ATMOSPHERE, WINDS, WEATHER
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I. WEATHER AND CLIMATE
A. Atmospheric properties
- Air always contains some water vapor
- Air is more dense at sea level than higher up
- -Temperature and humidity control density
- -Warm air less dense than cold (less mass, less molecules)
-Humid air less dense than dry... Water molecules weigh less
than Nitrogen and Oxygen molecules that they displace.
-Temp changes; look at PV=nRT -Air cools when it expands (P
drops, so T drops)
- -Air heats up when it compresses (P up, so T goes up)
• So air heats up as it descends from upper to lower
atmosphere and compresses -Warm air holds more water vapor
than cold air
• As result, when warm air rises and gets colder it can no longer
hold water vapor--the water vapor condenses into clouds
(aggregates of tiny water droplets). If cooling continues, droplets
coalesce and grow into rain drops or snowflakes.
• These tall to earth as precipitation
B. Definitions of some terms
-Weather- "state of atmosphere at specific place and time"
-Climate-"lonq term average of weather in an area"
I
7(2)
Influences on weather and climateinclude
• Solar radiation
• Terrain
• Nearby large lakes or ocean
Weather can be
predict ...currently can't be
too long term)
climate tougher to
predicted (too large an area and
II. ATMOSPHERIC CIRCULATION (controls weather and climate)
A. Wind -mass movement of air over Earth
(Air doesn't remain stationary)
B. Uneven solar heating and latitude
-more sunlight in equatorial regions than at polesdue
to angle of Incidence
Light filtered through more atmosphere at polesless energy gets through to Earth
-This change in incoming energy is in mid- latitudes,
3 x solar energy per day in June than December
-Due to 23 1/2 degrees of the Earth's axis
C. Uneven heating and seasons
This tilt causes a chance of seasons
-North America leans toward sun in summer, leans away
from sun in winter
-Causes Southern Hemisphere summer to be
opposite Northern Hemisphere
-Causes Alaska to light 24/hrs/day in summer,
dark 24 hrs/day in winter
7(3)
Sun looks high in sky during summer, low in sky during
winter Sun never appears overhead North of 23 1/2 degrees
North lot. or south of 23 1/2 degrees South latitude
Most incoming solar energy is concentrated between these
two bands of latitude
Atmospheric Circulation due to uneven solar heating
process
-Convection process transfers heat due to temperature
differences (just like in Earth's mantle) -Heat energy moves
from high to low (hot to cold) This sets up a circulation
pattern. Hot air moves toward cold, and cold air moves
back toward heat source.
Hot air rises at tropics, moves toward poles, cools and sinks near
poles moves back toward heat source in tropics along earth
surface - does this in reality using 3 convection cells per
hemisphere
If not for earth's rotation, this simple picture would suffice.
But, because of rotation, we must consider a force that
complicates the picture. The result of this force is the
Coriolis Effect.
7(4)
1.
II.
Coriolis Effect
Wind Patterns
I. Coriolis Effect
From earthbound perspective, any object moving freely above
earth appears to curve to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere. This is due to the fact
that movement is the sum of vector quantities.
A.
Eastward velocity--look at Eastward velocity vectors due
to Earth rotation
Buffalo, NY vs Quito, Equador
Quito travels further/day, has higher Eastward velocity
than Buffalo
B. Now, combine Eastward velocity with North-South motion
Fire cannonball from:
Quito north toward Buffalo
Conclusion: Viewed from the point of origin ( or, differently put,
from the tail of the arrow) both of the cannonball paths have
deflected to the right relative to the Earth's surface.
C. Coriolis Effect and Atmospheric Circulation
Now modify the original model to account for right and
left deflections due to Coriolis Effect
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Air does not move from tropics straight up to poles-Instead, there are 3 atmospheric circulator cells per hemisphere
0-30 degrees -- air makes it up about 1/3 of the way from the
equator to North and South poles, where it cools, becomes more
dense, falls back toward Earth. Air then starts flowing back down
toward equator (toward the tropical heat source, like the radiator in
the house
- But rather than flowing straight toward equator, air is deflected by
Coriolis Effect into the "Tradewinds". This air warms and becomes
more humid as it approaches the equator. It rises and once again
moves poleward to complete the cycle. -The cells on each side of
the equator are known as Hadlev cells (Hadley, 1735).
a
30-60 degrees-Another pair of cells is at mid-latitudes
-Some air flows Poleward at the outer edges of Hadley
cells. This air is deflected to form the "westerlies".
These cells are known as Ferrel(Ferrel, mid 1800s).
a
60-90 degrees-Polar easterlies move equator-ward from poles
along surface of earth, air warms as it moves away from poles,
rises at about 60 degrees.
-This is a polar call. Each of these Cells IS:
I Driven by uneven solar heating
2. Influenced by Coriolis effect
-An aside on remembering these cells and their circulation:
a
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Both the polar cells and the equatorial Hadley cells flow from
colder to warmer along the Earth's surface, rise when the air
warms. This air is deflected to the right in the Northern
Hemisphere and the left in the Southern Hemisphere.
