ii4 The Realm of Nature CHAP. 



of water. The minute globules formed in this way fall 

 very slowly, and in the absence of wind may remain sus- 

 pended in the air for a long time. This accounts for the 

 black winter fogs of great cities where the specks of soot 

 are very numerous and are only thinly coated with water. 

 Over the open sea, when a broad stream of warm air carrying 

 saturated water-vapour crosses a cold current of water or 

 meets an iceberg, the sudden cooling of the vapour necessi- 

 tates an enormous condensation, and the dust which is 

 abundant even far from land, enables the condensation to 

 take place in the form of a bank of mist. The famous 

 " fogs " of Newfoundland are so produced. Fog differs 

 from mist in not wetting solid objects with which it 

 comes in contact. The light mists formed at night 

 over low - lying meadows or valleys are usually very 

 thin sheets, and as soon as the Sun appears the water 

 particles are heated up and evaporated again, so that the 

 mist clears quickly away. When a mass of warm air rises 

 in the atmosphere its temperature falls ( 163). On 

 reaching a certain height the vapourjDecomes saturated, and 

 as it still rises, and the temperature continues to fall, the 

 vapour condenses upon the dust -motes forming a mist. 

 Clouds, which are mists at high altitudes, often hang over 

 a mountain or sail slowly through the -air for hours. In 

 such a case, though the form of the cloud does not change, 

 the water globules composing it are always falling as fast 

 as the friction of the air allows ( 161) ; when they reach 

 the warmer air below they are evaporated again and vanish, 

 while new globules are as rapidly condensed on the dust 

 above. 2 



1 68. Classes of Clouds. The differences between 

 clouds arise mainly from the height of the layer of mist 

 composing them. Three types of cloud are distinguished 

 by characteristic forms and by their usual elevation, and all 

 other kinds may be classed as a mixture of two or more of 

 them. The highest form of cloud is a mist of minute ice- 

 specks, usually forming at a height of about 5 J miles above 

 sea-level. It appears like tufts or curls of snow-white hair, 

 and is named Cirrus. In certain conditions this cloud 



