44 PHYSIOGRAPHY. [chap. 



arbitrary scale. Thus, a clear blue sky is represented by 

 a cipher, while a sky completely obscured is designated 

 by id; the intermediate numbers from o to lo being used 

 to express varying proportions of cloudiness. 



It has been shown that, when watery vapour condenses in 

 the upper regions of the atmosphere, it gives rise to the 

 formation of clouds. But, if the condensation occurs near 

 to the surface of the earth, it produces those visible vapours 

 which are known as mist or fog. Beyond the difference in 

 the place of origin there is really little or no distinction 

 to be drawn between a fog and a cloud. A fog is a cloud 

 resting on the earth ; a cloud is a fog floating high in 

 the air. 



Whenever moist air near the surface of the earth has its 

 temperature sufficiently reduced, the moisture may be con- 

 densed as mist or fog. Thus fogs almost constantly hang 

 over the banks off the coast of Newfoundland, where they 

 are produced by the warm moist air of the Gulf Stream^ 

 coming in contact with the cold air of the Labrador 

 current. In like manner, icebergs are often attended by 

 fogs, simply because the mass of ice cools the surrounding 

 air, and thus precipitates its moisture. Mountains, again, 

 are frequently enveloped in mist, since the warm air, on 

 being driven up the slope of the hill, becomes chilled to 

 the point at which its moisture is partially condensed. So 

 too the position of a river is often marked by mist ; and 

 this whether the water be colder or warmer than the 

 overlying air : in the former case, the air is cooled down 

 by contact with the water, and its moisture discharged ; in 



1 It will be explained in Chapter XI. that the Gulf Stream is a body 

 of warm water which flows from the Gulf of Mexico across the Atlantic 

 Ocean ; the Labrador current is a body of cold water coming down 

 from the north along the coast of Labrador. 



