Circulation of the Atmosphere

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Circulation of the Atmosphere
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A. Scales of Atmospheric Motion

• Winds are classified according to the size of the
  system and the time frame in which they occur.

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Large- and Small-Scale Circulation
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a. Macroscale Winds The largest wind patterns

• __________Scale Winds
• Last _________________
• Can extend the entire globe (1,000 to 40,000 km
• Examples are the westerlies and trade winds.

Planetary
weeks or longer
Trade Winds
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(2) ___________ Scale Winds (Also called
weather-map winds)
Synoptic

1. Last ______________
2. Have horizontal dimensions of 100 to 5,000 km
3. Well-known examples include

days to weeks
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Wave Cyclones
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Hurricanes
Hurricane Nora Sept. 22, 1997
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b. Mesoscale Winds

• Have a strong ________component
• Last for ___________________________
• __________.
• (3) Are usually less than 100 km across.
• (4) Include

vertical
several minutes and may exist
for hours.
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Thunderstorms
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Tornadoes
Greensburg, KS on May 5, 2007
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Land Breezes and Sea Breezes
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(No Transcript)
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c. Microscale Winds

• (1) ________scale air motion (less than 1 km)
• (2) Last for seconds or at most a few minutes
• Include
• (a) Simple Gusts

Smallest
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(b) Dust Devils
Arizona

• Form on clear days unlike tornadoes that are
  associated with convective clouds
• Form from the ground upward
• Unstable air rises
• Rotation increases due to conservation of angular
  momentum.
• As air rises it carries sand, dust, and other
  loose debris dozens of meters into the air.
• Can be undetected over vegetated surfaces.

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Dust Devils on Mars
05/15/2005
Credit Mars rover Spirit

• 10 times larger than any tornado on Earth
• Kilometers high and 100s of meters wide
• Wind speeds 30 m/sec (70 mi/hr)

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2. Structure of Wind Patterns

• a. Global winds are a composite of all
  ________________.
• b. This is analogous to a meandering river whose
  current flows consistently in _____direction but
  contains many large _______ (swirling currents)
  which may contain even smaller eddies.

scales of motion
one
eddies
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c. Flow associated with hurricanes is an example.

• (1) From space hurricanes appear as a large
  whirling cloud moving slowly across the ocean.

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(2) Hurricanes often have a net motion from
__________ indicating that they are larger eddies
embedded in a larger macroscale flow.
east to west
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(3) Hurricanes have several scales of motion.

1. Mesoscale thunderstorms and tornadoes
2. Microscale disturbances are imbedded in the
   mesoscale motions.
3. The counterclockwise synoptic scale flow is
   imbedded in the larger-scale planetary winds
   (_________ and ____________).

trades
westerlies
westerlies
Trades
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B. Local Winds

• 1. Land and Sea Breezes
• Caused by daily temperature contrast between
  land and water

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a. Sea Breeze During the Day

• By Mid-Afternoon

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Sea Breeze (Develops during the day)
Warmer, Less dense air
Cooler, Denser Air
Land
Water
Land heats faster and is warmer
Water heats slower than the land and is cooler
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Sea Breeze Showing Horizontal and Vertical Airflow
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b. Land Breeze at Night

• The reverse of the sea breeze forms after sunset

L
H
Land Breeze (Develops at night)
Cooler, denser air
Warmer, Less dense Air
Land
Water
Land cools faster and is cooler
Water cools slower than the land and is warmer
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Land Breeze Showing Horizontal and Vertical
Airflow
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2. Mountain and Valley Breezes
Valley

• _____________ Breeze
• (1) Heating during the day causes air______.
• (2) Also referred to as ________.
• (3) Often recognized cloud development on
  mountain peaks.

rise
thermals
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Valley Breeze

• Cloud development on mountain peaks from a
  daytime upslope (valley) breeze
• Can develop into mid-afternoon thunderstorms

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b. ________ Breeze
Mountain
Cooling

1. __________at night
2. ________air drains into the valley

Denser
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3. Chinook (Foehn) Winds

1. Strong downslope winds from mountains.
2. Caused by a significant difference in pressure on
   the windward side vs. the leeward side.
3. Air rises, and cools on the windward side and
   then heats due to compression as it descends on
   the leeward side
4. Can cause a temperature increase of 10 to 20
   degrees Celsius in a matter of minutes.
5. Common in the Rockies (where they are called
   chinooks meaning snow-eater) and the Alps (where
   they are called (foehns).

