The Atmosphere in Motion Ch 19'

1
The Atmosphere in MotionCh 19.

• What is air pressure and what causes wind?
• How does surface friction and the Earths
  rotation affect the wind?
• How does the Earths rotation affect the global
  wind patterns?
• How do continents, seasons, and topography affect
  wind?

2
The air is made up of molecules.
What type of molecules?
3
Like all matter, air molecules have mass or
weight.
1.2 kg/m3
4
Gravity pulls the air molecules toward the earth,
giving them weight. The weight of the air
molecules all around us is called the air
pressure.
5
Air pressure is equal in all directions.
Pressure force per unit area
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Low altitudes higher pressure
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A Barometer is used to measure air pressure
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Torricellis barometer used a glass column
suspended in a bowl of mercury. The pressure of
the air molecules pushed the mercury up into the
glass tube.
The weight of the mercury in the tube was equal
to the weight of the air pressing down on the
mercury in the dish.
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As atmospheric pressure increases
The mercury in the tube rises.
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The Mercury Barometer
Good
Bad

• Simple to construct
• Highly accurate

• Glass tube is fragile
• Mercury is very toxic!

Is there a better way to measure air pressure?
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The Aneroid Barometer!!

• No fragile tubes!
• No toxic chemicals.
• Small and portable

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An aneroid barometer uses a cell which has had
most of the air removed.
As the air pressure around the cell increases, it
presses on the cell, which causes the needle to
move.
Television weather forecasters usually give
barometric pressure in inches of mercury(the
height of a mercury barometer). However,
meteorologists measure atmospheric pressure in
millibars.
13
Since pressure changes with altitude
how does changing altitude affect a barometer?
14
Two types of barometric pressure measurements
Station pressure is the actual pressure at the
recording location. It is affected by the local
altitude.
Sea level pressure is referenced to sea level, so
it has the same altitude anywhere in the world.
15
Most aneroid barometers have a needle which can
be set to remember the previous reading.
Why would this be important?
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Changing Pressure
A rising barometer increasing air
pressure. This usually means nice weather or
improving weather. Clear skies, dry weather. Not
necessarily warm.
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Changing Pressure
A falling barometer decreasing air
pressure. This usually means poor weather,
higher humidity, clouds, possibly rain, windy.