The sea surface is smoother than the land surface, so winds at sea are stronger than those over land. When the wind blows off the land (offshore), friction reduces as the wind passes over the coast. This accelerates the wind, making it veer in the northern hemisphere, and back in the southern hemisphere. However, coastlines are rarely uniform so this phenomenon is usually masked by other coastal influences.
Wind will follow the path of least resistance. Hills will tend to block the wind while valleys will funnel and accelerate it. If the wind is blowing off a cliff, then there will be a band of stronger, fairly consistent wind approximately a nautical mile or more offshore. In shore of this, the winds are likely to be lighter and inconsistent. Turbulent eddies close to the cliff face can also lead to vigorous onshore gusts. The opposite effect can be observed when the wind is blowing onshore and perpendicular to a cliff face. Because the wind will begin to ascend before it reaches the cliff face, turbulent eddies close to the shore can result in strong offshore gusts.
Katabatic winds can also impact coastal regions, especially overnight. Katabatic winds can be more simply referred to as drainage winds and occur when air that has cooled overnight falls down from higher altitudes because it is heavier than the warmer air below it. Localized katabatic winds (unlike large scale katabatic winds like the Bora in Croatia) are most likely to be a problem overnight at anchor, especially along coastlines near mountainous terrain. Katabatic winds usually occur in gusts that can be violent and will not be predicted by a weather model or forecast.
Water and land heat up and cool down at different rates because they have a different specific heat capacity. It takes more energy to heat up the ocean, so it tends to heat up and cool down slower than the land surface. During the day, the land heats up faster than the surrounding sea. This heat is then transferred to the air above the land which in turn rises, creating a localized region of low pressure. Denser, cooler air from the ocean flows toward this localized low pressure, creating an onshore flow of air known as a sea breeze. The sea breeze can vary in strength depending on the temperature difference between the land and sea and also the size of the land mass. When the temperature difference is large, the sea breeze will be stronger. Likewise, the sea breeze will be stronger near a large landmass (like a continent) and weaker around islands. On continental coastlines, the sea breeze can be felt as much as 100 nm off shore. Sea breezes are impacted by the rotation of the earth, and once they are well established, tend to veer in the northern hemisphere and back in the southern hemisphere due to the Coriolis force. They are also influenced by the prevailing wind. For example, if there is a strong prevailing off shore wind, the sea breeze may not be felt at all.
As the land surface begins to cool in the late afternoon, the sea breeze will begin to weaken. Overnight, the land surface cools more than the surrounding ocean so that the air flow is reversed, known as a land breeze. Because the temperature difference between the land and sea overnight is less than during the day, the overnight land breeze effect is almost always smaller than the sea breeze.
Perhaps the most important thing to remember about coastal winds is that they are heavily influenced by local factors. Because of this, many coastal effects on wind speed and direction will not be reflected in the wind forecast. The best forecasts in coastal regions will be from weather models with the highest resolution.
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