Pressure Air 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!!!
P
force area
weight of the air area
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 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.
neglects:
Ideal Gas Law
temperature T and density
P T
C
The pressure P of an ideal gas is proportional to its . 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))
Assumptions:
Simple model of atmospheric pressure
Column of air molecules 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
P T
From high to low pressure
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
C
Daily pressure variations
The weight of the Hg column is balanced
Mercury (Hg) barometer.
How do we measure pressure?
Can we measure the atmospheric
by the weight of the atmosphere above the open air surface.
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
Elements: isobars, high (H) and low (L) pressure regions
It is an example of a constant height chart (sea-level)
Sea-level pressure chart
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
They move on constant pressure surfaces
Flying on a constant pressure surface
Airplanes measure altitude based on pressure readings
calibrated often with actual altitude measurements.
High to Low, Look Out Below
This is a problem when T changes. The altimeter needs to be