ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 227 



vapor at a higher temperature than the average, then it is conceivable 

 that momentary deposition and reevaporation may occur. The result 

 would be haze. With fairly homogeneous masses of air, as with a 

 west wind, the contact of warm and cool air occurs here and there on 

 a much larger scale and at once produces massive clouds, owing to the 

 quick growth of particles in a moist air brought in block below the 

 dew-point by ascent or otherwise. The interchange between differing 

 air masses is in this case by large columns instead of by infiltration 

 and filaments. The steam leaving the escape valve of a boiler at high 

 pressure is at first invisible, then bluish and semitransparent, like haze, 

 then opaque and white, like cloud. The influences which cause haze 

 maintain the vapor in the second stage; it passes perpetually from 

 molecular invisibility to the verge of particulate visibility and back to 

 invisibility by swift evaporation. Clouds, on the contrary, result from 

 cooling in large masses, as by ascent, and the humidity is too great to 

 permit so rapid a return to the condition of vapor within their borders. 

 When they evaporate they become invisible at the edge without per- 

 ceptibly passing through the stage of haze. 



Why the jirocess f change of size of the particles differs so much 

 in different states of weather is by no means clear. 



Haze has long been a meteorological problem. If it be vapor, why 

 does it so frequently occur in the driest weather? If it be dust, 

 why should dust continue to affect the atmosphere in such excessive 

 quantities during particular periods, often in calm weather, and with a 

 gentle wind from uninhabited areas, either sea or land? The moistest 

 winds are generally the clearest, the driest are the haziest. 1 More- 

 over, there is a thick haze which sometimes persists for many days in 

 spring or summer in England, and neither increases nor diminishes per- 

 ceptibly during the night, when radiation is active. In such weather 

 the air is dry, and the wind, if any, commonly a light air from between 

 east and north. Since neither the sun's heat nor the nocturnal cold 

 affects it, we must ascribe it to one of two things — the presence of a 

 large quantity of dry dust in an unusual state, or the development of 

 vapor condensation in some unusual way, so as to depend little on the 

 general temperature. On the top of Snowdon, 3,300 feet, the present 

 writer has observed haze as thick as on the ground level, and extend- 

 ing 1,000 or 2,000 feet above the summit. It was similar, though less 

 in degree, to the obscuration described in the annals of last century 

 as having covered Europe for months after the great eruption of a 

 volcano in Iceland in 1783. Mr. Conway has recently observed high 

 above the Himalayas a sudden haze overspreading the sky like the 

 smoky haze seen near a large city in England. The explanation prob- 

 ably is that the haze depends on the relative temperature of mixed 

 portions of strata of air, and much less on the general air temperature. 



Aitkeu has shown that when the wind blows from inhabited places 



1 In England. 



