228 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



there is both more haze and more dust than when it blows from the sea 

 or from uninhabited country, aud in Switzerland a thick veil of haze 

 seemed to hang in the air between the observer and the mountains 

 on all days when the number of particles was great, and it became 

 very faint when the number was small. When the wind blew from 

 the plains the air was thick ; when from the Alps, clear. Similarly, at 

 Ben Nevis, on the northwest coast of Scotland, a northwest wind was 

 clearest, a southeast wind haziest, and the dust particles were gen- 

 erally more numerous according to the amount of haze. " Of ' purify- 

 ing areas' the Mediterranean gave for lowest values 891, the Alps 381, 

 the Highlands 141, and the Atlantic 72 particles per cubic centimeter. 

 Dampness of the air was found to increase the effect of dust, so" that 

 nearly double the number of particles are required to produce the same 

 amount of haze when it is dry than when it is dampish." When the 

 depression of the wet-bulb thermometer below the dry bulb was 2° or 

 more the transparency was roughly proportional to the wet bulb 

 depression) that is, to the dryness of the air. "The nearness of the 

 vapor to the dew-point seems to enable the dust particles to condense 

 more vapor by surface attraction and otherwise, and thus by becoming 

 larger they have a greater hazing effect." The number of dust parti- 

 cles in square centimeter lengths of 10 to 250 miles required to pro- 

 duce complete haze in air giving different wet-bulb depressions was 

 calculated to be as follows : 



Wet-bulb 

 depression. 



Number of parti- 

 cles to produce 

 complete haze. 



Degrees. 

 2 to 4 



4 to 7 



7 to 10 



12, 500, 000, 000 

 17, 100, 000, 000 

 22, 600, 000, 000 



Since more particles are required to produce haze in dry than in 

 damp weather, it becomes the more remarkable that thick haze is so 

 common in dry weather and generally absent in a moist atmosphere. 



The observations of the present writer for many years have shown 

 that haze is most apt to occur when there is infiltration or mixture of 

 differing air currents, and indeed that it generally expresses the juxta- 

 position and mixture of winds. A steady wind extending to the 

 upper clouds is very seldom hazy, and, on the other hand, haziness may 

 be taken as a sign of the existence of another wind above that pre- 

 vailing near the ground, or of variable currents. So much is this the 

 case that in southern England a hazy or misty east wind signifies gen- 

 erally a rather short period of its prevalence, but a clear east wind 

 means continuance. Of course care must be taken to be situated on 

 the windward side of thickly inhabited districts in making such fore- 

 casts. It seems, therefore, that when haze is not due to a large amount 



