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Bay Area Earth Science Institute (BAESI)

Pressure

San Jose State University, January 24, 2004

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

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Principle forcesPrinciple forces

Q: What drives the weather in the atmosphere?Q: What drives the weather in the atmosphere?

Q: What causes the weather to change?Q: What causes the weather to change?

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

pressure and pressure gradients, pressure and pressure gradients, gravitygravity rotation of the earthrotation of the earth friction friction

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PressurePressure

Measured in millibars Measured in millibars (mb) or inches of (mb) or inches of mercurymercury

Surface Pressure:Surface Pressure:

Sea level pressure:Sea level pressure:1012 mb or1012 mb or29.9 in of mercury29.9 in of mercury

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What causes pressure to What causes pressure to change?change? Temperature changeTemperature change Density changeDensity change

Formally, the equation of state describes this Formally, the equation of state describes this relationship:relationship:

p=p=RTRT

p-Pressure (Pa)p-Pressure (Pa) - density (g m- density (g m-3-3))R - Gas Constant (287 J KR - Gas Constant (287 J K-1-1 kg kg-1-1))T - Temperature (K)T - Temperature (K)

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Pressure DifferencesPressure Differences

Determines the direction and speed of windsDetermines the direction and speed of winds

Can help explain general circulation of Can help explain general circulation of atmosphere.atmosphere.

General guidelines:General guidelines:– High pressure: clear skies/fine weatherHigh pressure: clear skies/fine weather– Low pressure: cloudy skies/changeable Low pressure: cloudy skies/changeable

weatherweather

Why is this?Why is this?

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Pressure Gradient ForcePressure Gradient Force

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Coriolis Force

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The rotation of the EarthThe rotation of the Earth

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

The Earth’s rotation causesThe Earth’s rotation causes– Rotational movement that deflects the winds.Rotational movement that deflects the winds.

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

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Coriolis ForceCoriolis 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.

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Geostrophic WindGeostrophic Wind

Northern Hemisphere