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4. Santa Ana Wind

• A chinook-like wind that occurs when a strong
  high pressure system settles to the NE of
  southern California with low pressure to the SW.
  Clockwise flow forces desert air from Arizona and
  Nevada westward towards the Pacific. It is
  funneled through the canyons of the Coast Ranges,
  compresses and heat the region to temperatures
  that can exceed 100 degrees F.

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5. Katabatic or Fall Winds

• Cold and dense air cascades over a highland area.
• The air does heat as it sinks but its still
  colder than the air it displaces due to its very
  cold original temperature.
• Occurs on ice sheets of Greenland and Antarctica.

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Called a mistral from the French Alps to the
Mediteranean Sea
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Called a bora from the mountains of Yugoslavia to
the Adriatic Sea
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The General Circulation of the Atmosphere

• Large Scale Air Flow - Caused by
• ___________________by the Sun resulting in
  pressure differences.
• Earths _________________________

Unequal heating
rotation (the Coriolis Effect)
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2. A Nonrotating Earth

1. A simple convection system produced by unequal
   heating.
2. Greatest heating in________________ region
3. Polar regions __________________
4. Convection cell model first proposed by George
   Hadley in 1735

Equatorial
coldest
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3. The Three Cell Model for the__________Earth
rotating

• a. Accounts for the maintenance of Earths heat
  balance and conservation of angular momentum

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b. Tropical Hadley Cell (0o to 30o latitude)

1. Near the equator warm air rises and releases
   latent heat and upper flow moves poleward
2. Upper flow starts to descend between 20o and 35o
   latitude due to (1) radiational cooling and (2)
   increased Coriolis effect causing deflection to
   nearly west to east flow. This causes convergence

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Hadley Cells
(3) At the surface a region of higher pressure
exists at about 30o latitude. These are
referred to as the horse latitudes due to the
generally weak and variable winds. (4)
Air flows towards the equator. This equatorward
flow is deflected by the Coriolis effect
forming the trade winds
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• 5. Doldrums

At the equator there is a weak pressure gradient
with light winds and humid conditions.
H
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Hadley Cells
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6. Intertropical Convergence Zone (ITCZ)
ITCZ
Z

• The ITCZ is the equatorial region where the trade
  winds converge.
• This region has rising, hot air with abundant
  precipitation

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Satellite Image of the ITCZ
ICTZ

• The ITCZ is seen as the band of clouds across the
  equatorial ocean and Central America

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c. Ferrel Cell (mid-latitude indirect cell)
Ferrel Cells

1. Not all the air that converges at around 30o
   North and South latitudes (at the subtropical
   high pressure zones) moves equatorward. Some
   moves towards higher latitudes.
2. Between 30o and 60o latitude the net surface flow
   is poleward.
3. The Coriolis force causes winds to have a strong
   westerly component resulting in the prevailing
   northwesterlies. (Aloft, due to cold polar air
   and warm tropical air the poleward directed
   pressure-gradient force is balanced by an
   equatorward-directed Coriolis force with the net
   result being a prevailing flow from east to west.)

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d. Polar Cell
Sinking Air
L

• Polar regions (from about 60o north and south)
  and extending to each pole.
• Polar Easterlies Prevailing winds are from the
  northeast in the Arctic and the southeast in the
  Antarctic.
• Caused by the subsidence of cold dense air at the
  poles.
• Eventually this cold polar air collides with the
  warmer westerly flow from the mid-latitudes
  resulting in the polar front.

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Ideal Pressure Belts vs. The Real World

1. An imaginary uniform Earth with idealized,
   continuous pressure belts.
2. The real Earth with disruption of the zonal
   pattern caused by large landmasses. This causes
   the formation of semipermanent high and low
   pressure cells.

