© 2007, John Wiley and Sons, Inc. Physical Geography by Alan Arbogast Chapter 6 Atmospheric Pressure, Wind, & Global Circulation Lawrence McGlinn Department of Geography State University of New York - New Paltz

© 2007, John Wiley and Sons, Inc. Air Pressure Weight of air exerting pressure on surface of Earth Air pressure decreases with altitude Warm air is lighter than cool air, so it tends to rise – cool air sinks  Vertical and horizontal pressure differences that lead to atmospheric motions.

© 2007, John Wiley and Sons, Inc. Air Pressure & Altitude Lower Density, Lower Pressure Higher Density, Higher Pressure

© 2007, John Wiley and Sons, Inc.  Pattern of pressure: a non-linear decrease with height  Why? Vertical Changes in Pressure

© 2007, John Wiley and Sons, Inc. Measuring Air Pressure Barometer measures air pressure in millibars or inches

© 2007, John Wiley and Sons, Inc. Atmospheric Pressure Systems High-pressure system -also called anticyclone -circulating body of air -descending air -clockwise circulation in northern hemisphere Low-pressure system -also called cyclone -circulating body of air -rising air -counterclockwise circ. in northern hemisphere

© 2007, John Wiley and Sons, Inc. Atmospheric Pressure Map Isobars – lines of equal air pressure on a map

© 2007, John Wiley and Sons, Inc. Direction of Air Flow Unequal heating of land surfaces Pressure gradient force – air flows from high to low Coriolis force – deflection or change in direction caused by Earth’s rotation Frictional forces – places a drag on that air flow

© 2007, John Wiley and Sons, Inc. Isobars and Pressure Gradient The closeness of isobars decides the gradient between high and low pressure The closer the isobar, the higher the winds and vice versa Winds at the surface always blow at right angles to isobars

© 2007, John Wiley and Sons, Inc. Pressure Gradient HL isobars 1028 mb1024 mb1020 mb1016 mb1012 mb1008 mb1004 mb1000 mb

© 2007, John Wiley and Sons, Inc. Pressure Gradient Force

© 2007, John Wiley and Sons, Inc. Coriolis Force Due to Earth’s rotation Pulls wind to right in northern hemisphere Pulls wind to left in southern hemisphere Strongest at poles None at equator

© 2007, John Wiley and Sons, Inc. Geostrophic Wind Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force. Low Pressure High Pressure 500 mb 504 mb 508 mb 512 mb PGF

© 2007, John Wiley and Sons, Inc. Frictional Forces Near surface, friction (F) works against pressure gradient force (PGF), so resulting wind direction is between pressure gradient force and coriolis force (CF)

© 2007, John Wiley and Sons, Inc. Three Forces Combined

© 2007, John Wiley and Sons, Inc. Rossby Waves  Great waving undulations within the westerlies flow of geostrophic winds.  I  Instrumental to the latitudinal transportation of energy.  Play an important role in determining divergence and convergence areas of the upper atmosphere.

© 2007, John Wiley and Sons, Inc. Rossby Waves Figure 6.17

© 2007, John Wiley and Sons, Inc. Jet Stream

© 2007, John Wiley and Sons, Inc.  Strong boundaries often occur between warm and cold air. In the mid-latitudes, the polar front marks this thermal discontinuity at the surface. The Polar Front and Jet Streams

© 2007, John Wiley and Sons, Inc. Physical Geography by Alan Arbogast Chapter 6 con’t Atmospheric Pressure, Wind, & Global Circulation Lawrence McGlinn Department of Geography State University of New York - New Paltz

© 2007, John Wiley and Sons, Inc. LL H H Global Pressure & Atmospheric Circulation Unequal heating of tropics and poles Global circulation on non-rotating, uniform Earth

© 2007, John Wiley and Sons, Inc. Global Circulation Model

© 2007, John Wiley and Sons, Inc. Tropical Circulation Intertropical Convergence Zone – ITCZ Zone of low pressure around equator Warmed by direct sunlight Also called Equatorial Trough Converging surface air called Trade Winds – from NE or SE Trades winds form Tropical Easterlies

© 2007, John Wiley and Sons, Inc. Tropical Circulation Subtropical High Pressure Systems (STHP) Air rising from ITCZ sinks at 25-30º N & S Warm, dry air leads to deserts This convection loop called Hadley Cell Diverging air on equatorial side of STHs drives trade winds

© 2007, John Wiley and Sons, Inc. Seasonal Changes in Circulation ITCZ migrates with subsolar point Trade winds, STHs, westerlies, all follow the ITCZ north & south with seasons

© 2007, John Wiley and Sons, Inc. Midlatitude Circulation Westerlies blow from poleward side of STHPs Polar Front separates cold air poleward from warm air equatorward Polar Jet Stream – high altitude wind driven by temp gradient – defines polar front Rossby Waves – undulations in polar front – N-S heat exchange

© 2007, John Wiley and Sons, Inc. Polar Circulation Cold, descending air over or nearly over the poles Low precipitation, low humidity, and low temperatures Forms the Polar High Polar high drives polar easterlies

© 2007, John Wiley and Sons, Inc. Seasonal Changes in Circulation Monsoon Seasonal shift of prevailing wind dir. due to land/water contrasts Happens throughout subtropical regions Most significant in South & SE Asia

© 2007, John Wiley and Sons, Inc. South Asian Monsoon Winter Monsoon Cold air over Asia Sinking air/High pressure Cool, dry NE winds ITCZ far south Summer Monsoon Warm air over Asia Rising air/Low pressure Warm, moist wind – rain ITCZ swings north

© 2007, John Wiley and Sons, Inc. Local Wind Systems Sea Breeze Sun-heated air over land rises – Low pressure Cooler air over water sinks – High pressure Land Breeze Night air over land cools fast – High pressure Air over water stays warmer – Low pressure

© 2007, John Wiley and Sons, Inc. Local Wind Systems – Mountain-Valley Breeze

© 2007, John Wiley and Sons, Inc. Oceanic Circulation Ocean currents driven by friction from wind Gyres form under high pressure areas in oceans – steered by landmasses Gulf Stream runs along western edge of STHP in N. Atlantic – warm water far north Warm, Caribbean water saltier and cooler as it flows north – Thermohaline Circulation

© 2007, John Wiley and Sons, Inc. Subtropical High-pressure Cells Figure 6.14

© 2007, John Wiley and Sons, Inc. Global Oceanic Circulation

© 2007, John Wiley and Sons, Inc. Oceanic Circulation Thermohaline Circulation – warm water in gulf stream cools and evaporates as it heads north making it heavier – downwelling, then a long trip to the Pacific where upwelling occurs

© 2007, John Wiley and Sons, Inc.ENSO El Nino Southern Oscillation Reversal of normal water flow in the Pacific Occurs every 3-8 years Underlying cause unknown Long-reaching climatic effects Called El Niño by Peruvian fishermen in honor of Christ (occurs near Christmas)

© 2007, John Wiley and Sons, Inc. El Niño NormalEl Niño