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Sea breezes and land breezes are similar in that they are both created by changes in temperature near the ground. With a sea breeze, the air over land warms (during daytime), pressure drops, and air accelerates from sea to land in response. With a land breeze, the air over land cools (during nighttime), pressure rises, and air accelerates from land to sea in response.
Generally, sea breezes (and to a lesser extent, land breezes) are marked by a front, or a sudden air mass change, at their leading edge. In a sea breeze, the advancing front causes the temperature to drop and the wind to shift.
The front is a consequence of the response of the flow to heating. Imagine an initially uniform temperature and pressure distribution. Then, by heating some air near the surface, a zone of temperature gradient near the surface forms separating warm air from cold air. Where a temperature gradient exists, a pressure gradient is present also; where the temperatures are uniform, no pressure gradient is found.
The air that feels this pressure force accelerates, driving a wind blowing from cold to warm air. Near the warm edge of the temperature gradient there is convergence, causing the temperature gradient to increase; on the cold side there is divergence, and the temperature gradient weakens.
Most sharp atmospheric fronts, including land breezes, sea breezes, and coastal fronts, are density currents. Density currents have a sharp leading edge, called a "head", where the air ascends rapidly. Behind the head is a region of turbulent mixing between the warm and cold air.
Density currents are found in all sorts of circumstances: oil spills and airborne powder avalanches are also density currents. The theory of density currents was developed by T. Brooke Benjamin and published in 1968. Making simplifying assumptions and applying the principles of conservation of energy and momentum, he found that the speed of a density current should depend only on the density (or temperature) difference and the depth of the two fluids.
The equation he obtained was
c = speed of front (m/s)
vw = ambient wind speed in warm air (positive when directed away from cold air (in m/s)
k = dimensionless, depends on the depth of warm air, always pretty close to 1
g = gravity: 9.8 m/s2
Hc= depth of cold air (m)
Tw = temperature of warm air (K)
Tc = temperature of cold air (K)
It's easy to produce a density current in the laboratory, and experiments in the late 1970's by John E. Simpson and coworkers confirmed the theoretical predictions of density current speeds.
Figures are from J. E. Simpson and R. E. Britter, 1980: A Laboratory Model of an Atmospheric Mesofront, published in the Quarterly Journal of the Royal Meteorological Society.
A typical speed of a sea breeze front, compared to the ambient wind, is 25 knots, or 12 m/s. A typical temperature difference, for a strong sea breeze, is 10 C, or 18 F. Figure out how deep this sea breeze should be, using the density current equation provided above. Or, to save time, guess.
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