Pressure Centers and Winds Chapter 19.2 Pressure Centers and Winds

Highs and Lows Lows, or cyclones are centers of low pressure. High, or anticyclones, are centers of high pressure. In cyclones, the pressure decreases from the outer isobar toward the center. In anticyclones, the pressure increases from the outer isobar toward the center.

Cyclonic and Anticyclonic Winds When pressure gradient and the Coriolis effect are applied to pressure centers in Northern Hemisphere, winds blow counterclockwise around a low. Around a high they will blow clockwise. In either hemisphere friction causes a net flow of air inward around a cyclone or outward around an anticyclone.

Weather and Air Pressure A low pressure center will cause air to accumulate and increase pressure so the air will rise. The opposite is true in a high center. Because of this low pressure centers will produce clouds and precipitations and give stormy weather.

Weathering Forecasting Low pressure centers can produce bad weather. Lows will move from west to east across the United States. They can have an unpredictable path.

Global Winds The atmosphere can act as a giant heat- transfer system. This system moves warm air toward high latitudes and cool air toward the equator.

Non-Rotating Earth Model In this model we have a theoretical non- rotating planet that has a completely smooth surface of either all land or water. There would be 2 large thermally produced cells. The air at the equator will rise and move toward the poles. This air would cool down and move back toward the equator.

Rotating Earth Model Because the Earth rotates these 2 cells break down into smaller cells. Near the equator, the rising air produces a pressure zone called an equatorial low (produces a lot of precipitation). This warm air moves up 20 to 30 degrees, north or south latitude then sinks toward the surface. This produces hot, arid condition called the subtropics. We find deserts in this area. As air moves from this area it is deflected and makes the trade winds. Trade winds are 2 belts of winds that blow almost constantly from an easterly direction. The prevailing westerlies is the air that is deflected from the subtropics northward. Polar easterilies are winds that blow from the polar high toward the subpolar low. These are not constant winds. The meeting of warm and cool air masses makes a polar front which is an area with many storms.

Influence of Continents Large landmasses change how air flows and will produce changes in wind directions called monsoons. This is were areas like India get a flow of warm, wet air from the Indian Ocean so they have a very rainy season.