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This chapter discusses the characteristics of pressure centers, cyclones, and anticyclones, as well as the wind patterns associated with them. It also covers global atmospheric circulation and the influence of the Coriolis effect on wind direction.
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Chapter19 Air Pressure and Wind
Highs and Lows 19.2 Pressure Centers and Winds Cyclones are centers of low pressure. Anticyclones are centers of high pressure. In cyclones, the pressure decreases from the outer isobars toward the center. In anticyclones, just the opposite is the case—the values of the isobars increase from the outside toward the center.
Cyclones and Anticyclones Regions of local pressure highs and lows have characteristic circulation patterns Cyclones: local low pressure centers: Air spirals inward and upward Anticyclones: local high pressure centers: Air spirals outward and downward
Air Movement at a Cyclone (Low) Warm (or humid) air is less dense than cold air and therefore exerts lower pressure Rising warm air undergoes expansional (adiabatic) cooling which causes clouds and rain. Low surface pressure associated with unsettled weather and rain.
Pressure and wind Cool (or dry) air is denser than warm This air exerts a higher pressure than warm air and will tend to sink, Compressional (adiabatic) warming prevents saturation and cloud formation High pressure often associated with good weather
Highs and Lows 19.2 Pressure Centers and Winds Cyclonic and Anticyclonic Winds • When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counterclockwise around a low. Around a high, they blow clockwise. • In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow of air outward around an anticyclone.
Highs and Lows 19.2 Pressure Centers and Winds Weather and Air Pressure • Rising air is associated with cloud formation and precipitation, whereas sinking air produces clear skies. Weather Forecasting • Weather reports emphasize the locations and possible paths of cyclones and anticyclones. • Low-pressure centers can produce bad weather in any season.
Global Winds 19.2 Pressure Centers and Winds The atmosphere balances these differences by acting as a giant heat-transfer system. This system moves warm air toward high latitudes and cool air toward the equator. Non-Rotating Earth Model • On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form.
Global Winds 19.2 Pressure Centers and Winds Rotating Earth Model • If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells. • Trade winds are two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of the subtropical highs. • Westerlies are the dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.
Global Winds 19.2 Pressure Centers and Winds Rotating Earth Model • Polar easterlies are winds that blow from the polar high toward the subpolar low. These winds are not constant like the trade winds. • A polar front is a stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin.
Global Circulation and the 3-cell model • Global circulation patterns are created by differential heating and modified by the Coriolis Effect. • Idealized atmospheric model: 3 convection cells in each hemisphere: • Hadley Cell (tropical) • Ferrel Cell (mid-latitude) • Polar Cell • Note warmer air at surface for all cells
Global atmospheric circulation ITCZ The equatorial low pressure is due to rising warm equatorial air Adiabatic expansions causes the frequent rainfall. It rains a lot in the tropics! Returning air from the Hadley Cell converges at the Intertropical Convergence Zone (ITCZ). Since air is rising up after converging at the ITCZ, there is little wind, hence the sailor’s term: “the “doldrums”.
Global atmospheric circulation –Trade Winds Leg of Hadley Cell closest to Earth’s surface is pushed west by Coriolis Effect. results are winds that curve in from the east and converge at the ITCZ. These are the easterly trade winds (coming from NE in the northern hemisphere; from SE in the southern hemisphere). Trade winds drive surface equatorial ocean currents in the tropics.
Global atmospheric circulation – subtropical high pressure ~30o N/S latitude, air from the Hadley Cell lost some heat and much moisture, so falls Adiabatic compression causes hot dry air and high pressure at the surface – a subtropical high pressure zone in both northern and southern hemispheres. Much of the world’s deserts are located in this part of the world.
Global Circulation in the Mid-latitudes The boundary between Hadley and Ferrel Cells is located at about 30° N/S latitude. Warm, dry air descending at this junction diverges NE/SE trade winds go towards the Equator. Other branch goes towards the N/S pole and is deflected to the East by the Coriolis Force. Since they blow from the west, these winds are called the Westerlies.
The cold pole creates permanent high pressure at the N/S Pole) Polar easterlies from descending polar air Rising air from the junction of the Ferrel and Polar Cells create a region of stormy, unsettled weather at about 60° N/S Polar jet stream– forms along the polar front. Global Circulation– The Polar Cell
Global Winds 19.2 Pressure Centers and Winds Influence of Continents • The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. In the Northern Hemisphere, where land masses break up the ocean surface, large seasonal temperature differences disrupt the pressure pattern. • Monsoons are the seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea. In summer, the wind blows from sea to land.
