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# Air Pressure and Winds I - PowerPoint PPT Presentation

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!!!. Units: 1 bar=1000 mbar.

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Snow

Drizzle

Sleet

Freezing rain

Fog

Atmospheric pressure and density decrease with altitude exponentially!!!

Units: 1 bar=1000 mbar

1 Standard atmosphere: 1013 mbar

• 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.

• 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))

• 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.

• 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

• 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?

• 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

• Elements: isobars, high (H) and low (L) pressure regions

• It is an example of a constant height chart (sea-level)

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 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

• 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

• Airplanes measure altitude based on pressure readings

• They move on constant pressure surfaces

• This is a problem when T changes. The altimeter needs to be calibrated often with actual altitude measurements.