Atmospheric Pressure and Wind - DOC

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					                     Chapter 5: Atmospheric Pressure and Wind

I.    The Impact of Pressure and Wind on the Landscape
      Pressure has an intimate relationship with wind: spatial variations in
      pressure are responsible for air movements
II.   The Nature of Atmospheric Pressure
      A. Intro
       1. atmospheric pressure: force exerted by the gas molecules on some
           area of Earth’s surface or on any other body
         a. pressure is omnidirectional
       2. atmospheric pressure at sea level = 14.7 pounds/in2
         a. atmospheric pressure decreases with increasing altitude
      B. Pressure, Density, and Temperature
       1. Density and Pressure
         a. density: amount of matter in a unit volume
         b. density of gas varies with location: a gas expands as far the
             environmental pressure will allow
           1) pressure a gas exerts is proportional to its density
           2) the denser the gas, the greater the pressure it exerts
         c. atmospheric pressure is directly proportional to the air density at that
           1) lower altitudes: atmospheric gas molecules are packed more
               densely because of stronger gravitational pull  more molecular
               collisions  higher pressure
           2) higher altitudes: less dense air  decrease in pressure
       2. Temperature and Pressure
         a. heated air  increase molecular agitation  increase speed 
             greater collision force  higher pressure
         b. if other conditions remain the same, an increase in gas
             temperature  increase in pressure
         c. a decrease in temperature  decrease in pressure
       3. Complications
         a. intricate relationship between atmospheric pressure, air density, and
             air temperature
         b. generalizations about surface pressure:
           1) very warm surface conditions often produce low pressure at
               the surface – a thermal low
           2) strongly rising air often produces low pressure at the surface –
               dynamic low
           3) very cold surface conditions often produce high pressure at
               the surface – thermal high
           4) strongly descending air often produces high pressure at the
               surface – a dynamic high
      C. Mapping Pressure with Isobars
       1. millibars: unit of measure of atmospheric pressure
         a. 1 bar = 1,000 millibars = 14.7 pounds/in2

Chapter 5: Atmospheric Pressure and Wind – p. 1 of 9
          b. 1 millibar = 0.0147 lb/in2
        2. barometer: instrument used to measure pressure
        3. isobar: isolines of equal pressure
        4. pressure is relative
          a. pressure is higher or lower than that of surrounding areas
          b. a ridge of high pressure: separates 2 isobars of low pressure
          c. a trough of low pressure: separates 2 isobars of high pressure
        5. air pressure readings usually adjusted to represent pressures at
            sea level
        6. pressure gradient: horizontal rate of pressure change
          a. relative closeness of isobars indicates the pressure gradient
          b. closely spaced isobars indicate steep pressure gradient
          c. direct influence on wind speed
III.   The Nature of Wind
       A. Intro
        1. atmosphere is always in motion
        2. wind: horizontal air movement
        3. updrafts and downdrafts/ascents and subsidences refer to vertical air
       B. Direction of Movement
        1. intro
          a. insolation is ultimate cause of wind
            1) uneven heating of different parts of Earth’s surface 
                temperature gradients  pressure gradients  wind
            2) wind is nature’s attempt to even out the uneven distribution of air
          b. air flows from area of high pressure to low pressure
            1) if Earth did not rotate and there were no friction
          c. direction of wind movement determined by interaction of:
            1) pressure gradient
            2) Coriolis effect
            3) friction
        2. Pressure Gradient
          a. air moves from areas of high pressure toward low pressure
          b. pressure gradient force acts at right angles to the isobars in the
              direction of the lower pressure
        3. The Coriolis Effect
          a. results from Earth’s rotation on its axis
          b. appears to deflect flowing objects to the right in the northern
              hemisphere and to the left in the southern hemisphere
          c. geostrophic wind
            1) combined effect of pressure gradient and Coriolis effect
            2) wind moves parallel to the isobars
            3) upper atmosphere
        4. Friction

