786 The Outline of Science 



The Cause of Mists and Fogs 



When warm air blows over cold sea the air becomes chilled in 

 contact with the cold water ; the water-vapour in the air then con- 

 denses out as fog, which is thus really a layer of cloud resting 

 on the earth's surface. In somewhat similar manner, the rapid 

 cooling of the land on a clear night gives rise to valley-mist and 

 dew. In winter-time, when condensation takes place below the 

 freezing-point, we have falls of sleet or snow in place of rain, 

 and hoar-frost instead of dew forms on the ground, while a warm 

 wind setting in suddenly after a hard frost may cover trees and 

 walls with a glassy coating of ice. 



Large hailstones, if examined, often prove to be built up of 

 several distinct layers of ice, due to the fact that they are carried 

 up and down many times by the powerful vertical currents of air 

 in the cloud from which they fall, and by this process they receive 

 successive additions of water which become frozen when the hail- 

 stone again reaches the higher regions of the cloud. 



The atmosphere everywhere contains dust in suspension. 

 Sand blown from the desert, salt from the ocean spray, soot from 

 factory chimneys, ashes hurled from volcanoes, the debris of 

 meteorites, and pollen from the flowers all contribute to form the 

 veil that lies between us and the sun. Were it not for these dust 

 particles in the atmosphere we should never see those gorgeous 

 colour effects that delight the eye. Our azure skies would be 

 almost as black as they are at night, and we should see the sun 

 and stars shining with undimmed lustre; for it is really the dust 

 that produces the lovely limpid blue of the sky. The floating 

 dust particles are exceedingly minute, especially those found in 

 the upper air; they scatter the blue light-rays from the sun, and 

 thus cause the sky to assume the colour that it does. 



The Colour in the Clouds; the Aurora 



The soft shimmering light of twilight is merely the lingering 

 illumination by the sun of the dust particles in the higher regions 



