Lecture 9

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Wind Pressure gradient force

Weather map

WindAtmospheric

Motion

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Atmospheric pressureQ: what produces wind?

wind is produced by pressure difference in horizontal~10 mb/100 kmrecall vertical variations of pressure

~1000 mb/12 km

Q: what are two primary factors affecting atmospheric pressure?

DensityTempera

ture

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Idea gas law

Pressure ~ density * Temperature

by increasing/decreasing density

by increasing/decreasing temperature

Q: So, from the ideal gas law, how can change the pressure of a gas?

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What is this?

This is the model of a person, but not real.

We are to use a model to understand how an atmospheric circulation is produced.

So far as the laws of mathematics refer to reality, they are not certain.

And so far as they are certain, they do not refer to reality.

-- Albert Einstein

Wind is produced by pressure differenceHow? play a model

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Assume two columns have same density, same temperature, …

Air pressure = total air weight per unit area of the atmospheric column above z

0P

1P

0 1 ?P P

P0=total air weight

P1=total air weight

0 1P P

Air pressure = total air weight per unit area of the atmospheric column above z

0P

1P

2P

3P

4P

0 1 2 3 4P P P P P

Pressure decreases with increasing of height

Mount Everest 8,848 m

Wind is produced by pressure differenceHow? play a model

Q: What happens if the column 1 cools and column 2 warms?

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Fig. 6-2, p. 143

Q: At which location (A or B), pressure

will be higher?

A BQ: As a result, will air move

from the cold to warm column or from the warm to cold column?

Wind is produced by pressure difference

How? play a model

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Fig. 6-2, p. 143

air moves from the warm column to cold

column due to pressure difference

Rulesurface pressure is high as the air sinks above the

surface, and surface pressure is low as the air rises above the surface

Wind is produced by pressure difference

How? play a model

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RuleCool surface air sinking high surface pressure

Warm surface air rising low surface pressure

Wind is blowing from high pressure to low pressure

CoolHigh P

WarmLow P

Wind

Where is clear sky?

Surface pressure chartisobars (lines of constant pressure)

are plotted every 4 mbQ: how is a pressure chart created from observations around the country?

a problem is that not all stations are at sea level

Q: how can we correct this problem?

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Surface pressure chart

Rulein the lower part of atmosphere,

pressure changes about 10 mb for every 100 meters of

elevation changes

use this rule, we adjust all pressure

measurements to sea level,

producing a constant

elevation sea-level pressure

chart, commonly refer to surface

weather map

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Tropopause height varies with latitude

Tropopause height is proportional to mean tropospheric temperature

200mb

200mb

Q: what percent of air is below 200mb level?1000 200 800

100% 100% 80%1000 1000

500 mb

700 mb 500 mb

700 mb

Q: What would the pressure change from EQ to NP at level of Z

= 5km?

5 km

pressure would decrease as you move northward

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Isobaric Charts 500mb level 17

Each contour line tells us the elevation above sea level at which pressure is 500 mb Q: what is difference between isobar and contour line?

Isobaric Charts 500mb level

Wavelike patterns of surface chart

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Isobaric Charts

Q: where is the ridge? Where is the trough?Ridge Trough

Warm air aloft is associated with

a ridge.

Cold air aloft is associated with

a trough.

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Ridges and Troughs

N

ERidge

Warm air aloft

TroughCold air

aloft

Ris

ing

air,

Clou

ds, P

reci

p.

Sinking air,

Clearing skies

Isobaric Charts

The table below gives the “approximate” altitudes of common isobaric charts used in meteorology

Isobaric Surface (mb) Approximate Elevation (m)

1000 120

850 1,460

700 3,000

500 5,600

300 9,180

200 11,800

100 16,200

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