A Mighty Wind a Blowin

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A Mighty Wind a Blowin - Atmospheric Circulation
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Put in notes
Solar heating is the source of energy for the
Earths surface including the oceans and
atmosphere. Transfer of heat from regions of
greater heating at the equator to lesser heating
at the poles drive movement of air and
water, control our seasons, and control
our weather.
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The amount of heat a point on the Earths surface
depends on the position of the Sun above the
horizon
Energy density (watts/m2)Solar input / sin(angle)
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The Sun is closer to the Earth in December than
in June
The southern hemisphere has its winter in
June and the northern hemisphere has its winter
in December.
WHY?!!!
Distance from the Sun cannot explain the seasons
because it does not explain why the northern
and southern hemispheres are different.
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The Earths axis of rotation is tilted 23.5
relative to its orbit around the Sun
Mar. 21
Dec. 21
June 21
Sept. 21
1. In December, the northern hemisphere is tilted
away from the Sun, so it gets less heat input.
2. In June, the northern hemisphere is tilted
towards the Sun, so it gets more heat input.
3. The pattern is reversed in the southern
hemisphere.
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We can see the effect of this tilt in the
temperatures at the Earths surface through the
seasons
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If the peak solar heating occurs in June, why
are our hottest months in July- September (and
conversely, the coldest months after the minimum
in December)?
(Hint does a pot of water heat up instantly on
the stove?)
Thermal inertia - the Earths surface takes
time to absorb the heat and warm, especially the
oceans (WHY?).
High heat capacity of water!
In December, the oceans are still losing heat
and keeping the climate warmer than it would be
without them.
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Consequences of uneven heating?
If the Sun is constantly pumping heat in,
why doesnt the surface temperature steadily
increase?
Earth
Heat In
Heat Out
In ? Out
On average, the Earth re-radiates as much heat
in as it gets from the Sun.
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Processes complicated, but remember Heat In
Heat Out.
We will talk about this further when we discuss
global warming.
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So, what happens if we have a net surplus at the
equator
and a net deficit at the poles?
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HEAT IS TRANSFERRED!
.but how?
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Molecules in warm air are farther apart because
of higher vibrational energy (temperature), so
density is less and hot air rises.
WHY DOES HOT AIR RISE?
GRAVITY!!!
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A simple pattern of heat transfer would look like
this
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Being the smart people you are, you probably are
thinking..
What happens to the air that rises to cause it to
sink later?
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KNOW THIS!!
Air pressure is a fancy term for the weight of
the column of air above you. As air rises,
pressure decreases. When the pressure
decreases, air expands. It takes work (energy)
for the air molecules to move farther apart, and
that energy has to come from the vibrational
energy of the molecules (temperature). Thus,
air cools as it rises.
This is called adiabatic (no heat in or out)
cooling air sinking and warming undergoes
adiabatic heating.
A useful factoid. dry air cools 3-5C for every
1000 foot change in elevation.
but wait, theres more!
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The ocean at the equator is also warmer, so more
water is evaporating and the rising air has more
water vapor (humidity)
What happens to humid air?
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Humid air is less dense than dry air, so it also
wants to rise!
Water vapor molecules displace other molecules,
and these are heavier than water!
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Air is made mostly of nitrogen (molecular
weight 28 g) and oxygen (molecular weight32 g).
Molecules of water vapor (molecular weight18
g) displaces oxygen and nitrogen in air,
making humid air less dense.
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Some of the air escapes north, warming as it
moves across the surface. It, too,
eventually gets enough heat to rise at about 60
Actually, the cold dry air does not make it all
the way to the pole before it is heavy enough to
sink. Instead, it sinks at about 30 latitude.
As it warms, it forms hot, dry air at the surface
that is capable of absorbing lots of water
vapor without precipitating.
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Summary so far Uneven solar heating across
Earths surface leads to an excess of heat at the
equator and a deficit of heat at the poles.
