Chapter 4: Pressure and
Wind
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Pressure, Measurement, Distribution |
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Hydrostatic Balance |
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Pressure Gradient and Coriolis Force |
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Geostrophic Balance |
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Upper and Near-Surface Winds |
Slide 2
Units of Atmospheric
Pressure
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Pascal (Pa): a SI (Systeme Internationale) unit for air
pressure. |
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1 Pa = force of 1 newton acting on a surface of one square meter |
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1 hectopascal (hPa) = 1 millibar (mb)
[hecto = one hundred =100] |
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Bar: a more popular unit for air
pressure. |
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1 bar = 1000 hPa = 1000 mb |
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One atmospheric pressure = standard
value of atmospheric pressure at lea level = 1013.25 mb = 1013.25 hPa. |
Measurement of Atmos.
Pressure
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Mercury Barometers |
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Height of mercury indicates downward
force of air pressure |
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Three barometric corrections must be
made to ensure homogeneity of pressure readings |
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First corrects for elevation, the
second for air temperature (affects density of mercury), and the third
involves a slight correction for gravity with latitude |
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Aneroid Barometers |
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Use a collapsible chamber which
compresses proportionally to air pressure |
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Requires only an initial adjustment for
elevation |
Pressure Correction for
Elevation
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Pressure decreases with height. |
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Recording actual pressures may be
misleading as a result. |
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All recording stations are reduced to
sea level pressure equivalents to facilitate horizontal comparisons. |
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Near the surface, the pressure
decreases about 100mb by moving 1km higher in elevation. |
Slide 6
Isobar
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It is useful to examine horizontal
pressure differences across space. |
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Pressure maps depict isobars, lines of
equal pressure. |
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Through analysis of isobaric charts,
pressure gradients are apparent. |
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Steep (weak) pressure gradients are
indicated by closely (widely) spaced isobars. |
Slide 8
Slide 9
Pressure Gradients
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Pressure Gradients |
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The pressure gradient force initiates
movement of atmospheric mass, wind, from areas of higher to areas of lower
pressure |
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Horizontal Pressure Gradients |
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Typically only small gradients exist
across large spatial scales (1mb/100km) |
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Smaller scale weather features, such as
hurricanes and tornadoes, display larger pressure gradients across small
areas (1mb/6km) |
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Vertical Pressure Gradients |
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Average vertical pressure gradients are
usually greater than extreme examples of horizontal pressure gradients as
pressure always decreases with altitude (1mb/10m) |
Hydrostatic Balance in
the Vertical
What Does Hydrostatic
Balance Tell Us?
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The hydrostatic equation tells us how quickly air pressure drops
wit height. |
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čThe rate at which air pressure
decreases with height (DP/ Dz) is equal to the air density (r) times the
acceleration of gravity (g) |
The Ideal Gas Law
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An equation of state describes the
relationship among pressure, temperature, and density of any material. |
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All gases are found to follow
approximately the same equation of state, which is referred to as the “ideal
gas law (equation)”. |
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Atmospheric gases, whether considered
individually or as a mixture, obey the following ideal gas equation: |
Hydrostatic Balance and
Atmospheric Vertical Structure
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Since P= rRT (the ideal gas law),
the hydrostatic equation becomes: |
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dP = -P/RT x
gdz |
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č dP/P = -g/RT x dz |
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P = Ps exp(-gz/RT) |
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P = Ps exp(-z/H) |
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The atmospheric pressure decreases
exponentially with height |
Temperature and Pressure
Wind Changes with Height
Pressure Gradient Force
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PG = (pressure difference) / distance |
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Pressure gradient force goes from high
pressure to low pressure. |
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Closely spaced isobars on a weather map indicate steep pressure
gradient. |
Balance of Force in the
Horizontal
Force that Determines
Wind
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Pressure gradient force |
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Coriolis force (Earth’s Rotation) |
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Friction (near Earth’s surface) |
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Centrifugal force |
Coriolis Force
Coriolis Force
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Coriolis force causes the wind to deflect to the right of its
intent path in the Northern Hemisphere and to the left in the Southern
Hemisphere. |
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The magnitude of Coriolis force depends
on (1) the rotation of the Earth, (2) the speed of the moving object, and (3) its latitudinal location. |
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The stronger the speed (such as wind
speed), the stronger the Coriolis force. |
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The higher the latitude, the stronger
the Coriolis force. |
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The Corioils force is zero at the
equator. |
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Coriolis force is one major factor that
determine weather pattern. |
Coriolis Force Change
with latitudes
Upper Atmospheric Winds
Geostrophic Balance
Upper Atmosphere
Geostrophic Flow
Slide 26
Frictional Force
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A force of opposition which slows air
in motion. |
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Initiated at the surface and extend,
decreasingly, aloft. |
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Important for air within 1.5 km (1 mi)
of the surface, the planetary boundary layer. |
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Because friction reduces wind speed it
also reduces Coriolis deflection. |
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Friction above 1.5 km is negligible. |
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Above 1.5 km = the free atmosphere. |
Surface Winds
Surface Geostrophic Flow
Slide 30
Slide 31
Centrifugal Force
Gradient Wind Balance
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The three-way balance of horizontal pressure gradient, Coriolis
force, and the centrifugal force is call the gradient wind balance. |
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The gradient wind is an excellent approximation to the actual
wind observed above the Earth’s surface, especially at the middle latitudes. |
Super- and
Sub-Geostrophic Wind
Slide 35
Measuring Winds
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Wind direction always indicates the
direction from which wind blows. |
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An aerovane indicates both wind speed
and direction. |