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There is a diurnal (daily) contrast in the heat-ing of local water and land areas similar to the seasonal variation of the monsoon. During the day, the land is warmer than the water area; at night the land area is cooler than the water area. A slight variation in pressure is caused by this temperature contrast. At night the wind blows from land to sea and is called a land breeze. Dur-ing the day, the wind blows from water areas to land areas and is called a sea breeze. The sea breeze usually begins during midmorn-ing (0900-1100 local time) when the land areas become warmer than adjacent ocean waters (see fig. 3-3-3). This temperature difference creates an area of slightly lower surface pressures over land compared to the now cooler waters. The result is a wind flow from water to land. The sea breeze starts with a shallow flow along the surface;

Figure 3-3-1.—Northeast monsoon (January).

Figure 3-3-2.—Southwest monsoon (July).

however, as maximum heating occurs, the flow increases with height. The height varies from an average of 3,000 feet in moderately warm climates to 4,500 (or more) in tropical regions. The effects of the sea breeze can be felt as far as 30 miles both onshore and offshore. In extreme cases, the sea breeze is felt 100 miles inland depending upon ter-rain. By midafternoon (maximum heating) the sea breeze will reach its maximum speed and may be strong enough to be influenced by the Coriolis force, which causes it to flow at an angle to the shore. The sea breeze is most pronounced in late spring, summer, and early fall when maximum temperature differences occur between land and water surfaces. The start of a sea breeze is marked by a decrease in temperature and an increase in humidity and wind speed.

The sea breeze continues until the land area cools sufficiently to dissipate the weak low pressure. After sunset, the land cools at a faster rate than the adjacent waters and eventually pro-duces a complete reversal of the winds.

As the land continues to cool through the evening hours, a weak area of high pressure forms over the land. The water area, with its warmer temperatures, has slightly lower pressure and again a flow is established; however, the flow is now from land to water (offshore). (See fig. 3-3-3.)

The land breezes, when compared to the sea breezes, are less extensive and not as strong (usually less than 10 knots and less than 10 miles offshore). This is because there is less temperature contrast at night between land and water surfaces as compared to the temperature contrast during daytime heating. Land breezes are at maximum development late at night, in late fall and early winter. 

In the tropical land regions, the land and sea breezes are repeated day after day with great regularity. In high latitudes the land and sea breezes are often masked by winds of synoptic features.

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