The Sea-Breeze is a circulation that develops due to differential heating of air over land and sea. As the sun heats the boundary layer over land, the resulting pressure gradient causes the movement of low-level air from the sea to land (Sea-Breeze) with a return flow aloft (return current).

The existence and intensity of the Sea-Breeze depends strongly on seasonal and latitudinal factors as well as on the time of day. On many tropical and subtropical coastal regions the Sea-Breeze is a regular phenomenon throughout the year, while in cooler regions the Sea-Breeze is a common feature during spring and summer, when the temperature difference between land and sea is at its maximum.

Daily life cycle

A simplified overview is provided using the following model:

A Sea-Breeze normally starts in the morning, a few hours after sunrise, when the solar radiation heats the boundary layer over land. A classical explanation for the development of a Sea-Breeze is the "Upwards" Theory: The differential heating between land and sea leads to the development of a horizontal pressure gradient, which causes a flow from land towards sea. This flow is called a "return current", even though it may develop before the actual Sea-breeze. The mass divergence and resulting pressure fall over land and the convergence and pressure rise over the sea initiate the Sea-Breeze close to the surface

There are other explanations for the initiation of Sea-Breeze:

• Sea-Breeze is initiated at low levels, or
• Both Sea-Breeze and return current (representing the air flowing from land to sea above Sea-Breeze) develop practically at the same time.

The return current aloft carries the excess of air towards the sea. Cloud development frequently occurs in the ascending part of the circulation, while clouds tend to dissipate over the sea, where the air is sinking. When the air is very dry, as often is the case in spring and early summer, the cumulus clouds may not appear at all. In these cases the use of satellite imagery is clearly problematic for the detection of Sea-Breezes, while it may still be detectable using other remote sensing means, such as sensitive weather radars.

In the afternoon, when the boundary layer heating over land is at its maximum, the Sea-Breeze is normally at its most intense, and can penetrate tens of kilometres - in some cases, even over a hundred kilometres - inland.

If the large-scale flow is weak, the direction of the Sea-Breeze often veers with time. This is a result of the Coriolis force having an impact on the air current. Another factor influencing the wind direction along the coast is the regular existence of thermal lows over land in the afternoon.

Later in the day, as solar radiation decreases, the Sea-Breeze dies out, the thermals weaken and the cumuliform clouds gradually disappear.

Factors influencing the Sea-Breeze development

Sufficient temperature contrast between the land and sea surfaces is needed for the initiation of the Sea-Breeze. In this respect the physical background differs for Coastal Convergence (see Coastal Convergence ) in which the differential friction over land and over the sea is the main component for the development.

The coastline shape can either enhance or inhibit sea-breeze development. For example, merging of two Sea-Breezes originating from both sides of a peninsula enhances convection. On the other hand, over land areas adjoining bays the Sea-Breezes tend to diverge, which enhances low-level descent of air.

The image below shows a case over Finland, in which Sea-Breezes (barely visible as a line of cumulus) merge along the Finnish west and south coasts, producing some intensive, local Cb Clouds in the south-western corner of Finland. Meanwhile, along the coast adjacent to the eastern Gulf of Finland the wind field is more divergent and no clouds are produced.

The low-level gradient wind ("background flow") steers the development of a Sea-Breeze:

• If the large-scale flow is weak, the Sea-Breeze circulation dominates.
• If opposing (offshore) large-scale wind exists, it can either inhibit the Sea-Breeze totally or slow down the development of the Sea-Breeze. Under these circumstances the Sea-Breeze may develop late in the afternoon and only slowly advance further inland. In extreme cases the whole Sea-Breeze circulation may be transported over the sea. The most intensive Sea-Breeze fronts develop in association with an opposing light large-scale flow. Strong low-level convergence within the Sea-Breeze front may result in strong convection restricted to a narrow line along the front.
• If large-scale onshore winds exist, the Sea-Breeze circulation often has no possibilities to develop, as the large-scale flow advects the cool sea air over land, reducing the temperature and pressure gradients.

Generally it can be said that a Sea-Breeze is more likely to occur in a non-frontal environment, in which the thermal wind (the change of wind by height) is small and there is no frontal cloudiness.

Additionally, capping temperature inversion above the boundary layer tends to intensify the Sea-Breeze circulation.