

 wind showing a true haze, beyond the influence of Lon- 

 don, but have often observed the extraordinary clearness 

 •of such days, and the apparently dissipative action of the 

 air on London smoke. 



Generally, the density of the haze is less as the strength 

 of the wind increases. A gale from the north-east is 

 seldom accompanied by much haze inland, although on 

 the east coast the combination is not uncommon. Haze 

 appears to diminish as the north-east wind grows more 

 •established, and in winter a long period of this wind may 

 be experienced without the continuance of haze. It is 

 also important to observe that, when high upper clouds 

 are seen to be moving from a direction between east and 

 north inclusive, but especially from north-east, the air is 

 usually clear, and a long continuance of the polar wind 

 may be expected. It is a sign of the firm establishment 

 of the north-east wind when high cirro-cumulus is seen 

 passing over from that direction, whatever deviations 

 may take place temporarily on the earth's surface. The 

 extension of the north-east wind to a great altitude seems 

 to deprive it of its accustomed haziness. When, on the 



other hand, thick haze accompanies the north-east wind, 

 if upper clouds are in view, they are generally seen to be 

 borne by a different current, and in winter the lower wind 

 does not, in such conditions, often remain long in the 

 same quarter. Hence we have the means of making 

 forecasts with tolerable safety as follows ; — 



(i) If the lower air be clear, whether clouds at a high 

 level be seen to move from the north-east or none be 

 visible, the lower wind from north-east will probably last 

 some days, perhaps some weeks. 



(2) If the lower air be very thick and misty, the north- 

 east wind is not strongly established, and is likely soon 

 to be succeeded either by variable airs and calms, or by 

 breezes from a different quarter. 



In spring and summer, haze prevails sometimes for 



many days together, with a dry atmosphere, over the 



whole or a large part of Great Britain. The wind is 



either easterly or variable, the barometer high, t'impera- 



ture high by day and low by night, and the deposition of 



dew either small or heavy. The haze seems to be uni- 



i formly distributed through the atmosphere, and varies 



\ neither from one day to another, nor from day to night. 



j The sky is pale blue, the sun rises and sets red and ray- 



! less, and the moonlight reveals the blue mist unchanged 



by the absence of the sun's rays. 



Haze has been known to affect a great part of Europe 

 during a period corresponding with the prevalence of 

 drought. 



The formation of haze seems to be more common and 

 more sudden in mountainous regions than on the plain. 

 I had once an opportunity of observing the rapid pro- 

 duction of a very dense haze from the top of Cader idris, 

 in Wales. The morning was bright, fine, and clear, but 

 the heat very oppressive. About midday, signs were seen 

 of an approaching thunderstorm, which, however, spent 

 its force at some distance down the valley. Before the 

 storm, a haze quickly gathered, and completely obscured 

 even the nearer ranges. This haze resembled that which 

 prevails sometimes during many hours before the occur- 

 rence of a thunderstorm in the level country. 



The conditions favourable to the production of haze 

 may be conveniently summed up as follows : — 



(i) A gentle wind from east-south-east to north-east 

 inclusive, and east wind in general, especially with dry 

 weather in spring and summer. If the east wind be 

 established up to a great height, the lower air is usually 

 clear, but if the upper current is from a westerly direc- 

 tion, haze prevails. 



(2) Fine settled weather, with variable currents, a dry 

 air, and little dew. 



(3) Opposition of currents — such as occurs when several 

 shallow barometric depressions exist over the country — 

 and the atmospheric state preceding thunderstorms. 



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

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



Turning to those conditions which are most unfavour- 

 able to the production of haze, or in which the air is most 

 transparent, we find them to be — 



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

 occurs often before bad weather, the wind being between 

 south and west. 



(2) Strong winds and showery weather. 



(3) Winds between south-west and north. 



(4) Fine settled summer weather, with westerly or 

 southerly winds. 



(5) Settled easterly or northerly winds, with either clear 

 sky, or high clouds moving from those directions. 



(6) Easterly or northerly winds, with a high continuous 

 cloud canopy moving in the same direction, small range 

 of temperature, and steady conditions ; or, with detached 

 cumulus in the daytime, and clear nights. 



(7) North-west following a wind between north-west 

 and south is particularly clear, except in thundery 

 weather. 



