ahrens 9th chapter8

7/28/2019 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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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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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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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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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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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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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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ahrens 9th chapter8

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