A week ago I arrived at Salisbury Beach at 11AM on a cloudless day and was surprised there was no sea breeze. High tide was scheduled for 12:13 that day. By 12:30 the sea breeze developed strongly. Why (or how) would a tide affect the formation of a sea breeze?
I thought I understood the physics of a sea breeze (sun heating the sand which heats the air causing it to rise and be replaced by the more dense cooler air over the ocean). But this event has me perplexed.
Allan Dunn – Gardner, MA
Good question Allan…the sea breeze is a constant part of forecasting here in New England. We feel them most often in the summer months, and they usually send ashore a refreshingly cool blast of air (maybe not so refreshing when we get them in April!)
To answer your question, the tides do not have a lot to do with the timing or strength of a sea breeze. The only instance in which the tides may affect the sea breeze that I can think of is in places where the tidal fluctuations are large and wide swaths of tidal flats get exposed/covered with time. A place like Brewster on Cape Cod may be a good example of this, with the tide going wayyy out when it’s low.
In general, the sea breeze is all about differential heating between the land and the ocean. On a sunny day, the land will heat up much faster than the ocean, since the heat capacity of the ocean is much greater. This means the sun can keep on shining on the surface of the water, but it won’t heat up too much. The land, however, will heat up quickly as it absorbs solar radiation. This causes air to rise, and a low pressure cell forms over the land as air heads up and out over the shore.
In return, a high pressure cell forms over water and the ocean air rushes toward land to ‘fill the gap’ left behind by the rising warm air (air always flows from high pressure to low pressure). And since the water is cooler, the air above it is cooler, and you get the idea!
There are some other factors besides the tides that may change around the timing of a sea breeze. The main one is how strong the synoptic winds are. For example, a day with high pressure directly overhead will feature very light winds, and the sea breeze will start up quickly. But if there’s a light wind, say 3-8mph, it may take a bit longer to overtake.
Another one is the time of year. Sun angle and the season will determine how quickly the land heats up, and therefore the timing of when the temperature gradient becomes large enough to bring on the breeze. It may take a little longer in October to get one started than it will in July when the sun is strong and nearly directly overhead.