The Sea Breeze

Living on Cape Cod, it's not uncommon to hear the term "cooling sea breeze" during the warmer seasons. The sea breeze can be both friend and foe. During the summer months, it can help keep things warm and refreshing while interior locations bake under stifling conditions. However, during the spring, when interior New England is experiencing a sunny and mild afternoon, the Cape can be stuck in the 40s and 50s. But what is a true sea breeze? Sometimes the term is misused to define any wind blowing in off the water. Here on Cape Cod, any wind direction other than west and northwest is essentially blowing in from the water - does this mean that every wind is a sea breeze? Of course not. A true sea breeze is:

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 it as heat to warm the air. The warmed air, with its decreased density, begins to rise (1). The rising air creates a weak low pressure area (called a thermal low) due to a decrease in air mass at the surface (2). Typically, from 3,000 to 5,000 feet (1,000 to 1,500 meters) above this low pressure, as the air cools, it begins to collect resulting in an increase in pressure, creating a "high" (3).

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 (4 and 5). 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 (6) and surface onshore flow, called the sea breeze (7).

Because of the temperature difference between the surface temperature over land and over water are typically large, a sharp boundary can develop. This boundary, called a sea breeze front, acts the same as the cold front we are use to experiencing. Just like cold fronts, the air temperature drops significantly after the sea breeze front passes a location, sometimes as much as 15-20°F. The skies also clear after the sea breeze front pass by. Another change that takes place with the passage of the sea breeze front is an increase in humidity. Finally, there is a significant change in wind direction and/or speed.

On Cape Cod, the sea breeze front as described above is not all that common. We often will see back door cold fronts, which act much the same as a sea breeze front, but the traditional sea breeze frontal passage is more common in warmer climates. On some hot summer afternoons, when the sea breeze is quite strong, sometimes locations over eastern Massachusetts, near Boston, will experience a more traditional sea breeze front. - image courtesy NOAA Jet Stream