When the weather has been clear and moderately warm for two or more
days, and the winds are light and variable, there may occur on the third
day a moderate wind from the east, known as the sea-breeze. This occurs
during anticyclonic conditions. Preceding the sea-breeze, the winds are
very light, there are no clouds, and the temperature rises rapidly
during the forenoon. This heating is due to a slow dynamic compression
as the air slowly descends and the surface air does not flow away. There
is no cooling because there is no evaporation due to air movement. The
absolute humidity is low, often less than ten grams per cubic metre.
Cumulus clouds do not form because there is no uplift of the lower air
and consequently no chance for condensation of whatever water vapor may
be present. No thunder-heads form notwithstanding the heat. The heat,
while dry, is nevertheless extremely trying to men and animals. Relief
comes in the early hours of the afternoon by the arrival of the
sea-breeze.
The usual explanation of the origin of the sea-breeze is that the land
being excessively warm, the air over a relatively cool ocean moves in to
take the place of the warm and therefore lighter air, which it is
assumed has risen. Unfortunately for this explanation, the air over the
land has _not_ risen; but on the contrary is falling slowly. Again the
sea-breeze does not begin at the place where the temperature contrast is
greatest, namely, just inside the shore line; but comes in from the sea.
Nor does the flow extend far inland, which would be the case if there
were up-rising currents. The sea-breeze is very shallow, generally not
extending upward more than 200 metres, and often not above 100 metres.
It does not penetrate far inland, as a rule not more than 15
kilometres, 9 miles.
The sea-breeze is probably caused by a slow descent of dry, warm air, on
an incline sloping from northeast to southwest. As it reaches the
surface it is twisted more to the right; that is, becomes an east wind.
It carries inland with it some of the air over the ocean which is much
cooler and heavily saturated.