© 2011 pearson education, inc. the atmosphere in motion chapter 18

© 2011 Pearson Education, Inc.

The Atmosphere in MotionChapter 18


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


• 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


A mercury barometer


• Instruments for measuring• Barometer

• Aneroid barometer • “Without liquid” • Uses an expanding chamber

• Barograph - continuously records the air pressure


Aneroid barometer


Aneroid barograph


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


Isobars on a weather map


• 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


The Coriolis effect


• 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


• 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)


The geostrophic wind


Surface and upper-level winds


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


• 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


Surface cyclones and anticyclones


Cyclonic and anticyclonic winds

mygeoscienceplace.com animation


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


• 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


• 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


• 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


Idealized global 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• Example, Asia: winter = cold = subsiding air = high

pressure system = dry wind direction off land


Average pressure and winds for January


Average pressure and winds for July


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


Local winds

• Produced from temperature differences• Small scale winds • Types

• Land and sea breezes• Mountain and valley breezes• Chinook and Santa Ana winds


Illustration of a sea breeze and a land breeze


Illustration of a valley breeze and a mountain breeze


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


• 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


• Changes in wind direction • Associated with locations of

• Cyclones• Anticyclones

• Often bring changes in • Temperature • Moisture conditions


Questions?

© 2011 pearson education, inc. the atmosphere in motion chapter 18

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