OCEAN SEASONS 



247 



months the surface waters become warmed and we have 

 seen on page 222 that owing to the bad conductivity of the 

 water this heat is only very slowly dissipated into the 

 deeper layers. The 

 result is that a layer 

 is formed at the sur- 

 face with a tempera- 

 ture two or three 

 degrees higher than 

 that of the water 

 beneath. Between 



these two water 

 masses there is a very 

 narrow layer in which 

 the temperature 

 changes very abruptly r 

 from that of the upper 

 water mass to that of 

 the lower. This is 

 known as the " dis- 

 continuity layer " and 

 owing to certain j 

 physical properties of j 

 water it becomes ex- 

 tremely difficult for 

 the upper and lower 

 water masses to mix 

 one with another, thus 

 forming an almost im- 

 penetrable boundary, 

 and only under the 

 action of very severe 

 gales can it be broken down. The depth at which the 

 layer is formed becomes deeper as the summer advances, 

 but never gets much below twenty metres. 



WINTER" 



SPRING 



Fig. 52. — Diagram to show the relation between 



the number of diatoms and the amount of 



phosphates and nitrates in solution in the sea in 



temperate regions during the year. 



