236 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



resembles that which produces haze in summer; a slow infiltration of 

 currents of different temperatures brings different laminae into contact. 

 A cold earth and a sky clear above the low clouds increase the inten- 

 sity of such a fog, but are not necessary to its existence. A southerly 

 wind is too warm to produce fog by itself unless it meets with a cold 

 surface, and a northerly wind is too dry by itself to be reduced below 

 the dew-point. When, however, two opposite curreats, one of which is 

 colder than the other, diffuse into each other slowly, as when the colder 

 current over an extensive area sinks into the warmer current below it, 

 a fog may be produced which is less thick than a radiation fog, but 

 may continue with little change through several days and nights, and 

 commonly declares its character by the height to which it extends 

 and by its moisture. It deposits much more moisture on trees, etc., 

 than most radiation fogs, and, though no visible mist or rain may fall, 

 the ground under trees often becomes very wet. Thus precipitation of 

 moisture is increased in forests. In cold climates or at high levels 

 every exposed object accumulates ice. A wet or mixture fog disap- 

 pears under cover, and is thinner in large towns than in the country, 

 for the particles of which it is composed are almost pure water and 

 evaporate when the air is a little raised in temperature. On moun- 

 tains in Great Britain wet fogs are very common, and may occur with 

 strong wind; moisture or ice is deposited on the windward side of all 

 objects. Continuous damp mist may be produced in Great Britain by 

 a northeast wind blowing beneath a damp southwest or south current, 

 and such mists produce very disagreeable weather. In September and 

 the first half of October, 1894, southern England was immersed for 

 weeks in a mist so produced. The northeast wind was not of very 

 distant origin, and, not being dry, its mixture with the very damp 

 southerly current overlying it produced dense mist, cloud, and occa- 

 sional rain. 



Many fogs, such as those over rivers or valleys, and over the cold 

 ocean current near the Bank of Newfoundland, are due partly to mix 

 ture and partly to radiation. The sea fog originates in the cooling of 

 air by contact with the colder surface of water and by mixture with 

 the cold air which lies near the water. At many coast places on a hot 

 summer day a sea fog frequently comes up on a cool breeze which 

 mixes with the warm air above it from the land. On the other hand, 

 when a sheet of water is much warmer than the air above it, a thick 

 mist or fog may be formed, which is largely condensed steam. 



Fog is less common in summer in the interior of continents or of large 

 islands than on the coast, but in winter, owing to the greater loss of 

 heat by the surface of the earth than by the surface of the sea, fog is 

 more common inland. In many countries in the temperate zone the 

 stratum of cloud or fog does not lie often upon the ground, but at a 

 height of hundreds or thousands of feet; the sky remains quiet and 

 overcast for days and weeks together. The elevation of the cloud, 



