TEMPERATURE 71 



face and propagated downwards ; but when the 

 temperature of the water is raised, evaporation 

 tends to check the rapidity of the rise ; and 

 when the temperature of the water is lowered, 

 condensation of the water-vapour in the air 

 resting upon the sea-surface checks the rapidity 

 of the fall; and if the temperature becomes 

 very low freezing sets in, again retarding the 

 fall. 



The daily variation of temperature in the 

 air resting upon the dry land may in places 

 be very large, as for instance in the desert 

 regions of the tropics, where during the day 

 the temperature of the air may rise to 125° F., 

 while during the night the temperature may 

 fall below the freezing point. This is in 

 striking contrast to what holds good for the 

 air resting upon the ocean, and in still greater 

 contrast to what takes place in the actual 

 surface waters of the sea. For example, dur- 

 ing 126 days when the " Challenger " made 

 observations in the North Atlantic Ocean the 

 mean amplitude of the daily variation in the 

 temperature of the air over the sea was 3-21° F. 

 and on 76 of these days when the " Chal- 

 lenger " was near land the mean daily range 

 was 4*38° F., indicating a larger range in the 

 temperature of the air over the sea when near 

 land than when out in the open ocean. During 

 these same 126 days the mean daily range of 



