Thermal Low - Role in Sea Breeze Formation

Role in Sea Breeze Formation

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 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 (15 km/h) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.

Along the California coast, the cooler water sets up a surface marine layer that is much cooler than inland areas during the summer. At the same time, the intense heating inland creates a pronounced thermal trough aligned with the Great Central Valley and typically linked to the broader thermal low across the North American deserts. As a consequence, a strong pressure gradient is created which draws cool marine air landward. As temperatures plummet, fog and stratus stream in and through the gaps of the Coast Ranges, and especially through the Golden Gate at San Francisco (see San Francisco fog). The same thermal trough is sometimes pushed toward the coast, especially in late Fall as higher pressure develops to the east due to cooling further east. This setup often brings the warmest temperatures of the year to the normally cool coastline as the seabreeze stops or is even replaced by a dangerously dry land breeze.