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

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Fig. 11.18

Snow

Drizzle

Sleet

Freezing rain

Fog


Atmospheric pressure p
Atmospheric pressure P

Atmospheric pressure and density decrease with altitude exponentially!!!

Units: 1 bar=1000 mbar

1 Standard atmosphere: 1013 mbar


Ideal gas law
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 law1
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
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
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
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
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
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

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


Isobaric charts
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
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
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
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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