Wx Theory Part 1

• All wx is the result of uneven heating/cooling of the Earth’s surface

Composition of the Atmosphere

• 78% Nitrogen

• 21% Oxygen

• 1% other gases

Water vapor in the Atmosphere

• varies from 0% to 4% by volume

Layers of the atmosphere (bottom —> up)

• Troposphere

• Tropopause

• Stratosphere

• Mesosphere

• Thermosphere

Troposphere

• from sea level up to 20,000’ over the poles + up to 56,000’ over the equator

• Majority of the wx, clouds, storms occur here

• Temperature lapse rate:

  • decreases ~2*C per 1,000’ gained

• Pressure lapse rate:

  • decreases ~1” Hg per 1,000’ gained

Tropopause

• altitude: varies w/ latitude and season — elliptical shape

• boundary of the troposphere

  • traps moisture and associated wx in the troposphere

• commonly associated w/ the location of the jet stream and possible clear air turbulence

• abrupt change in temp

Stratosphere

• altitude: from tropopause to ~160,000’

• little wx exists here

• air remains stable although certain types of clouds extend into this layer

• Temp: increases w/ height

• non turbulent

• may have ice clouds

Atmospheric Pressure

• Atmospheric pressure: the weight of all the air molecules above you

• the unequal heating of Earth’s surface causes changes in air pressure

• pressure differs w/ altitude, temperature, and density of the air

• measured in in-Hg

• @ sea level:

  • standard pressure: 29.92” Hg

  • standard temp: 15*C

• pressure lapse rate:

  • decreases 1” Hg per 1,000’ PA gained

Convection

• results from uneven heating of air by contrasting surface temps

• warm air becomes less dense + rises

• cold are becomes more dense + falls

• As warm air rises it eventually cools off, sinks, and warms up, rises, etc

Pressure Gradients

• pressure gradients

  • the force created by pressure differences that drive the wind

• this force travels from high pressure to low pressure perpendicular to isobars

• isobars - lines that connect areas of equal pressure

• the closer the isobars = the stronger the pressure gradient = the higher the flow = the stronger the wind

  

Coriolis Force

• created by the rotation of the Earth

• affects bodies that move over the great distances such as air masses

• northern hemisphere — air deflects to the right

• southern hemisphere — air deflects to the left

• three-cell circulation pattern created from Coriolis:

  • Hadley

  • Ferrel

  • Polar

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• Pressure gradient flow perpendicular to the isobars, Coriolis deflects the wind 90* to the right

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• pressure gradient forces balances Coriolis force

Friction

• friction between surface and the wind — slows wind down

• surface friction allows the wind and reduces Coriolis force, but does not affect pressure gradient force

• winds near the surface are deflects across the isobars towards lower pressure

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