A sea breeze is a wind that blows from the sea onto the land on sunny days, because the land heats up faster than the water. It usually starts in late morning, is strongest in mid-afternoon and dies away around sunset.
On a sunny day the land warms up much faster than the sea. The air over the land is heated, becomes lighter and rises, and cooler air from over the water flows in underneath to replace it. That inflow is the sea breeze, blowing from the sea towards the land.
A typical sea breeze day goes like this:
- Morning: light winds, or the general weather-map (gradient) wind, with the land still cool.
- Late morning: as the land heats, the sea breeze sets in near the coast, often first as a dark line of ripples moving out across the water.
- Afternoon: it builds to its strongest, commonly 10 to 15 knots and sometimes more.
- Evening: as the land cools, it fades, often leaving a calm around sunset.
Sea breezes are strongest on clear, warm days with light gradient winds, so they are most common in spring and summer. Cloud cover or a strong gradient wind can stop one forming. If the gradient wind already blows onshore, the sea breeze adds to it and the afternoon can be much windier than the morning. If the gradient wind blows offshore, the two can cancel out, leaving a band of calm near the coast before the sea breeze wins.
At night the opposite can happen. The land cools faster than the sea, and a lighter land breeze may blow off the shore.
For planning a day out, remember that a calm morning does not mean a calm afternoon, and the sea breeze tends to blow up harbours and inlets. A sail out in light air can turn into a beat or a lively reach home in a fresh breeze, so plan to reef if needed and allow more time for the return.