© 2011 pearson education, inc. the atmosphere in motion chapter 18
TRANSCRIPT
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The Atmosphere in MotionChapter 18
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Atmospheric pressure
• Force exerted by the weight of the air above in all directions
• Weight of the air at sea level (1 atm)• 14.7 lbs per in2, or• 1 kg per cm2
• Decreases with increasing altitude• Your body was built to withstand 1 atm• Units of measurement
• Millibar (mb)—standard sea level pressure is 1013.2 mb
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• Units of measurement • More common then mb is inches of mercury—
Standard sea level pressure is 29.92 in of mercury
• Instruments for measuring air pressure• 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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• Instruments for measuring• Barometer
• Aneroid barometer • “Without liquid” • Uses an expanding chamber
• Barograph - continuously records the air pressure
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Aneroid barometer
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Aneroid barograph
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Factors affecting wind
• Wind is horizontal movement of air • Out of areas of high pressure • Into areas of low pressure• Unequal heating of the Earth causes these
pressure difference
• Controls of wind• Pressure gradient force (PGF)
• Isobars—Lines of equal air pressure – Wind moves at right angles to the isobars
• Pressure gradient—Pressure change over distance that different isobars indicate
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Isobars on a weather map
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• 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 (like ocean currents, not water going down a drain)
• The stronger the wind, the larger the deflection (does not affect wind speed)
• Strongest at the poles and weakens towards the equator where it is nonexistent
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The Coriolis effect
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• Controls of wind• Friction
• Only important near the surface (below 2,000 ft) • Acts to slow the air’s movement (lowers Coriolis
effect)• Alters wind direction• Roughness of terrain determines the angle of
airflow across the isobars
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• Upper air winds • Lack of friction with the Earth’s surface allows
them to blow fast = higher Coriolis effect• Generally blow parallel to isobars— called
geostrophic winds• Jet stream
• “River” of air • High altitude• High velocity (75 to 150 mph)
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The geostrophic wind
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Surface and upper-level winds
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Highs and lows
• 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
• Associated with rising air • Often bring clouds and precipitation
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• Anticyclone• A center of high pressure• Pressure increases toward the center• 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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Surface cyclones and anticyclones
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Cyclonic and anticyclonic winds
mygeoscienceplace.com animation
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General atmospheric circulation
• Underlying cause is unequal surface heating– Tropical regions = more solar radiation
received than lost; polar regions = less solar radiation received than lost
• On a non-rotating Earth there is one atmospheric cell that redistributes the heat– Upper level air flows poleward– Surface air flows equatorward
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• On 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• Reaches to 20 - 30 degrees latitude
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• 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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• 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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• 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• Example, Asia: winter = cold = subsiding air = high
pressure system = dry wind direction off land
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Average pressure and winds for January
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Average pressure and winds for July
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The Westerlies
• Complex pattern in the midlatitudes (30-60 degrees)
• Air 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 differences• Small 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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Illustration of a valley breeze and a mountain 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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• Direction • Direction indicated by either
• Compass points (N, NE, etc.) • Scale of 0 degrees to 360 degrees
• Prevailing wind comes more often from one direction
• Speed—Often measured with a cup anemometer
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• Changes in wind direction • Associated with locations of
• Cyclones• Anticyclones
• Often bring changes in • Temperature • Moisture conditions
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Questions?