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Bay Area Earth Science Institute (BAESI). Pressure San Jose State University, January 24, 2004 http://www.met.sjsu.edu/baesi/. *. Q: What drives the weather in the atmosphere? Q: What causes the weather to change? We must analyse the predominant forces in the atmosphere, namely:

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Bay area earth science institute baesi l.jpg
Bay Area Earth Science Institute (BAESI)

Pressure

San Jose State University,

January 24, 2004

http://www.met.sjsu.edu/baesi/

Weather and Climate



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*


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Q: What drives the weather in the atmosphere?

Q: What causes the weather to change?

We must analyse the predominant forces in the atmosphere, namely:

pressure and pressure gradients,

gravity

rotation of the earth

friction

Principle forces

Weather and Climate


Pressure l.jpg
Pressure

  • Measured in millibars (mb) or inches of mercury

  • Surface Pressure:

Sea level pressure:

1012 mb or

29.9 in of mercury

Weather and Climate


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Temperature change

Density change

Formally, the equation of state describes this relationship:

p=RT

p-Pressure (Pa)

 - density (g m-3)

R - Gas Constant (287 J K-1 kg-1)

T - Temperature (K)

What causes pressure to change?

Weather and Climate


Pressure differences l.jpg

Determines the direction and speed of winds

Can help explain general circulation of atmosphere.

General guidelines:

High pressure: clear skies/fine weather

Low pressure: cloudy skies/changeable weather

Why is this?

Pressure Differences

Weather and Climate




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Pressure Gradient Force

Weather and Climate


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Coriolis Force

Weather and Climate


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The rotation of the Earth

  • Rockets, migrating birds, and large scale weather systems are all deflected due to the rotation of the Earth.

  • The Earth’s rotation causes

    • Rotational movement that deflects the winds.

  • The Coriolis Force is the name of this rotational force that deflects motion.

Weather and Climate


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Coriolis Force

  • Affects direction, not speed of object

  • Maximum at the poles

  • Zero at the equator

Always acts to the right of the motion in Northern Hemisphere.

Always acts to the left of the motion in the Southern Hemisphere.

Weather and Climate


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Geostrophic Wind

Northern

Hemisphere

Weather and Climate