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Atmospheric Circulation
Causes of Atmospheric Circulation
1) Uneven solar energy inputs
2) Rotation of the Earth
Global Circulation Patterns
Local Events and Storms
Uneven Solar Energy Inputs: Earth is heated
unevenly by the sun due to different angles of
incidence between the horizon and Sun.
This angle of incidence is
affected by two factors:
1) Latitude: solar inputs
are most dense when the
sun is overhead in the
tropics; reflection is low.
The reverse holds true in
polar regions.
2) Season: Due to Earth’s
annual orbit around the
Sun on an axis tilted by
23.5º.
Vernal equinox
Mar 22nd
Summer solstice
June 22nd
Autumnal equinox
Sept 22nd
Winter solstice
Dec 22nd
The Global Heat Budget with Latitude:
Losses by long-wave radiation is fairly constant at most
latitudes, whereas inputs are variable and greatest in the tropics.
Heat inputs from the
tropics are delivered
to the poles via
atmospheric and
ocean circulation,
each delivering about
50% of the tropical
heat.
These circulation
patterns are partly
due to heat gradients,
or more accurately
pressure gradients.
Atmospheric pressure gradients
= winds
Low Pressure: warm,
moist, low density air,
is lighter and rises.
High Pressure: cool,
dry, high density air is
heavier and sinks
Net force from high to low
pressure is called the pressure
gradient force and initiates
air movement, i.e. wind.
Convection cells establish themselves due to
the pressure gradient with latitude and
altitude. The flow of air due to pressure
differences generates winds.
The intense heating of the
tropics by solar radiation
sets up a strong convection
cell near the equator. As
water evaporates from the
ocean, the overlying air
becomes less dense .
Air looses moisture and
cools as it rises and moves
poleward. It becomes more
dense and sinks at poles
Winds would blow across
the surface of Earth from
the poles toward the tropics.
Idealized pattern without
accounting for Earth’s rotation.
Rotation of the Earth: Coriolis Effect
The idealized model of a
single convection cells
driving atmospheric
circulation in each
hemisphere is incorrect.
The eastward rotation of
the Earth causes a
deflection in wind flow
(Coriolis Effect) that
results in three smaller
atmospheric convection
cells in each hemisphere.
Perspective of earthbound observer.
Gaspard Gustavede Coriolis, 1835.
Global Circulation Patterns
• Again, there are three atmospheric convection cells per
hemisphere (Hadley, Ferrel, and Polar).
• Low pressure belts are where air rises and climate is wet
(Intertropical Convergence Zone (0º) and Polar Front (60º)).
• High pressure belts are where climate is dry (Horse Latitudes
(30º) and Polar High (90º)).
• Surface winds converge at low pressure belts:
– ITCZ: NE & SE Trade Winds (easterlies).
– Polar Front: Westerlies and Polar Easterlies.
• There are latitudinal shifts in these patterns with season.
Monsoons:
Caused by:
1) difference in specific heat
of land and sea; and 2)
seasonal shift in the ITCZ.
Winter: weaker NW and NE
monsoons of SE Asia and N.
Australia. Cool and dry.
Summer: strong SW and SE
monsoon of India, SE Asia
and parts of Africa. Intense
heating of Asian continent and
warm moist Indian Ocean air
create extremely wet, warm
and stormy conditions.
There are localize
patterns of air
circulation
superimposed or
superseding those of
global scale.
These too are the result
of differential heating
causing air pressure
gradients, but their
cause is due to
difference in the specific
heat of land versus sea,
not solar intensity.
Sea Breezes and Land Breezes
Cyclonic Storms
• Extratropical Cyclones (nor’easterns)
–
–
–
–
Created at the Polar Front.
Involves two air masses.
Meandering (wave) creates low pressure center.
Air from both sides flow inward and begin rotating due to Coriolis effect
(c.cw. in N.hemi.)
– Rising warm moist air condenses, fueling storm energy and creating strong
wind and rains.
– Usually during winter when differences in air masses are extreme.
– Strongest winds hit northeast US and eastern Canada from the northeast
(hence nor'eastern)
• Tropical Cyclones (hurricanes, typhoons, cyclones, willi-willies)
– Created by disturbances within a single tropical air mass.
– Genesis involved thunderstorms during periods of low wind shear.
Ash Wednesday Storm, March 7, 1962
Extratropical Cyclone on Mars
Tropical Cyclones:
Willi-willies
Move westward and poleward.
• Begins as a tropical depression
over warm water (>25ºC).
• Energy (latent heat of
condensation) given off during
condensation of rising moist air
in vertical thunderstorms.
• Creates intense low pressure.
• Coriolis effect initiates rotation
of air as it moves inward toward
the low pressure center = "eye".
• Upgrades to a hurricane when
the wind velocity increases
above 74 mph.
• Winds circulate around the eye
in a counter-clockwise direction
in the northern hemisphere.
• Weakens when they travel
over colder water or land.
Category
Winds
(mph)
Storm Surge
(feet)
1
74-95
4-5
2
96-110
6-8
3
111-130
9-12
4
131-155
13-18
5
155 +
18 +
Low wind
shear favors
development!