lecture9 oct9-bb
TRANSCRIPT
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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