Warm, humid air at the equator rises and
circulates poleward in convection cells. These
cells transfer heat from the equator towards the
poles. This convection explains the high rates
of precipitation at the equator and at 60
latitude and the desert conditions at 30 and
poles.
2/3 of heat transfer to poles occurs this way
but wait, theres more!
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Everything on the Earth is moving eastward at a
speed determined by its location north or south
of the equator
Slower
Faster
You do not notice this motion because you are
moving too!
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In fact, you are moving eastward in your seat
RIGHT NOW at 1414 km/hr (848 mph)!
but so is the Earth beneath you so you do not
notice it.
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But, Buffalo is moving only 783 mph east, so you
the object moves east FASTER!
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. and circulation cells are tilted instead of
aligned N-S.
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The actual wind patterns are affected by
continents because land heats up faster (low P)
and cools
faster (high P).
WHY???
Wind speed (blueslow, yellowfast) and
direction on 9/21/96
Remember, land has a much lower heat
capacity than water. Also, land has mountains to
block air flow.
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Summary Average wind directions are set by the
interaction of two processes transfer of heat
from the equator to the poles and rotation of
the Earth. The latter results in a Coriolis
force that deflects air masses clockwise in the
northern hemisphere and counterclockwise in the
southern hemisphere.
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This is all about average winds and
directions. What about storms?
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Storms are powered by solar energy!
Two main types
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Extratropical cyclones usually develop in
winter when differences across the polar front
are especially large.
Polar cell
Storm develops here
Ferrell cell
Remember, warm, humid air is less dense and it
rises!
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Notice that the air currents are going
counterclockwise (OPPOSITE OF CORIOLIS FORCE)!
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As warm air rises and cools, it produces rain or
snow (or sleet, or ice storms, )
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Northeasters are examples of extratropical cyclone
s in the Atlantic Ocean - most violent ones in
North America.
A Perfect Storm actual event
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Extratropical cyclones can be extremely destructiv
e
3/7/92 Fire Island, NY
NOT a good time to own a weekend beach getaway!
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Not to be outdone, tropical cyclones can also be
very destructive!
The destruction of Galveston, TX 9/8/1900
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Tropical cyclones develop in a single warm,
humid air mass from tropical depressions (areas
of low pressure in easterly trade winds).
.but not always. WHY??
aka hurricane in the Atlantic, typhoon in
the Pacific
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Moving over water warmer than 26C, warm and
humid air begins to rise and produces rain. This
upward movement sucks in surface air. As that
air flows inward towards the center, it is warmed
by the water. Energy for the storm comes from
the heat released from the rain condensation and
is replenished by the surface air being warmed
by the ocean.
The energy released goes into wind, but
fortunately only 2-4 and these winds spiral
counterclockwise also.
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Hurricane Katrina was moving over VERY warm water
(3289F)
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A large tropical cyclone generates enough
energy that it could power the entire United
States for a year.
Oh, yeah, there is the minor detail of how to
extract this energy and store it (the storm does
not last all year long)..
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High winds and rapid rainfall ( 2.5 cm/hour) can
cause damage.
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but most of the damage is from storm surge
A mound of water beneath a storm may be as much
as 1 m high. This may not sound like much, but
when it comes onshore, the shallow coast may
amplify it and make it 12 meters high.
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Storm surge occurs because the rising air in the
eye of the hurricane creates a low pressure
zone. The water is sucked up by this low
pressure.
L
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While the Coriolis effect does not predict the
ccw rotation of the cyclone, it DOES predict its
track
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Changing gears to more local events..
The great Los Angeles heat wave and why it
happened.
Lets begin with normal conditions..
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Summer low pressure over continents because of
heating
pulls in cooler air from ocean.
LA
UCR
We often get afternoon onshore breezes (wind from
west).
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At night, the air over the continent cools
faster and sinks, producing offshore breezes.
These offshore breezes warm from the ocean and
rise to make clouds
LA
UCR
Cooling us at night also.
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