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        a. frictional drag of Earth’s surface slows wind and modifies its
        b.  angle of wind flow across isobars ~ 90o
        c. lower atmosphere – impact of friction extends to only ~ 5,000’
        d. higher atmosphere most winds are geostrophic
      5. Focus: The Coriolis Effect
        a. all freely moving objects on Earth’s surface or in Earth’s
            atmosphere appear to drift sideways as a result of Earth’s
          1) can significantly influence long-range movements
        b. basics of the Coriolis effect
          1) objects are deflected to the right in the northern hemisphere; to
              the left in the southern hemisphere
          2) apparent deflection strongest at the poles; decreases
              progressively toward the equator where there is zero deflection
          3) fast moving objects deflected more than slower ones
          4) influences direction only; speed not influenced
        c. Coriolis impact on climate:
          1) all winds are affected
          2) ocean currents are deflected
            a)  cold water upwelling along continental coastlines
        d. does not affect water draining out of bathtub
     C. Cyclones and Anticyclones
      1. High-Pressure Circulation Patterns (Fig 5-8)
        a. anticyclone: high-pressure center
        b. 4 anti-cyclonic wind circulation patterns:
          1) northern hemisphere upper atmosphere: clockwise flow parallel to
              the isobars
          2) northern hemisphere lower atmosphere: divergent clockwise
              flow; air spirals out away from center of anticyclone
          3) southern hemisphere upper atmosphere: counterclockwise flow
              parallel to the isobars
          4) southern hemisphere upper atmosphere: air diverges in a
              counterclockwise direction
      2. Low-Pressure Circulation Patterns (Fig 5-8)
        a. cyclones: low pressure centers
        b. 4 anti-cyclonic wind circulation patterns:
          1) northern hemisphere upper atmosphere: counterclockwise flow
              parallel to the isobars
          2) northern hemisphere lower atmosphere: converging
              counterclockwise flow
          3) southern hemisphere upper atmosphere: clockwise flow parallel to
              the isobars
          4) southern hemisphere lower atmosphere: air converges in a
              clockwise spiral
      3. vertical component of air movement associated with pressure centers

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         a. air descends in anticyclones
           1) air sinks in center of high
           2) diverges near ground surface
         b. air rises in cyclones
           1) air converges horizontally into cyclone
           2) then rises
      D. Wind Speed
       1. wind speed is determined primarily by the pressure gradient
         a. steep gradient = strong winds
         b. gentle gradient = weak winds
         c. spacing of isobars indicates steepness of pressure gradient
       2. variations
         a. average surface winds are relatively gentle
         b. annual average North American wind speed = 6-12 knots
           1) 1 knot = 1 nautical mile/hour
           2) 1 nautical mile = 1.15 statute miles (6,076 ft)
         c. Cape Dennison, Antarctica = windiest place on Earth: annual
             average wind speed = 38 knots
         d. most persistent winds: coastal and high mountain areas
      E. Focus: Wind Chill
       1. wind chill: how cold weather feels at various combinations of low
           temperature and high wind
         a. rise in wind speed causes body heat to dissipate much more rapidly
             as the protective layer of warmer molecules surrounding the body is
             more rapidly removed
         b. no wind chill if wind ≤4 knots
       2. wind scorch: heating effect of wind on a body’s temperature when the
           air temperature > 95oF
IV.   Vertical Variations in Pressure and Wind
      A. pressure variations
       1. atmospheric pressure decreases rapidly with height
       2. pressure change is most rapid at lower altitudes
      B. wind variations
       1. wind speed usually increases with height
       2. wind moves faster above the friction layer
       3. strongest tropospheric winds
         a. intermediate levels: jet streams
         b. violent storms at Earth’s surface
V.    The General Circulation of the Atmosphere
      A. intro
       1. determinants of world climates
         a. global pattern of insolation is primary determinant
         b. general circulation pattern is principal mechanism for longitudinal and
             latitudinal heat transfer
       2. hypothetical non-rotating Earth of uniform surface (Fig 5-12)

