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Pressure Systems and Cyclones ESS 111 – Climate and Global Change Air Pressure • Force per unit area exerted against a surface by the weight of the air above that surface Atmospheric Pressure (882 mb) Hurricane Wilma October 2005 Altitude corrections with pressure readings • Adjusted to an altitude of 0 meters (sea level) Pressure gradient • Difference in pressure from one location to another – This establishes pressure gradient force – Tighter gradient = faster winds (isobars closer together) How the wind would act if PGF were the only force influencing the movement of air The Coriolis Effect • Objects in the atmosphere are influenced by the Earth’s rotation – Rotation of Earth is counter-clockwise looking down from N. Pole. • results in an ‘apparent’ deflection (relative to surface) • deflection to the right in Northern Hemisphere (left in S. Hemisphere) • Greatest at the poles, 0 at the equator • Increases with speed of moving object and distance • Coriolis changes direction not speed Coriolis Force • Due to earth’s rotation • Applies to any freely moving object – Bullets, rockets, airplane, cars – Wind & ocean circulation • Magnitude of Coriolis effect Coriolis force • Does not control the way water drains out of a sink or tub. Winds near the Earth’s Surface Surface Winds = Pressure Gradient Force + Coriolis Force + Friction Upper air w/out Friction (geostrophic balance) Near surface w/ Friction Clockwise airflow in NH (opposite in SH) Characterized by descending/converging air which warms creating clear skies Counterclockwise in NH (opposite in SH)characterized by ascending/diverging air which cools to form clouds/precipitation Where would the strong winds be located? A B Cyclones, Anticyclones, Troughs and Ridges Upper air: isobars usually not closed off • Troughs (low pressure areas) • Ridges (high pressure areas) Near surface: isobars usually closed off due to surface friction • Cyclones (Low pressure areas) • Anticyclones (High pressure areas) Mid-latitude Cyclones • Low pressure systems are a characteristic feature of midlatitude temperate zones • They form in well defined zones associated with the polar front – which provides a strong temperature gradient – and convergent flow 14 Polar Front Theory • First published after World War I • 6 stages of life cycle Life Cycle of Mid-latitude Cyclone • Stage 1 • Stationary front – Cold air to the north – Warm air to the south Life Cycle of Mid-latitude Cyclone • Stage 2 • Kink develops along stationary front • Wave forms Life Cycle of Mid-latitude Cyclone • Stage 3 • Open wave • Pressure decreases – What happens to wind? • Warm front – What is causing the precipitation ahead of the warm front? • Cold front Life Cycle of Mid-latitude Cyclone • Stage 4 • Colder air is advancing faster than warm air Life Cycle of Mid-latitude Cyclone • Stage 5 • Occlusion forms – What kind of occlusion is pictured? • Cyclone soon dissipates – Why? • What is this point called? • Warm sector removed from center of the storm Cool Air Very Cold air Life Cycle of Mid-latitude Cyclone • Stage 6 • Storm continues to dissipate and becomes stationary Mid-latitude Cyclones & the US • What is cyclogenesis? – Development of a cyclone – Bomb = cyclone with pressure decreasing by at least 24mb in 24hrs. Mid-latitude Cyclone • General path of motion is from the west to the east (B) (D) (A) (E) (C) (F) B C A D E C B A D E