2) The mid-latitude Ferrel cell is different. I think of it as "caught"
between the polar and equatorial cells, and it's flow direction is
different. If flows from warmer to colder along the surface,
opposite from the other cells.
II. Wind Patterns
Direct result of atmospheric circulation cells and their edges
,Doldrums at equator--calm area--Low pressure due to
ascending air-calm area between Hadley cells; aka Intertropical
Convergence Zone or ITCZ
•Tradewinds from 0 degrees to 30 °-steady east to west winds
from Hadley cell circulation.
 Horse latitudes at 30 n- calm area -High Pressure due to
descending air-name due to Spanish practice of throwing
their dead horses over board when ships were becalmed and
ran out of food and water for horses.
• Westerlies from 30 °, to 60 u,-steady west -to-east wind
• Polar from 60 °,- calm- Low pressure due to ascending air
• Polar easterlies from 60-90 ° steady east to west wind
,Polar high-Hi pressure due to descending air
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STORMS
Definition: STORM ='regional atmospheric disturbance characterized by
strong winds and often accompanied by precipitation"
CYCLONE- huge rotating mass of low pressure air in which winds
converge and ascend. (tropical and extratropical)
TORNADO- smaller funnel of fast-spinning wind associated with severe
thunderstorm.
A. Air Masses Air Mass: "a large body of air with nearly uniform
temperature, humidity, and density throughout" Air pausing
over water or land will tend to take on the characteristics of
the surface below
• Land: Cold dry air; dense; high pressure zones
• Water: Warm humid air; less dense; low pressure zones.
Air masses can move within or between circulation cells. FRONT:
boundary between air masses of different density (Vilhelm Bjerknes,
after WWI battlefronts)
-Cyclones can be tropical (form within one tropical air mass) or
extratropical (form at front between air masses)
B. Extratropical Cyclones -form at the boundary between each
hemisphere's polar cell and Ferrel cell- the polar front. Occur mainly in
the winter hemisphere
-Development of "e.t." cyclone1) cold wind poleward of front moves from east; warmer air equatorward
of front moves from west.
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2) " twist "forms along front due to difference in the wind direction
3) Cold dense air mass slides beneath warmer lighter one.
4)Twist circulates counterclockwise in the Northern Hemisphere
- Extratropical cyclones driven eastward by westerlies.
Typically 1000 to 2500 km ( 600-1600 mi) in diameter lasting
2-5 days
- -Lifting , expansion , cooling of air mass causes precipitation
[Fig 8.15, 8.17).
- - When cold air advances and lifts warm air, "cold front"
occurs. - "Warm front" occurs when warm air is blown on top
of retreating edge of cold air. - Wind and precipitation from
these fronts are called Frontal Storms.
- Extratropical cyclones are principal cause of weather in
mid-latitude regions.
- -Nor'easters are most violent extratropical cyclones. 30
times/yr, nor'easters on coasts generate erosion of beaches
and offshore barrier islands. Every 100 yrs, will devastate
coastal settlement.
C. Tropical Cyclones
-Warm, humid rotating air mass, originates in tropics
-Types: "hurricanes" -in Atlantic and E. Pacific
"typhoons" - in W Pacific
"tropical cyclones"- in Indian Ocean
"willy-willies" - in Australia
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-Criteria for classification as hurricane
-Winds of greater than 74 mph; can go as high as 250
mph (book-good image-clinging to airplane wing)
-Lesser storms known as tropical depressions and
tropical storms.
-Structure of tropical cyclone
-counterclockwise rotation
- 'eye' of storm calm
-Genesis-not well understood
-forms from tropical depression
-need warm air (water temp greater than or equal to 79° F),
which rises and precipitation develops
-can form in pairs or trios
-Source of power-energy released during condensation that was
stored as latent heat of evaporation is released
this energy drives strong winds
-Some statistics:
-20 billion tons of water can tall from a tropical cyclone in 1
day
-I day of tropical cyclone generates 2,400 billion KW of
power, equivalent to US electrical energy usage for I year!
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-Destructive elements of cyclones
1) Wind (up to 250 mph)
2) Rain (cause flooding)
3) 3) Storm surge-most dangerous element "dome" of seawater
which rises due to low atmospheric pressure.
Can be as high as 40 ft.
Final analogy " Nature's safety valves, flinging solar energy poleward
from the tropics"
D.
Coriolis effect--still at work?
-At first glance, no
Air moves clockwise (when viewed from tail) in N.
Hemisphere
-Cyclones move counterclockwise What gives?
Coriolis effect does cause the counter clockwise spin on Low
Pressure cell. Key is to realize that airflow is moving toward the
low pressure cell from all directions, since wind energy moves
from high to low energy.
Air flow deflects to the right, sets up counterclockwise spin.