Download - Earth Science, 10e
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Earth Science, 10e
Edward J. Tarbuck & Frederick K. Lutgens
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Air Pressure and WindChapter 17
Earth Science, 10e Stan Hatfield and Ken Pinzke
Southwestern Illinois College
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Atmospheric pressure
Force exerted by the weight of the air aboveWeight of the air at sea level
• 14.7 pounds per square inch • 1 kilogram per square centimeter
Decreases with increasing altitudeUnits of measurement
• Millibar (mb) – standard sea level pressure is 1013.2 mb
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Atmospheric pressure
Units of measurement • Inches of mercury – standard sea level pressure
is 29.92 inches of mercuryInstruments for measuring
• Barometer • Mercury barometer
• Invented by Torricelli in 1643 • Uses a glass tube filled with mercury
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A mercury barometer
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Atmospheric pressure
Instruments for measuring• Barometer
• Aneroid barometer • "Without liquid" • Uses an expanding chamber
• Barograph (continuously records the air pressure)
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A recording aneroid barometer
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Wind
Horizontal movement of air • Out of areas of high pressure • Into areas of low pressure
Controls of wind• Pressure gradient force
• Isobars – lines of equal air pressure • Pressure gradient – pressure change over distance
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A weather map showing isobars and wind speed/direction
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Wind
Controls of wind• Coriolis effect
• Apparent deflection in the wind direction due to Earth's rotation
• Deflection is the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Friction • Only important near the surface • Acts to slow the air's movement
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The Coriolis effect
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Wind
Upper air winds • Generally blow parallel to isobars – called
geostrophic winds• Jet stream
• "River" of air • High altitude• High velocity (120-240) kilometers per hour
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The geostrophic wind
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Comparison between upper-level winds and surface winds
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Cyclones and anticyclones Cyclone
• A center of low pressure • Pressure decreases toward the center • Winds associated with a cyclone
• In the Northern Hemisphere • Inward (convergence)• Counterclockwise
• In the Southern Hemisphere • Inward (convergence)• Clockwise
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Cyclones and anticyclones Cyclone
• Associated with rising air • Often bring clouds and precipitation
Anticyclone • A center of high pressure• Pressure increases toward the center
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Cyclones and anticyclones Anticyclone
• Winds associated with an anticyclone• In the Northern Hemisphere
• Outward (divergence) • Clockwise
• In the Southern Hemisphere • Outward (divergence) • Counterclockwise
• Associated with subsiding air • Usually bring "fair" weather
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Cyclonic and anticyclonic winds in the Northern Hemisphere
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Airflow associated with surface cyclones and anticyclones
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General atmospheric circulation Underlying cause is unequal surface heatingOn the rotating Earth there are three pairs
of atmospheric cells that redistribute the heat
Idealized global circulation • Equatorial low pressure zone
• Rising air• Abundant precipitation
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General atmospheric circulation Idealized global circulation
• Subtropical high pressure zone • Subsiding, stable, dry air• Near 30 degrees latitude • Location of great deserts• Air traveling equatorward from the subtropical high
produces the trade winds • Air traveling poleward from the subtropical high
produces the westerly winds
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General atmospheric circulation Idealized global circulation
• Subpolar low pressure zone • Warm and cool winds interact • Polar front – an area of storms
• Polar high pressure zone • Cold, subsiding air • Air spreads equatorward and produces polar easterly
winds • Polar easterlies collide with the westerlies along the
polar front
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Idealized global circulation
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General atmospheric circulation Influence of continents
• Seasonal temperature differences disrupt the• Global pressure patterns • Global wind patterns
• Influence is most obvious in the Northern Hemisphere
• Monsoon• Seasonal change in wind direction
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Average surface pressure and associated winds for January
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Average surface pressure and associated winds for July
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General atmospheric circulation Influence of continents
• Monsoon• Occur over continents
• During warm months
• Air flows onto land • Warm, moist air from the ocean
• Winter months • Air flows off the land • Dry, continental air
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Circulation in the mid-latitudes The zone of the westerliesComplexAir flow is interrupted by cyclones
• Cells move west to east in the Northern Hemisphere
• Create anticyclonic and cyclonic flow • Paths of the cyclones and anticyclones are
associated with the upper-level airflow
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Local winds Produced from temperature differencesSmall scale winds Types
• Land and sea breezes• Mountain and valley breezes• Chinook and Santa Ana winds
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Illustration of a sea breeze and a land breeze
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Wind measurement Two basic measurements
• Direction• Speed
Direction • Winds are labeled from where they originate
(e.g., North wind – blows from the north toward the south)
• Instrument for measuring wind direction is the wind vane
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Wind measurement Direction
• Direction indicated by either• Compass points (N, NE, etc.) • Scale of 0º to 360º
• Prevailing wind comes more often from one direction
Speed – often measured with a cup anemometer
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Wind measurement Changes in wind direction
• Associated with locations of • Cyclones• Anticyclones
• Often bring changes in • Temperature • Moisture conditions
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El Niño and La Niña El Niño
• A countercurrent that flows southward along the coasts of Ecuador and Peru
• Warm• Usually appears during the Christmas season • Blocks upwelling of colder, nutrient-filled water,
and anchovies starve from lack of food • Strongest El Niño events on record occurred
between 1982-83 and 1997-98
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El Niño and La Niña El Niño
• 1997-98 event caused • Heavy rains in Ecuador and Peru• Ferocious storms in California
• Related to large-scale atmospheric circulation • Pressure changed between the eastern and western
Pacific called the Southern Oscillation • Changes in trade winds creates a major change in
the equatorial current system, with warm water flowing eastward
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Normal conditions
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El Niño
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El Niño and La Niña El Niño
• Effects are highly variable depending in part on the temperatures and size of the warm water pools
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El Niño and La Niña La Niña
• Opposite of El Niño • Triggered by colder than average surface
temperatures in the eastern Pacific • Typical La Niña winter
• Blows colder than normal air over the Pacific Northwest and northern Great Plains while warming much of the rest of the United States
• Greater precipitation is expected in the Northwest
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El Niño and La Niña Events associated with El Niño and La Niña
are now understood to have a significant influence on the state of weather and climate almost everywhere
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Global distribution of precipitation
Relatively complex pattern Related to global wind and pressure
patterns • High pressure regions
• Subsiding air• Divergent winds• Dry conditions • e.g., Sahara and Kalahari deserts
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Global distribution of precipitation
Related to global wind and pressure patterns • Low pressure regions
• Ascending air • Converging winds • Ample precipitation • e.g., Amazon and Congo basins
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Average annual precipitation in millimeters
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Global distribution of precipitation
Related to distribution of land and water • Large landmasses in the middle latitudes often
have less precipitation toward their centers• Mountain barriers also alter precipitation
patterns • Windward slopes receive abundant rainfall from
orographic lifting • Leeward slopes are usually deficient in moisture
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End of Chapter 17