Air Pressure and Winds I

Review: precipitation types

Sample weather map (Fig. 13.11)

Fog

Sleet

Snow

Drizzle

Freezing rain

Fig. 11.18

Atmospheric pressure PAtmospheric pressure and density decrease with altitude exponentially!!!

area

airtheofweight

area

forceP

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 . C is a constant of proportionality – air gas constant.

•Examples:♦ T increases, constant -> P increases (tea kettle)♦ increases, T constant -> P increases (blow a

balloon)♦ T decreases, decreases -> P decreases (climb a

mountain)♦ P constant -> T increases, decreases (example in

the book: Fig. 8.2 (a) and (b))

CTP

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

CTP

Daily pressure variations

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)

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

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

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.