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        a. insolational heating at equatorial region  girdle of low pressure
            around globe
        b. radiational cooling at poles  cap of high pressure at poles
        c. surface winds would flow from poles to equator
        d. convection cell: air rising at equator, descending at poles
      3. Hadley cells (Fig 5-13)
        a. vertical equatorial circulation cells
        b. gigantic convection system:
          1) warm air rises at the equator
              a)  low pressure at surface: converging, rising air
          2) rising air cools, moves poleward, descends at ~ 30oN and S
              a)  high pressure at surface: descending, diverging air
          3) pressure gradient: air flows from high to low pressure
              a) air flows from 30oN/S  equator
              b) air flows from 30oN/S  poleward
      4. 7 surface components of basic pattern of circulation from pole to
          equator; replicated north and south of equator: (Fig 5-14)
        a. polar high
        b. polar easterlies
        c. subpolar low
        d. westerlies
        e. subtropical high
        f. trade winds
        g. intertropical convergence zone
      5. subtropical latitudes of the 5 major ocean basins serve as source of
          major surface winds
     B. Subtropical Highs
      1. large semi-permanent high-pressure cells centered over each ocean
          basin at ~30o N/S
        a. 2 general high pressure ridges extend around globe at these latitudes
        b. broken up over continents, especially in summer
      2. general subsidence of air from higher altitudes
        a.  clear, warm, calm weather
        b. coincide with most of the world’s major deserts
        c. horse latitudes: becalmed sailing ships dumped horses overboard
      3. anticyclonic air circulation pattern
        a. divergent
        b. clockwise in northern hemisphere; counterclockwise in southern
        c. wind flow more pronounced on northern and southern sides than on
            eastern and western because of high pressure ridges in those
      4. source of 2 of the 3 world’s major wind systems:
        a. trade winds - equatorward
        b. westerlies - poleward
     C. Trade Winds
      1. major wind system of the tropics: between latitude 25oN and 25oS

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      2. easterlies (blow from east to west)
        a. northeast trades: northern hemisphere
        b. southeast trades: southern hemisphere
      3. most reliable of all winds – used by sailing ships/commerce
      4. trade winds and moisture
        a. originate as warm, drying winds
        b. evaporate enormous quantities of moisture over tropical oceans
        c. low-lying islands in trade wind zone: often desert
        d. some of wettest places on Earth are windward slopes in the
           trade winds – Hawaii
     D. Intertropical Convergence Zone
      1. northeast and southeast trades converge at equator
      2. position shifts seasonally
        a. follows high sun
        b. shift greater over land than sea
      3. globe-girdling zone of low pressure
        a. feeble and erratic winds
           1) doldrums – sailing ships often becalmed
        b. rising, unstable air  thunderstorms
           1) pumps enormous amount of sensible and latent heat into upper
           2)  poleward transfer of heat
        c. appears as well-defined, relatively narrow cloud band over
     E. The Westerlies
      1. great wind system of midlatitudes: between 30o and 60o N/S
      2. west winds on poleward side of subtropical highs
      3. surface westerlies less consistent and persistent than the trades
        a. surface friction
        b. topographic barriers
        c. migratory pressure systems
      4. winds aloft
        a. persistent geostrophic
        b. 2 jet streams: core of high-speed winds; 60 knots minimum speed
           1) polar jet
             a) follows meandering path located along the polar front
             b) 30,000-40,000 feet above surface
             c) located along area of greatest horizontal pressure gradient
                 (1) cold polar air to north
                 (2) warm tropical air to south
             d) Rossby waves: large north-south undulation of the upper air
                 (1) bring severe weather changes to midlatitudes
                 (2) redistribute cold air toward equator, warm air toward pole
           2) subtropical jet
             a) high in troposphere, just below tropopause

