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Pressure Systems and Cyclones
ESS 111 – Climate and Global Change
Air Pressure
• Force per unit area exerted against a surface
by the weight of the air above that surface
Atmospheric Pressure
(882 mb) Hurricane Wilma October 2005
Altitude corrections with pressure readings
• Adjusted to an altitude of
0 meters (sea level)
Pressure gradient
• Difference in pressure from one location to
another
– This establishes pressure gradient force
– Tighter gradient = faster winds
(isobars closer together)
How the wind would act if PGF were the
only force influencing the movement of air
The Coriolis
Effect
• Objects in the atmosphere are influenced by the Earth’s rotation
– Rotation of Earth is counter-clockwise looking down from N. Pole.
• results in an ‘apparent’ deflection (relative to surface)
• deflection to the right in Northern Hemisphere (left in S. Hemisphere)
• Greatest at the poles, 0 at the equator
• Increases with speed of moving object and distance
• Coriolis changes direction not speed
Coriolis Force
• Due to earth’s rotation
• Applies to any freely moving object
– Bullets, rockets, airplane, cars
– Wind & ocean circulation
• Magnitude of Coriolis effect
Coriolis force
• Does not control the way water drains out of a
sink or tub.
Winds near the Earth’s Surface
Surface Winds = Pressure Gradient Force + Coriolis Force + Friction
Upper air
w/out Friction
(geostrophic balance)
Near surface
w/ Friction
Clockwise airflow in NH (opposite in SH)
Characterized by descending/converging air
which warms creating clear skies
Counterclockwise in NH (opposite in
SH)characterized by ascending/diverging air
which cools to form clouds/precipitation
Where would the strong winds be located?
A
B
Cyclones, Anticyclones, Troughs and Ridges
Upper air: isobars usually not closed off
• Troughs (low pressure areas)
• Ridges (high pressure areas)
Near surface: isobars usually closed off due
to surface friction
• Cyclones (Low pressure areas)
• Anticyclones (High pressure areas)
Mid-latitude Cyclones
• Low pressure systems are a
characteristic feature of midlatitude temperate zones
• They form in well defined zones
associated with the polar front –
which provides a strong
temperature gradient – and
convergent flow
14
Polar Front Theory
• First published after World War I
• 6 stages of life cycle
Life Cycle of Mid-latitude Cyclone
• Stage 1
• Stationary front
– Cold air to the north
– Warm air to the south
Life Cycle of Mid-latitude Cyclone
• Stage 2
• Kink develops along
stationary front
• Wave forms
Life Cycle of Mid-latitude Cyclone
• Stage 3
• Open wave
• Pressure decreases
– What happens to wind?
• Warm front
– What is causing the
precipitation ahead of the
warm front?
• Cold front
Life Cycle of Mid-latitude Cyclone
• Stage 4
• Colder air is advancing
faster than warm air
Life Cycle of Mid-latitude Cyclone
• Stage 5
• Occlusion forms
– What kind of occlusion is
pictured?
• Cyclone soon dissipates
– Why?
• What is this point called?
• Warm sector removed from
center of the storm
Cool Air
Very Cold air
Life Cycle of Mid-latitude Cyclone
• Stage 6
• Storm continues to dissipate
and becomes stationary
Mid-latitude Cyclones & the US
• What is cyclogenesis?
– Development of a cyclone
– Bomb = cyclone with pressure decreasing by at least 24mb in 24hrs.
Mid-latitude Cyclone
• General path of motion is from the west to the
east
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