Sea Breezes

Consider the coastal region shown below.  Just before dawn, both the ground and the water surface are at the same low temperature.  As a result, the air above the ground and the air above the water -- crudely depicted here as two finite-height columns -- are at the same temperature.  The two green dots at high altitude represent two places where the air pressure is the same; that is, the amount of air above each point, weighing down on the air at that point, is the same for both points.

Once the Sun rises, things change.  Water has a high specific heat, meaning that it takes a lot of energy input to raise the temperature of 1 kg of water by even a small amount; rock and soil have much lower specific heats.  Since water is largely transparent, energy input from absorbed sunlight is spread throughout a deep layer; all sunlight absorbed by the ground is absorbed right at the surface.  The water's surface can cool itself by evaporation to the air above, whereas the ground, being drier than the water (!), can't do this as well.  The water's surface also can cool itself through convection (trading places with cooler water below); the ground can't flow, so convection is out of the question.

Result: The ground's temperature increases much faster than the water's does.  Heat flows (via conduction) from the warm ground to the air next to the ground, and this air expands and rises.  This is shown below as a tall column of warm air above the ground.

Remember the two high-altitude points shown in the first figure?  Since air above the ground has moved upward, there's more than before above the left point, weighing down on that point.  Hence air pressure has increased at this point, which now represents a high-pressure zone ("H" in the figure above) when compared to the lower pressure at the right point ("L").  Air at that altitude feels a greater push from the left than from the right -- a pressure gradient force -- causing a wind to flow from the left.

As air aloft thus flows from above the ground to above the water, there's more and more air weighing down on the air at the water's surface, and less and less weighing down on the air just above the ground.  In other words, a high-pressure zone develops at the water's surface, and a low-pressure zone develops at the ground (see figure below).  A pressure gradient force now causes surface air to flow from right to left, that is, from sea to land.

Since we live at the surface, this is the wind we experience: a sea breeze.  The convection cell is completed by the sinking motion of air above the water (see figure above).

Once the Sun has set, everything reverses: The ground cools faster than the water, and we get a land breeze.

Sea Breezes

Written by Chris Magri

Last modified on August 28, 2016

URL: https://sites.google.com/a/maine.edu/magri/phy101/weather/seabreeze