Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 201 



mean of sixty series of observations, that the water at the sea surface was always appre- 

 ciably denser (heavier) than that at 6 m depth and even at 1 5 m depth the water was still 

 specifically lighter than at the surface. Taking ^ = 100 g cm-^ sec-^ and h = 500 cm, 

 then E = — 16 x 10~^ ; this means that convection is initiated in this layer at this value 

 and not at £" = 0. If the turbulence becomes stronger the critical value of E increases 

 rapidly and strong density gradients are required for any start of convectional motion. 

 The long lines of foam often observed on the surface of the sea can be regarded as 

 "convection rolls" formed by a combination of a strong current in a single direction, 

 and circulations in convection cells in the above sense. Their frequent occurrence is an 

 indication that regular formations of Benard convection cells occur in the sea. 



