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Air Pressure and Winds IPowerPoint Presentation

Air Pressure and Winds I

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Atmospheric pressure P

Atmospheric pressure and density decrease with altitude exponentially!!!

Units: 1 bar=1000 mbar

1 Standard atmosphere: 1013 mbar

Ideal Gas Law

- A relationship between the pressure, the temperature, and the density of an ideal gas.
- Ideal gas: a simplified physical model for a gas. It neglects:
- the volume of the individual molecules
- the interaction between the molecules

- The ideal gas model is a very good approximation for the air at room temperature.

Ideal Gas Law

- 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.

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

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.

- 1 atmosphere = 76 cm.Hg = 29.92 in.Hg
- Can we measure the atmospheric 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)

Pressure variations are plotted at a fixed altitude

At higher altitudes, no need for 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

Constant height chartsIsobaric 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

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.

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