Discovering Physical Geography Third Edition by Alan Arbogast Chapter 6: Atmospheric Pressure, Wind, and Global Circulation Factors That Influence Air Pressure Air Pressure is the measured weight of air as it exerts pressure on Earth s surface Air pressure decreases with increasing altitude Air density is greatest near the Earth s surface Air pressure is influenced by air temperature: Warm air results in lower air pressure Cooler air results in higher air pressure Measuring and Mapping Air Pressure Measuring Air Pressure: Barometer Mapping Air Pressure: Air pressure changes with altitude Average air pressure at sea level = 1013.25 mb 1
Atmospheric Pressure Systems High-pressure system A circulating body of air that exerts relatively high pressure as air sinks toward the surface Air flow diverges Low-pressure system A circulating body of air where relatively less pressure is created as air rises away from the surface Air flow converges Atmospheric Pressure Systems Pressure systems create large-scale circulatory systems that are interconnected by airflow Advection is the process by which air flows horizontally from high-pressure to low-pressure Patterns of Atmospheric Pressure and Wind Direction Map of Atmospheric Pressure: Isobars indicate the geographic patterns of pressure systems Red arrows illustrate the path of airflow relative to pressure systems Wind Direction Winds are named for the direction in which they originate 2
The Direction of Airflow: Unequal Heating of Land Surfaces Variation in the amount of solar radiation received by latitude Air density and pressure differ from place to place Surface air flows from high to low pressure to balance the difference Convection causes motion in the atmosphere The Direction of Airflow: Pressure Gradient Force Air flows from areas of high pressure to low pressure Pressure gradient The greater the difference in pressure, the steeper the gradient The steeper the gradient, the faster the airflow The Direction of Airflow: Coriolis Force Due to Earth s rotation Deflects objects traveling in the atmosphere Earth s eastward rotation below Northern Hemisphere, deflection is to the right Southern Hemisphere, deflection is to the left 3
The Direction of Airflow: Frictional Forces Occurs at ground level Strongest at surface, diminishing at about 1500 m (5000 ft) Causes wind to slow down and move in irregular ways The Direction of Airflow: Integrating All Major Factors Pressure Gradient, Coriolis and Frictional Forces influence airflow of pressure systems Convection loops of spiraling descending and rising air are linked horizontally by advection Global Pressure and Atmospheric Circulation Unequal heating of tropics and poles drives airflow as the atmosphere balances the system Rotation and variation on Earth s surface complicates atmospheric circulation Latitudinal belts of high or low pressure and/or winds develop Convection loops develop over tropics and over midlatitudes 4
Global Pressure and Atmospheric Circulation Simplified system on an Earth that is nonrotating, nontilted, and has a uniform surface Actual system on an Earth that is rotating, tilted, and has a varied surface Global Atmospheric Circulation: Tropical Circulation Hadley Cell is the tropical convection loop Air at tropics is warmed by year-round direct sunlight Intertropical Convergence Zone (ITCZ) Warming creates a zone of low pressure at Equator as air rises into the atmosphere Winds converge into ITCZ by advection Subtropical High Pressure System (STH) Air rising from ITCZ eventually sinks at subtropics creating zones of high pressure Dry and warm winds diverge from STH Global Atmospheric Circulation: Midlatitude Circulation Ferrel Cell is the circulatory loop that mixes cool polar air with warm tropical air Polar Front is the line of contact between contrasting air masses at about 60 N/S Polar Jet Stream is formed by high-altitude winds that are formed with the temperature/pressure gradient Rossby Waves develop as undulations in the Polar Front and moderate significant temperature difference on either side 5
Global Atmospheric Circulation: Polar Circulation Polar Cell is the circulatory loop in the polar regions Polar High-Pressure System Air flowing northward from midlatitudes sinks, producing a weak high-pressure system Consists of masses of rotating, descending dry air that flows toward the Polar Front Global Atmospheric Circulation: Circulatory Loops and Wind Patterns Trade Winds ITCZ STH Westerlies and Trade Winds Westerlies and Polar Easterlies Polar Front Polar High Polar Easterlies Seasonal Migration of Pressure Systems ITCZ migrates with subsolar point, as the zone of most intense radiation and warming All large pressure systems migrate seasonally due to the consistent distance between them 6
Seasonal Migration of Pressure Systems: Monsoonal Winds Seasonal shift of the ITCZ and prevailing wind direction in the subtropics Asian monsoons Winter: ITCZ in south Cold air, high pressure Summer: ITCZ in north Warm air, low pressure Local Wind Systems: Land-Sea Breezes Sea Breeze Breeze blows from highpressure sea to lowpressure land Land Breeze Breeze blows from highpressure land to lowpressure sea Local Wind Systems: Topographic Winds Valley Breeze Breeze blows upslope as mountain slopes heat up Mountain Breeze Breeze blows downslope as mountain slopes cool off Katabatic Winds Extremely cold, dense air flows downslope under force of gravity Flow at great speeds 7
Local Wind Systems: Topographic Winds Chinook Wind Occurs when a steep pressure gradient develops in mountainous regions high pressure on windward side low pressure on leeward side Oceanic Circulation: Currents and Gyres Surface currents are driven by winds as energy transfers by friction Gyres form as continents block the movement of water Oceanic Circulation: Thermohaline Circulation Oceanic Conveyor Belt Slow vertical mix of water between layers of the ocean Downwelling currents caused by high-density water that is cooler and saltier Upwelling currents caused by low-density water that warms in tropical regions 8
Reversal of normal flow of currents and winds in tropical Pacific Occurs every 3-8 years Affects climate Changes ocean surface temperature Changes patterns of precipitation Oceanic Circulation: El Niño Human Interactions: Harnessing Wind Energy Potential wind energy varies across the United States Wind farms Collection of turbines used to harness wind power Conversion to clean usable energy Wind energy only accounts for ~1% of global and U.S. energy production 9