50 A TEXTBOOK OF OCEANOGRAPHY 



o'5° C, the maximum température being reached between 

 I and 2,30 p. m., the minimum between 5 and 8 a. m. 



At night the sea is appreciably warmer than the air above 

 it; in the daytime the température of the air slightly exceeds 

 that of the sea, but as a gênerai rule the température of the 

 surface of the sea is appreciably higher than the air above it. 



The conduction of heat plays a minor part in the thermal 

 condition of the sea. Acçording to Wegemann, if a volume 

 of sea-water 5,000 mètres deep with a température of 0° C. be 

 considered with a source of heat at the surface of 30° C, then 

 after 100 years there would be no increase of température at 

 100 mètres deep, after 1,000 years there would be no increase 

 at 300 mètres, but at 100 mètres an increase of 73° C. would 

 be registered, and at 200 mètres o'6° C. 



Heat conveyed to the upper layers by the sun's rays is 

 therefore only transmitted to lower layers by movements of 

 the water such as convection currents, or through changes in 

 spécifie gravity. In the latter case the surface layers are 

 rendered heavier by evaporation in the daytime, and at night 

 this disturbance of spécifie gravity is corrected by water move- 

 ments, this leading to the transmission of heat to lower layers.. 



The Discontinuity Layer. 



Where there is a marked différence of température in a 

 narrow range of depth we speak of a discontinuity layer 

 (American term, thermocline). This layer is more marked in 

 fresh-water lakes than in the open sea, since in the former there 

 is less disturbance due to waves and currents. In many mid- 

 European lakes there is at depths of from 11 to 13 mètres a 

 différence of 2° to 3° C. in a 20-centimetre layer. A sharp 

 discontinuity layer only occurs in confined seas such as the 

 Baltic, where Ekman found near Bornholm at 18 mètres a 

 température of 14° C, but at 20 mètres 8° C. only, althouo-h 

 the water in both cases was of the same salinity. 



The discontinuity layer is generally a boundary between 



