-3- 
layer mixing is inhibited so that a large temperature gradient can 
be produced by the heat exchange across the sea surface. A layer 
of nearly neutral equilibrium is easily mixéd by friction, and, if 
it is on the unstable side of neutral ecuilibrium, it is mixed by 
convection, so that practically always it is nearly isothermal. 
Such a layer on the unstable side of neutral is called a convection 
layer. 
Since density is a function of temperature and salinity, the 
stability of a layer is affected by any change of temperature or sa- 
linity which is not evenly distributed through the layer. Solar 
radiation acting alone, since its heating effect decreases from the 
sea surface less and less steeply to practically zero ina very few 
fathoms, would produce stability which decreases from a maximum 
value at the surface. If the resulting temperature and stability 
distributions were then subjected to evaporation alone, which in- 
P 
ereases the density of the surface water, a very thin convection 
layer would immediately form at the surface and then gradually deeven$ 
this layer would be nearly isothermal. Evaporation increases sur- 
face density not only by cooling, but also by increasing the salinity. 
Conversely precipitation tends to produce stability, since raii: water 
regardless of its temperature is always lighter than sea water of 
the salinities commonly found. 
Usually at night the density of the surface water is being in- 
ereased by nocturnal radiation and evaporation, forming a convection 
layer which is nearly isothermal. At noontime nocturnal radiation 
and evaporation are usually outweighted by solar radiation. Never- 
thetess, \sinee lattle of the’ solar radiation is absorbed in the sure 
face film, a thin convection layer often persists at the surface 
even though the stability and the magnitude of the temperature 
