Definitions

onshore breeze

Sea breeze

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 which create a pressure minimum over the land due to its relative warmth and forces higher pressure, cooler air from the sea to move inland.

Main cause

The sea is warmed by the sun to a greater depth than the land due to its greater specific heat. The sea therefore has a greater capacity for absorbing heat than does the land and 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 relatively 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.

Effects

A sea-breeze front is a weather front created by a sea-breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, cumulonimbus clouds, the front can sometimes trigger thunderstorms. If the flow aloft is aligned with the direction of the sea breeze, places experiencing the sea breeze frontal passage will be benign, or fair, weather for the remainder of the day. At the front warm air continues to flow upward and cold air continually moves in to replace it and so the front moves progressively inland. Its speed depends on whether it is assisted or hampered by the prevailing wind, and the strength of the thermal contrast between land and sea. At night, the sea-breeze usually vanishes.

Sea-breezes in Florida

Thunderstorms caused by powerful sea breeze fronts frequently occur in Florida, a peninsula surrounded on both the east and west by the Atlantic Ocean and Gulf of Mexico, respectively. No matter which direction the winds are blowing, they are always off the water, thus making Florida the place most often struck by lightning in the United States, and one of the most on Earth. On especially calm days with little prevailing wind, sea-breezes from both coasts may collide in the middle, creating especially severe storms down the center of the state. These storms also tend to produce significant hail due to the tremendous uplift it causes in the atmosphere. In Florida, a sea-breeze pushed by prevailing winds may also continue past the land and out over the water at night, creating spectacular cloud-to-cloud lightning shows for hours after sunset.

Land breezes

At night, the land cools off quicker than the ocean due to differences in their specific heat values, which forces the dying of the daytime sea breeze. If the land cools below that of the adjacent sea surface temperature, the pressure over the water will be lower than that of the land, setting up a land breeze as long as the environmental surface wind pattern is not strong enough to oppose it. If there is sufficient moisture and instability available, the land breeze can cause showers or even thunderstorms, over the water. Overnight thunderstorm development offshore can be a good predictor for the activity on land the following day, as long as there are no expected changes to the weather pattern over the following 12-24 hours. The land breeze will die once the land warms up again the next morning.

See also

References

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