Surface winds are often more a function of surface features or local thermal changes than the large area differences in barometric pressure that drive synoptic winds. Temperature differences between water and land and between mountain tops and valleys can cause the air to lift and descend and generate airflow parallel to the surface that will either add to or subtract from wind flow generated by overlying synoptic winds. We will briefly consider several sources of local surface winds: Sea/Land Breezes, Thunderstorms and Mountain (katabatic/anabatic) winds, caused by geography differences and local thermal differences.

Sea/Land Breezes: Sea/Land Breezes are formed as a result of temperature differences between large bodies of water and adjoining land masses, usually within a few miles of the coast. Water will typically retain heat longer than dry land creating temperature differences during the day as the land warms faster than the water thereby warming the overlaying air and creating lift over the land. The rising air decreases pressure over the land draws in the cooler air from the over the adjacent colder water causing a sea breeze. At night the land adjacent to a body of water cools faster than the water causing the warmer air over the water to lift and draw the cooler air over the land toward the water, a land breeze.

As you can see from the drawing in figure 2.2 during the day when the sun warms the land faster than the water (sea or large lake) the air over the land is lifted (remember warm air rises)from the low pressure (less dense air at the surface and cools as it rises (adiabatic lifting). Over the sea the warmer air aloft sinks and cools as it approaches the cool water, the surface wind is thereby caused to flow inland from the water to fill the low pressure area caused by the adiabatic lift of air over the land. During the night when the air over the land is cooled to temperatures below the temperature of the adjacent water the opposite flow occurs and surface air flows from the land toward the sea. This phenomenon is most noticeable in the summer time in the coastal areas and is often minimized or eliminated by strong synoptic winds flowing over the land, especially on the east coast of the U.S.

Point to Ponder: If synoptic wind is flowing in the same direction as the upper level circulation of a land or sea breeze does it increase the lower level , surface, wind flow or decrease it?

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