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Air Pressure and Winds I
Review: precipitation types
Sample weather map (Fig. 13.11)
Fig. 11.18
Snow
Drizzle
Sleet
Freezing
rain
Fog
Atmospheric pressure P
Atmospheric pressure and density decrease with altitude exponentially!!!
force weight of the air
P

area
area
Units: 1 bar=1000 mbar
1 Standard atmosphere: 1013 mbar
Ideal Gas Law
• temperature, and the density of an ideal gas.
A relationship between the pressure, the
•
Ideal gas: a simplified physical model for a gas. It
neglects:
♦ the volume of the individual molecules
♦ the interaction between the molecules
• the air at room temperature.
The ideal gas model is a very good approximation for
Ideal Gas Law
•
•
P  T  r C
The pressure P of an ideal gas is proportional to its
temperature T and density r. C is a constant of
proportionality – air gas constant.
Examples:
♦ T increases, r constant -> P increases (tea kettle)
♦ r increases, T constant -> P increases (blow a balloon)
♦ T decreases, r decreases -> P decreases (climb a
mountain)
♦ P constant -> T increases, r decreases (example in the
book: Fig. 8.2 (a) and (b))
Simple model of
atmospheric pressure
• Column of air molecules
•
•
•
Assumptions:
♦ Constant density
♦ Constant width
Atmospheric pressure P is
simply due to the weight of
the column.
P decreases with height
because there are less
molecules remaining above.
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From high to low pressure
Equal surface pressures in cities 1 and 2 result from
♦ Cold dense air in city 1
♦ Warm, less dense air in city 2
At higher altitudes the pressures are different (L vs H)
The air flow (due to the pressure gradient force) is from
High to Low -> expect to see the pressure dropping as the
air temperature increases
P  T  r C
Daily pressure variations
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•
How do we measure pressure?
Mercury (Hg) barometer.
The weight of the Hg column is balanced
by the weight of the atmosphere above
the open air surface.
•
• Can we measure the atmospheric
1 atmosphere = 76 cm.Hg = 29.92 in.Hg
pressure with a water barometer?
Altitude Corrections
• Pressure decreases with height.
• Altitude adjustment:
•
•
♦ Why: to compare pressure
readings from stations at
different altitudes.
♦ Convert all P readings to the
pressure at the Mean Sea
Level: sea-level pressure.
♦ For every 100 m add 10 mbar
♦ This is a rough correction.
Sea-level pressure chart
Height surface: surface of
constant height
♦ Pressure maps on constant
height surfaces show the
horizontal variation of the
pressure -> isobars
•
•
Sea-level pressure chart
Elements: isobars, high (H) and low (L) pressure regions
It is an example of a constant height chart (sea-level)
Constant height charts
• Pressure
variations are
plotted at a fixed altitude
• Atforhigher
altitudes, no need
altitude correction: what
you measure is what you plot
• Typical
values for the
atmospheric pressure at
various altitudes
♦ Sea-level: 1000 mb
♦ 3 km: 700 mb
♦ 5.6 km: 500 mb
Isobaric charts
• Constant
height chart: we fix the altitude and plot the pressure: the
map shows lines of constant pressure (isobars).
• Isobaric
chart: we fix the pressure and plot the altitude where it is
found: the map shows lines of constant height (contour lines).
• High pressure <-> High height on the isobaric chart
• Low pressure <-> Low height on the isobaric chart
The two types of pressure charts
• Surface map (constant height chart)
•
♦ Anticyclones (H) – centers of high pressure
♦ Cyclones (L) – centers of low pressure
Upper-air chart (isobaric chart)
♦ Pressure contour lines are parallel to the isotherms
♦ Winds flow parallel to the pressure contour lines
•
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Flying on a constant pressure surface
Airplanes measure altitude based on pressure readings
They move on constant pressure surfaces
•
High to Low, Look Out Below
This is a problem when T changes. The altimeter needs to be
calibrated often with actual altitude measurements.