Tuesday, September 3, 2013

Understanding Marine Weather – Sea Breeze

The ability of the ocean to absorb and store energy from the sun is huge. This is due to the transparency of the water that allows the sun’s rays to penetrate deep into the ocean. In clear, tropical water, light can reach a depth of 500-650 feet meaning that it takes a great amount of heat to raise the temperature over such a large volume of water. In addition the constant turbulence from wind and weather continually mix the water, distributing heat over large volumes requiring even more heat to raise the temperature.

In contrast to the ocean, the sun’s rays do not to penetrate deep into dry ground but are confined typically to just a few inches at the top. Consequently, the land on a daily basis, while it heats up many times more rapidly than the ocean, it can also lose that heat just as rapidly at night.

For coastal areas, this difference on heating can have a large impact on the weather by the formation of the sea and land breezes and is in fact the basic reason we observe wind in the first place. While sea breeze is generally associated with the ocean, they can occur along the shore of any large body of water such as the Great Lakes.

The sea breeze circulation is comprised of two opposing flows: one at the surface (called the sea breeze) and one aloft (which is a return flow). These two flows are a result of the difference in air pressure between the land and sea generated by the sun’s heating.

At the surface, the sun warms both the ground and ocean at the same rate. However, since the heat in the ground is not absorbed well it returns its heat to warm the air. The warmed air, with its decreased density, begins to rise. The rising air creates a weak low-pressure area (called a thermal low) due to a decrease in air mass at the surface. Typically, from 3,000 to 5,000 feet above this low pressure, as the air cools, it begins to collect resulting in an increase in pressure, creating a “high”.

These differences in pressures over land, both at the surface and aloft, are greater than the differences in pressures over water at the same elevations. Therefore, as the atmosphere seeks to reestablish equal pressure both onshore and offshore, two high-pressure to low pressure airflows develop: the offshore flow aloft and surface onshore flow, called the sea breeze.

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