CHAPTER 18
example occurs regularly off Cape Finisterre in Northwest Spain where low pressure over the Iberian Peninsula in opposition to the North Atlantic High results in strong to gale-force northeasterlies. In such places, a sailor would actually look for a steady change in pressure for relief.
The sea breeze
Sea and land breezes are local winds driven by temperature differences. They are noticeable around shorelines worldwide and have an important effect on coastal sailing. Once familiarity with a given area is established, they are to a large extent predictable. The mechanism of a sea breeze The process starts slowly, building steadily according to conditions. Land takes on heat from the sun more readily than water. Warm air is less dense than colder air, so on a hot day the heated air floats upwards. The lower pressure left at ground level is in effect a partial vacuum promptly filled by cooler, denser air from over the sea. As the wind blows in off the water, its place to seaward must be taken by air that has drifted offshore after rising from the surface. This subsides into the area left behind by the breeze blowing towards the shore. The sea-breeze engine is thus a circulation of air rising over the land, drifting out over the water, sinking down and blowing back onshore again.
• An undisturbed sea breeze begins by blowing directly onshore, but as the afternoon wears on it may well be veered (backed in the Southern Hemisphere) by Coriolis forces.
• Left to itself in still air, it can blow at force 4, or even 5 close in where it is usually at its strongest.
• The strength of a sea breeze is profoundly affected by any underlying wind due to larger pressure systems. Most winds are created by global air movements resulting from differences in pressure. A moderate overall wind drifting offshore does not, as you might expect, knock out the sea breeze. Instead, it picks up the air at medium altitude and accelerates its progress seawards. This makes room for more air rising off the land which in turn pipes up the sea breeze blowing in to fill the gap. Any sea-level offshore wind can soon be overcome by this effect, and the resulting sea breeze can blow as strongly as force 6. Perhaps surprisingly, by the same mechanism working in reverse, a moderate onshore wind can play down the tendency for sea breezes to develop.
• The sea breeze effect can be felt as much as 20 miles offshore in ideal conditions with essentially offshore airflow. In the evening, it fails first along the beach and cliff tops, being last felt offshore as its decaying remnants are carried out on the re-establishing general wind.
Land breezes Sea breeze Warm air rises from land
Air cools over sea & descends
Facts about sea breezes - • The temperature difference required to work the sea- breeze mechanism can be as little as 2°C. In Britain, the right conditions are often found in late spring when the sea is still cool after winter, although sea breezes can occur all summer long.
• It’s unusual for a sea breeze to start up much before noon, but thereafter it may blow until a short while after sunset. Large flat land masses are ideal, but rising ground can do the trick as well, so that even such coasts as West Norway enjoy a strong ‘sun wind’ in season.
Just as land absorbs heat more rapidly than the sea, it also sheds warmth quicker when the sun goes down. At night, this leaves a low-pressure area over the water that is filled by colder, heavier air draining down from higher land inshore. No mini-system like the sea-breeze engine is established, so the resulting air movements are often less dramatic than their sun-whipped opposite numbers, but in specific areas they are reliable and can make a useful contribution to a coastal passage.
Land breeze
Air descends over cooler land
Air rises from warm sea
MANUAL OF SEAMANSHIP | 151
Wind and wave
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