5 o A TEXTBOOK OF OCEANOGRAPHY 



o'5 C., the maximum temperature 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 temperature of the air slightly exceeds 

 that of the sea, but as a general rule the temperature 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. According to Wegemann, if a volume 

 of sea-water 5,000 metres deep with a temperature of o C. be 

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

 after 100 years there would be no increase of temperature at 

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

 at 300 metres, but at 100 metres an increase of 7-3 C. would 

 be registered, and at 200 metres 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 

 specific gravity. In the latter case the surface layers are 

 rendered heavier by evaporation in the daytime, and at night 

 this disturbance of specific 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 difference of temperature 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 u to 13 metres a 

 difference 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 metres a 

 temperature of 14 C., but at 20 metres 8 C. only, although 

 the water in both cases was of the same salinity. 



The discontinuity layer is generally a boundary between 



