Global Circulation and Weather Systems Notes

Atmospheric Pressure

Atmospheric pressure is the force exerted on you by the weight of the atmosphere.
Gravity pulls gasses down to earth, creating weight.
Atmospheric pressure decreases as altitude increases.
99% of the atmosphere is found in the first 32 km.
Standard atmospheric pressure:
- 15 \frac{lb}{in^2} (psi)
- 1013.2 mb (metric unit)

A barometer measures the rise and fall of atmospheric pressure.
- Aneroid Barometer: Expansion or contraction of a vacuum chamber, caused by a change in pressure, forces a pointer to move.
- Mercurial Barometer: Atmospheric pressure forces mercury upwards into a graduated glass tube.

Air masses rise and fall within the atmosphere, dependent on the cooling/heating of the Earth’s surface.
- Cooling air leads to sinking air and High Pressure.
- Warming air leads to rising air and Low Pressure.

Isobars: lines that connect equal points of pressure
Pressure Gradient: A change of atmospheric pressure measured along a line at right angles to the isobars
Pressure Gradient is perpendicular to isobar lines

The Coriolis Effect: Global winds, ocean currents, and airplanes are deflected due to the earth’s rotation.
Deflection occurs whether the object is moving north, south, east, or west.

High Pressure (anticyclone):
- Air spirals downward and outward (subsiding air mass).
- In the Northern Hemisphere, it rotates clockwise outwards.
- In the Southern Hemisphere, it rotates counter-clockwise outwards.
Low Pressure (cyclone):
- Air spirals inwards and upwards.
- In the Northern Hemisphere, it rotates counter-clockwise inwards.
- In the Southern Hemisphere, it rotates clockwise inwards.

Low Pressure (cyclones):
- Air rises (inward and outward).
- Adiabatic processes cause expansion of air.
- Air cools and condenses, typically forming clouds.
- Associated with cloudy, rainy weather (“Bad Weather”).
High Pressure (anticyclone):
- Air subsides (downward and outward).
- Adiabatic processes cause compression of air.
- Air warms, leading to no condensation.
- Associated with fair weather (“Good Weather”).

Angle of insolation, seasonal changes, and the earth’s albedo cause a difference in the amount of solar radiation the earth receives.
Warm air rises (Low Pressure) on the equator.
Cold air sinks (High Pressure) in the polar areas.

The Coriolis Effect breaks the convective flow of air into convective belts.
- Hadley cell: lies between the equator and 30° latitude; air deflected from the northeast to the southwest.
- Farrell cell: less well-defined set of convective cells between 30°-60°; air flows toward the north and is deflected to the right.
- Polar cell: cold frigid air flowing from the poles toward the equator (air rises at the 60° parallel); air that moves away from the pole is deflected to the right.
Intertropical convergence zone (ITCZ)

Winds originating high in the atmosphere that move parallel to a system of parallel isobars.
Pressure gradient forces wind to move from high pressure to low pressure.
Coriolis Effect “pulls” or deflects wind to the right (Northern Hemisphere).
Pressure gradient and Coriolis Effect reach equilibrium (balance), and winds follow parallel isobars.
GW forms streams of air that circumvent the earth.

Undulated meanders of waves that circumvent the earth.
Arise in zones of cold polar air converging with warm tropical air.
Responsible for variable mid-latitude weather patterns where pools of moist and dry air masses invade mid-latitude.

Narrow zones of high wind speeds.
Occur in strong atmospheric pressure gradients.
- Polar-front jet (polar jet): generally located between 35° and 65° latitudes and found at altitudes of 10-12 km (30,000-40,000 ft).
Typically follows the edges of Rossby waves.
Marks the boundary between cold polar and warm subtropical air.
Wind speeds range from 75-125 m/s (170-280 mi/hr).