Chapter 5: Atmospheric Pressure and Wind – p. 6 of 9
               b) located over poleward margin of subsiding air of the subtropical
               c) less influence on surface weather because less temperature
                    contrast associated with it
        5. midlatitudes experience greatest short-run variability of weather as a
            result of:
          a. Rossby waves
          b. migratory pressure systems
          c. storms associated with westerly flow
      F. Polar Highs
        1. high pressure cells situated over both polar regions
        2. anticyclonic: descending air that diverges horizontally at the surface
          a. clockwise in northern hemisphere
          b. counterclockwise in southern hemisphere
      G. Polar Easterlies
        1. between polar highs and 60o N/S
        2. cold, dry and generally from east to west
      H. Subpolar Lows
        1. zone of low pressure at ~ 50o – 60o N/S
        2. often contains the polar front
          a. zone of conflict between cold polar easterlies and warmer westerlies
        3. hemispheric differences
          a. southern hemisphere: virtually continuous over cold seas surrounding
          b. northern hemisphere: interrupted by continents; more prominent in
              winter than summer
        4. rising air, cloudiness, precipitation, unsettled/stormy weather
      I. Vertical Patterns of the General Circulation
        1. biggest surface/upper level difference over tropics: antitrade winds
            result from Coriolis effect at top of Hadley cell
VI.   Modifications of the General Circulation
      A. Seasonal Variations in Location
        1. 7 surface components shift latitudinally with the changing seasons
          a. greatest displacement in low latitudes
          b. ITC can range from 25oN (Jul) to 20oS (Jan)
        2. impact of shift
          a. extend summer-like conditions farther poleward during the summer
              and winter-like conditions farther equatorward during winter
          b. significant effect in midlatitudes and their fringes
      B. Monsoons
        1. most significant disturbance of the pattern of general circulation
        2. seasonal reversal of winds
          a. onshore (sea to land) flow in summer
          b. offshore (land to sea) flow in winter
        3. results in seasonal precipitation regime
          a. heavy summer rains – result of moist maritime onshore flow

Chapter 5: Atmospheric Pressure and Wind – p. 7 of 9
        b. winter dry season – continental air dominates
      4. cause
        a. thermally induced pressure differences
        b. unusually large latitudinal migrations of trade winds and westerly
            flow, associated with jet stream behavior
      5. human impact: developing regions of extremely large populations that
          rely on precipitation for agriculture
      6. regions impacted by monsoons
        a. major monsoonal systems
          1) South Asia
          2) East Asia
        b. minor monsoonal systems
          1) Australia
          2) West Africa
        c. regions with monsoonal tendencies
          1) Central America
          2) southeastern United States
VII. Localized Wind Systems
     A. Sea and Land Breezes
      1. daytime sea breeze (onshore flow from sea to land)
      2. nighttime land breeze (flow from land to sea)
      3. convectional circulation pattern develops:
        a. day: lower pressure over land which heats more rapidly than water
        b. night: high pressure over land which cools off more quickly than
     B. Valley and Mountain Breezes
      1. daytime valley breeze: slopes warm up faster  low pressure 
          upslope flow of cooler valley air
        a. rising air  cloud formation over peaks
        b.  afternoon showers common in high country
        c. prominent in summer
      2. night mountain breeze: mountain slopes lose heat rapidly through
        a. high pressure  downslope air movement
        b. more prominent in winter
        c. winter: cold air drainage even in areas of gentle slope
     C. Katabatic Winds
      1. winds originate in cold upland areas and cascade toward lower
          elevations under influence of gravity
      2. common in Greenland and Antarctica: flow off high, cold ice sheets
      3. specialized katabatic winds
        a. mistral: France’s Rhône Valley
        b. bora: Adriatic region
        c. taku: southeastern Alaska
     D. Foehn/Chinook Winds
      1. downslope winds

Chapter 5: Atmospheric Pressure and Wind – p. 8 of 9
       2. originates when a steep pressure gradient develops
         a. high pressure on windward side of a mountain
         b. low pressure on leeward side
       3. dry, warming wind
       4. foehn: Alps
       5. chinook: Rocky Mountains
         a. the “snow-eater”
       6. Santa Anas – California
         a. high speed, high temperature, extreme dryness
         b. wildfire threat
         c. late summer/early fall

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