'SEASON OF MIST' 51 



flowers and grasses, and the dust from the bark of trees. 

 The wear and tear of the whole fabric of the earth must 

 contribute to these innumerable floating atoms, which supply 

 the foundation for the fog-clouds. 



Land-fogs are produced either by the mingling of two 

 bodies of saturated air at different temperatures, which 

 causes condensation by cooling of the warmer mass, or by 

 the passage of a warm moist current over a cold land surface. 

 The former process is the commoner in the case of ordinary 

 low-lying fogs, though a moist sea-wind blowing against 

 a colder mountain-top often keeps its head wrapped in 

 cloud. Land-fogs, like clouds, can drift to a considerable 

 distance without dissolving, if the balance of temperature 

 keeps their vapour condensed ; and in autumn and winter 

 tracts of fog cover estuaries when the sea outside is clear, or 

 even join across the Channel, or drift off-shore in wandering 

 masses. Sea-fogs in the same way often overlap the land. 

 Sea-fogs are formed by the passage of warm moist air over 

 colder water, the chilly evaporation from which condenses 

 the vapour in the air. In regions where a cold sea-current 

 thrusts down into more temperate latitudes, fog is the normal 

 condition of the weather. Off our own coasts, where the 

 water is normally warm, sea-fogs are produced more 

 occasionally. They are commonest in spring and early 

 summer, when the sea still keeps a good deal of its winter 

 chill, and the temperature of the air is rapidly warming. 

 Sea-fogs provide the chief exception to the general rule, that 

 fogs occur in calms or light airs. There may be thick fog 

 along our own coasts with a fresh breeze blowing, while in 

 the seas round the Horn there is sometimes thick fog with 

 a heavy gale. This peculiarity of sea-fogs forms a striking 

 contrast with the way in which an autumn or winter mist 

 will often seem to melt by magic when a light draught 



