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weather - page 3

8.3 Local Winds

The wind systems of the world undergo local modifications due to the presence of topographical and geographical features and the unequal heating and cooling of land and sea. High hills will cause air to blow up the slope when the sun is heating the surface and down again at night when it is not.


The sea breeze

When an area of high pressure is well established in late spring and early summer it is likely that there will be clear skies, light winds and many hours of warm sunshine to heat the land. At this time of year the sea is still relatively cold after the winter as it absorbs heat more slowly than the land. This temperature difference can cause a breeze to blow off the sea during the hotter parts of the day in areas close to a large expanse of water.

During the morning the land heats, causing the air above it to be heated and to then rise. This leaves slack pressure above the land, so the cool air from the sea blows in to replace the displaced air. As the land gets hotter during the afternoon, this cycle will continue and speed up – causing the wind to veer. A line of cumulus cloud may form along the coastline as the damp air continues to blow in from the sea (photo 8.3.1). Everything will slow down in the late afternoon and early evening as the heat goes out of the sun. Once sunset comes, there may be no wind at all from the sea and the cumulus cloud will have dispersed over the land. A sea breeze does not normally exceed a Force 3; it may extend up to 15 miles offshore, and sometimes even further inland when the land is at its hottest. During the late summer the breezes are less pronounced as the differential between land and sea temperature narrows.

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Photo 8.3.1

Land breeze

In late summer the days are shorter and the temperature of the land falls more quickly than that of the sea as darkness falls (photo 8.3.2). This situation is perfect for a land breeze to blow. This is never as strong as the sea breeze and seldom felt more than 5 miles offshore, although topographical features may cause some variation.


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Photo 8.3.2

Downslope winds (katabatic)

In a mountainous region the cool land air can rush down the hill slopes at night and out into what started as a sheltered bay (photo 8.3.3). Ask those who have chartered a boat in Greece, Turkey or Thailand and they all have a tale about standing an anchor watch after choosing the wrong anchorage! This wind, known as a katabatic wind, normally blows for just a few hours until sea and land temperatures equalise, but it is wise to keep watch to check that the anchor is holding.



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Photo 8.3.3

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