Ahrens 9th Chapter8
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Transcript of Ahrens 9th Chapter8
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Chapter 8
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Atmospheric Pressure
What causes air pressure to change in
the horizontal?
Why does the air pressure change at the
surface?
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Atmospheric Pressure
Horizontal Pressure Variations
It takes a shorter column of dense, cold air
to exert the same pressure as a taller
column of less dense, warm air Warm air aloft is normally associated with
high atmospheric pressure and cold air aloft
with low atmospheric pressure
At surface, horizontal difference intemperature = horizontal pressure in
pressure = wind
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Atmospheric Pressure
Daily Pressure Variations
Thermal tides in the tropics
Mid-latitude pressure variation driven by
transitory pressure cells
Pressure Measurements
Barometer, barometric pressure
Standard atmospheric pressure 1013.25mbAneroid barometers
Altimeter, barograph
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Atmospheric Pressure
Pressure Readings
Temperature correction: the mercury
barometer is also a thermometer
Altitude corrections: high altitude addpressure, ~10mb/100m above sea level
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Surface and Upper Level
Charts Sea-level pressure chart: constant
height
Upper level or isobaric chart: constant
pressure surface (i.e. 500mb)
High heights correspond to higher than
normal pressures at a given latitude and
vice versa
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Surface and Upper Level
Charts Observation: Constant Pressure Surface
Pressure altimeter in an airplane causes
path along constant pressure not elevation
May cause sudden drop in elevation Radio altimeter offers constant elevation
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Forces that Influence Winds
Pressure Gradient Force: difference in
pressure over distance
Directed perpendicular to isobars from high
to low. Large change in pressure over s short
distance is a strong pressure gradient and
vice versa.
The force that causes the wind to blow.
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Forces that Influence Winds
Coriolis Force
Apparent deflection due to rotation of the
Earth
Right in northern hemisphere and left insouthern hemisphere
Stronger wind = greater deflection
No Coriolis effect at the equator greatest at
poles.
Only influence direction, not speed
Only has significant impact over long
distances
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Forces that Influence Winds
Geostrophic Winds
Earth turning winds
Travel parallel to isobars
Spacing of isobars indicates speed; close =fast, spread out = slow
Topic: Math & Geostrophic Winds
Vg = 1 x p
f d
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Stepped ArtFig. 8-29, p. 214
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Forces that Influence Winds
Winds on Upper-level Charts
Winds parallel to contour lines and flow west
to east
Heights decrease from north to south
Surface Winds
Friction reduces the wind speed which in
turn decrease the Coriolis effect.
Winds cross the isobars at about 30 into
low pressure and out of high pressure
Buys-Ballots Law
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Winds and Vertical Motion
Replacement of lateral spreading of air
results in the rise of air over a low
pressure and subsidence over highpressure
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