When sailing close to the coast it often seems that NOAA’s wind forecasts are maddeningly inaccurate. How could NOAA get it so wrong? The answer, of course, is that they didn’t. NOAA’s forecasts refer to prevailing winds, but as sailors we need to account for how the land affects the breeze.
After the sun goes down, the ocean retains heat longer than land. This heat rises, creating a low pressure area that gets filled in by the cooler air over the land. The flow of air from the land to the ocean creates a land breeze. Conversely, as the sun rises and heats up the land (the temperature of which changes more drastically between night and day), the low pressure area created by rising warm air is now over the land. The resulting flow of air from the sea to the land creates what is called a sea breeze.
No matter what direction it’s from, when the prevailing wind is light, a land or sea breeze can counteract it. Even in stronger prevailing winds, a land or sea breeze can have a noticeable effect. If, for example, you are on the eastern shore of any large land mass and there is an east wind, it will be augmented by the sea breeze during the day. At night, however, it will be decreased, or perhaps entirely counteracted, by the land breeze blowing against it. Conversely, a west wind will be augmented by a land breeze at night, but counteracted by the sea breeze during the day.
In less simplified situations, the land and sea breeze will have different effects depending on local geography. Learning how they behave in your cruising ground will help you make more informed decisions when you study weather forecasts.