A sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water; these create a pressure minimum over the land due to its relative warmth, and forces higher pressure, cooler air from the sea to move inland. Generally, air temperature gets cooler relative to nearby locations as one moves closer to a large body of water.
Main cause
The sea has a greater heat capacity than land and can therefore absorb more heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the land rises, the land heats the air above it. The warm air is less dense and so it rises. This rising air over the land lowers the sea level pressure by about 0.2%. The cooler air above the sea, now with higher sea level pressure, flows towards the land into the lower pressure, creating a cooler breeze near the coast. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If the environmental wind field is greater than 8 knots and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.
LAND BREEZES:
Land breezes usually occur at night. During the day, the sun will heat land surfaces, but only to a depth of a few inches. At night, water will retain more of its heat than land surfaces. Water has a high heat capacity which is one reason hurricane season officially extends through the chilly November months.
At night, the temperature of the land cools quickly without the insolation from the sun. Heat is rapidly re-radiated back to the surrounding air. The water along the shore will then be warmer than the coastal land creating a net movement of air from the land surfaces towards the ocean.
Why? The movement of the wind is a result of differences in air pressure over the land and the ocean. Warm air is less dense and rises. Cool air is more dense and sinks. As the temperature of the land surfaces cool, the warm air rises and creates a small area of high pressure near the land surface. Since winds blow from areas of high to low pressure, the net movement of wind is from the shore to the water as shown in the image to the right.
- Air temperatures decrease at night.
- Rising air creates a thermal low at the ocean surface.
- Cool air collects forming a high pressure zone above the surface of the ocean.
- A low pressure zone forms above the land surface from the rapid loss of heat.
- A high pressure zone forms as the cooler land cools the air immediately above the surface.
- Winds aloft flow from the ocean to the land.
- Winds at the surface flow from high to low pressure creating a land breeze.
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