Surface winds are often dominated by high or low pressure systems that typically move from west to east across the United States.  In the Northern Hemisphere, these winds flow counter clockwise and inward toward the center of a Low Pressure System and clockwise and outward from a high pressure system as indicated by the arrows depicting wind speed and direction in the below NOAA charts (go to http://adds.aviationweather.noaa.gov for current charts).  Also you can see in the NOAA charts that the surface winds generally move in the direction from a high pressure system to a low pressure system.  The greater the pressure gradient (closer the constant pressure lines are together on the chart) of a high or low pressure system the greater the wind speed as indicated by the number and size of “barbs” on the wind speed arrows. The large barbs indicate 10 knots (multiply knots by 1.15 to get MPH) of wind speed and smaller barbs 5 knots of wind speed and are added together to get total wind speed.  For example: two large barbs and one small barb indicate 25 knots of wind.  Think of the barbs as feathers on an arrow that points in the direction of the wind.

Point to Ponder: Why don’t these charts show differences in wind speed and direction that should occur as a result of terrain variations like mountains and bodies of water interfering with synoptic winds created by large area atmospheric pressure differences?

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.

Figure 2.2 LAND AND SEA BREEZES

SEA BREEZE (On shore in afternoon and evening)

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?

Copyright: Belfort Instrument Co. 12/29/08