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Air Pressure & Wind


    1.    Define air pressure.

    2.    Describe how changes in elevation, temperature, and humidity affect air pressure.

    3.    Explain what makes the wind blow.

Key Terms:

    air pressure        isobar        high-pressure area        low-pressure area        pressure gradient

Notes: (19-1)

We are constantly surrounded and buried in gas molecules.  They allow us to move freely through them while constantly filling in behind us.  We breath them in and out and use them to make energy within our bodies.  They do have mass and can create widespread damage if moving at a high enough velocity.  Below are some basic properties of gas molecules.

Gases have mass
Gases are compressible
Gases will fill the space available to them
Gases diffuse through each other very rapidly
Gases exert pressure
The pressure of a gas is directly related to its temperature - (high temp = high pressure)

Atmospheric Pressure

Atmospheric (air) pressure is the result of the mass of the molecules of gas and the gravity of the Earth.  

(library.tedankara.k12.tr/chemistry/ vol1/physics/trans60.jpg)

Pressure of gas at sea level is 760 mmHg - 1atm
The millibar is used by the National Weather Service - (1013 millibars = 1 atm at sea level)
Air pressure decreases with altitude
Can be measured using a mercury barometer

Mercury barometer(www.bom.gov.au/info/ftweather/ images/mercury.gif)

The atmospheric pressure acts on the mercury reservoir causing the mercury to rise in the tube.

Atmospheric pressure can also be measured using a aneroid manometer

HM79 Nylon cuff aneroid (www.europa-med.com/acatalog/ 08.347.00%20Aneroid%20palm%...)

Changes in Air Pressure in the Atmosphere

Changes in pressure systems occur constantly in our dynamically changing atmosphere.  Pressure changes drive the wind and establish weather systems.  General rules governing pressure systems.

Elevation - changes the amount of air molecules above the Earth
An increase in temperature decreases the atmospheric pressure - Air expands and moves up and away from the pressure system
Cold winds blowing into an area increases the pressure - adds more air 
An increase in humidity decreases the pressure - water is lighter than the gases in the air
Isobars are lines that connect areas of equal pressure on a weather map - the difference between two different isobars is called a pressure gradient
pressure gradients are used by meteorologists to predict weather

(vortex.plymouth.edu/dstreme/ images/sfc_map_00.gif)




Winds are created by the pressure gradients in the atmosphere - the greater the pressure change the greater the wind.

direction is measured us using a wind vein
speed is measured using an anemometer


weather station(weather.carleton.edu/instruments/ weather.jpg)





Last modified: March 24, 2003