Transcript Atmospheric Circulation and Weather

Atmospheric Circulation and
Weather

Composition and
Properties of the
Atmosphere

Lower atmosphere
nearly homogenous
mixture of nitrogen
78.1% and oxygen
20.9% (argon and
co2 = 1%).

Density of air is
influenced by its
temperature and
water content.
• Warm air is
less dense
than cold air,
(because of
molecular
movement) it
occupies more
space.
 Humid
air is less
dense than dry air
at the same
temperature (water
vapor less dense
than nitrogen and
oxygen molecules
it displaces).

Atmospheric Pressure
• Weight of the atmosphere – decreases as
you ascend into the atmosphere. Most
pressure is at the surface.
• Air near the surface is packed densely by
its own weight – when this air is lifted it
will expand – expanding air becomes
cooler because going from high to low
pressure.
• Opposite is also true, when air is
compressed it becomes warmer –
descending air warms because it is
compressed by higher atmospheric
pressure near earth’s surface.
• Warm air can hold more water vapor than
cold air. When air is rising expanding and
cooling water vapor condenses into
clouds. The cooler air can no longer hold
as much water vapor.
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Atmospheric
Circulation
Weather = the
state of the
atmosphere at a
specific place and
time.


Climate = a long
term average of
weather in an
area.
Both are
influenced by the
amount of solar
radiation the area
receives.
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Air does not remain
stationary over earth,
but flows in large
patterns shaped or
caused by unequal
solar heating of the
earth with latitude and
season, and by the
rotation of the
earth. The mass
movement of air is
known as wind.
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Uneven solar
heating and latitude

Poles have less
solar heating
because
•
sun reaches poles
at greater angle
energy spread
over greater
area. (lower
angle of
incidence).

Equator has more
solar heating
because
• equator has
high angle of
incidence
• sunlight
subjected to
less filtration
and surface
reflection
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Uneven solar heating and the seasons:
Mid latitudes receive about 3 times as
much solar energy per-day in June than
December – reason is the 23 1/2% tilt of
the earth, its orbital inclination. The
inclination of the axis causes the change
of seasons.
December
True color images
March
June
September
http://www.nasa.gov/vision/earth/features/blue_marble.html
Uneven solar heating and atmospheric
circulation
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most of suns heat is at the equator.
We know that warm air rises and cold air
sinks (convection in room example).
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The ideal model of atmospheric
circulation would be – air heated at
tropics expands, rises, and goes
poleward where it loses heat and sinks to
the surface and moves
equatorward. This large circuit of air is
called an atmospheric circulation
cell. But this is not what
happens. Global circulation is governed
by both uneven solar heating and the
rotation of the earth.
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The affects of the rotating earth on
atmospheric circulation

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The eastward rotation of earth deflects moving air
and water away from its initial course. The
deflection is to the right in the northern
hemisphere. The apparent deflection is called
coriolis effect.
With a rotating earth, wind and water get
deflected. Because of this coriolis effect a more
complex circulation model is needed to explain
atmospheric circulation.
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In this new model of
atmospheric
circulation, air still
warms and expands
at the equator. This
rising air loses
moisture and is now
drier and more
dense.
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The air will travel
poleward and will
become dense
enough to fall back
toward the surface at
about 30 degrees
north and 30 degrees
south latitude.
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Most of this air near
the surface will travel
back towards the
equator and it is then
deflected to the right
coming from the north
east (NE Trade Winds
or Easterlies).
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These winds warm as
they approach the
equator, evaporating
water and becoming
less dense,
completing the circuit.
This large
atmospheric
circulation cell is
called the Hadley
Cell.
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A more complex cell
– Ferrell Cell
operates in the mid
latitudes. Air
descending at 30
degrees N/S going
poleward is
deflected to the right
forming the
westerlies.
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The polar easterlies
are formed from air
returning from the
poles. This cold air
is deflected
westward. This
circuit is called the
Polar Cell.
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Where the polar cell
meets the Ferrell Cell
is called polar front –
which creates most
weather in mid –
latitudes.
Wind Patterns
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At the bands between circulation cells air is
moving vertically, and the surface winds are
weak and erratic.
At the equator where the two Hadley cells
converge is called the doldrums. Also called
Intertropical Convergence Zone (ITCZ).
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Air moving from equator loses precipitation
and cools. This cooler dry air moves
poleward but is dense enough to descend
around 30 degrees latitude between Hadley
and Ferrell Cells. This area of high pressure
and little surface winds because of vertical
movement is called the horse latitudes.

Sailors are most interested in surface winds
between cells, surface winds of Hadley cells
are the trade winds or easterlies 15 degrees
N and S. Surface winds of Ferrel Cells called
westerlies 45 degrees N and S.
Modifications of circulation due to
unequal distribution of land.
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
Local variations occur when land heats up
more in summer causing low pressure and
cools more in winter creating high
pressure.
Land and Sea
breezes (small
daily monsoons)
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coastal areas
during the day in
warm months,
land is heated
more than
water. This warm
low pressure air
rises and the
cooler air above
the sea replaces
it creating a sea –
breeze.
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At night the land cools
more rapidly than the
water creating high
pressure over
land. Air goes from
high to low pressure
forming land breeze.
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Monsoons
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Similar to land and sea
breezes but are
seasonal and occur on
a large land mass like
Asia.
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Land heated in summer
creates low
pressure. Cooler air over
ocean (high pressure)
moves in to replace the
lifting low pressure air
over land (remember air
goes from high to low
pressure). The moist air
from the ocean warms
over land rises and
condenses to form
precipitation. The rainy
seasons.
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The opposite occurs in autumn, as the
land cools more rapidly than adjacent
ocean. Air cools and sinks over land
(high pressure), and dry surface winds
move seaward. The dry
season. Monsoon = is a pattern of wind
circulation that changes with the
seasons.
Storms
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Storms form between or within air
masses. Air Mass = large body of air with
nearly uniform temperature, humidity, and
therefore density throughout. These are
determined by the land which the air
passes over.
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•
•
These air masses move within or between
circulation cells.
Different air masses do not mix well because of
different densities.
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The lighter air mass
is wedged upward
causing it to expand,
cool, and condense,
which contributes to
the turbulence
between boundaries.
The boundary
between air masses
of different density is
called a front.
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Two main types of storms – Extratropical Cyclone and
tropical cyclone.
Cyclone = huge rotating mass of low – pressure air in
which winds converge and ascend.
• Extratropical cyclones = form at the boundary
between each hemispheres polar cell and its
Ferrel cell – the polar front. (Occur mainly in
winter where temperature and density
differences across front are most
pronounced). Principle cause of weather in mid
– latitudes regions.
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Tropical Cyclones
These form from disturbances within one
air mass. These are great masses of
warm, humid rotating air.
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Large ones called hurricanes (winds of
74 mph)
• typhoons in Pacific,
• Tropical cyclone in Indian and
• willi - willis in Australian waters.
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Winds less than 74 mph called tropical
storms and tropical depressions.
Develops from a small tropical
depression which forms over a large
warm air mass (Africa). The air mass
goes over ocean waters that must be
79 degrees F or higher and warm
humid air is forced up. The storm
begins to develop.
The energy is from latent heat of
evaporation.