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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 |
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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. |
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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 |
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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. |
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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. |
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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) |
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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) |
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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: |
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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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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 |
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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. |
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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 |
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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. |
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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. |
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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. |
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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. |
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