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Semipermanent Pressure and Wind SystemsAverage
Surface Pressure and Global Winds for January and
July
Note the change in the position of the ITCZ the
semipermanent Highs
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Average Surface Pressure and Global Winds for
January

• Polar Highs are prominent features of Northern
  Hemisphere winter circulation
• Clear skies and divergent surface flow results
  from subsiding air resulting in the polar
  easterlies.
• Siberian high
• Strong high-pressure center position over
  northern Asia
• Weak polar high
• Over North America
• Azores high
• Subtropical high in the North Atlantic close to
  the northwest African coast

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Average Surface Pressure and Global Winds for
January

• Semipermanent low-pressure centers (absent in
  July)
• Aleutian and Icelandic lows
• A composite of numerous cyclonic storms that move
  through these regions. So many cyclones are
  present that these regions almost always
  experience low pressure.
• Cloudy conditions and abundant precipitation

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Average Surface Pressure and Global Winds for
July

• Lows replace winter highs
• Result from high surface temperatures over
  continents.
• Warm air rises resulting in inward directed
  surface flow.
• Strongest low develops over southern Asia
• A weaker low is found in the southwestern United
  States.

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Average Surface Pressure and Global Winds for
July

• Subtropical highs migrate westward become more
  intense than during the winter months.
• Pump warm most air onto continents that are west
  of the highs
• Increased precipitation oer parts of eastern
  North America and Southeast Asia results.
• Bermuda High
• The subtropical high found in the North Atlantic
• During the winter it is found near Africa and is
  called the Azoores high.

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D. Monsoons
Seasonal

• (1) ____________ change in Earths global wind
  circulation.
• Monsoon refers to a wind system that exhibits a
  pronounced seasonal __________________
• not just a rainy season. A monsoon could
  result in a dry season

reversal in direction

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3. Summer Monsoon
Cherrapunji, India
ITCZ migrates northward and draws warm Moist air
onto the continent
from the sea toward the land

• Warm moist air blows ____________________________
• Results in abundant precipitation.
• One of the worlds rainiest regions is found on
  the slopes of the Himalayas.
• Rising moist air from the Indian Ocean cools.
• Cherrapunji, India once had 25 m (82.5 ft.) of
  rain during a four-month period during the summer
  monsoon.

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4. Winter Monsoon
In January a strong high pressure develops over
Asia and cool, dry continental air causes the
winter monsoon.
blow off the continent

1. Winds ________________________.
2. Results in a _____winter

dry
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The North American Monsoon

• High summer temperatures over SW United States.
• A thermal low is created that draws moisture from
  the Gulf of CA and the Gulf of Mexico
• Produces precipitation over SW United States and
  NW Mexico, mostly as thunderstorms.

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E. The Westerlies
Upper level air flow in the middle latitudes has
a strong west-to-east component.

• The
  between the poles and the equator drives these
  winds.
• Air pressure decreases more rapidly in a column
  of cold air
• (
  ) than in a column of warm air.

temperature difference
denser and more compact
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3. At the Equator

1. Air pressure decreases more gradually than over
   the cold polar regions
2. At the same altitude, equatorial regions have
   pressure than over the poles.
3. Aloft, the pressure gradient is directed from the
   equatorial region of ________ pressure
   towards the polar region of
   pressure

higher
higher
lower
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• As air from the tropics move poleward the
  force changes the direction of airflow to
  the .
• Eventually the pressure gradient force and the
  Coriolis force are
• and winds flow
  geostrophically from west to east.

Coriolis
right
balanced
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F. Jet Streams

• Narrow and meandering belts of air found near the
  ____________.
• Width varies from less than 100 km to over 500
  km 60 mi. to 300 miles
• Altitude is 7500 to 12,000 meters 25,000 to
  40,000 feet.
• ____________winds speeds that range from 200
  km/hour to 400 km/hour (120 mi/hour to 240 mi/hr)

tropopause
High velocity
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Discovery

• Predicted as early as early as 1920 by Japanese
  meteorologist Wasaburo Ooishi.
• Dramatically affected American bombers during
  World War II.
• On return flights tail winds increased their
  speeds.
• Flying to targets they often made little headway,
  flying into the wind.

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3. Origin

1. Large surface temperature contrasts produce large
   temperature gradients aloft (and higher wind
   speeds).
2. In winter it can be warm in Florida and
   near-freezing a short distance away in Georgia.
3. Polar Jet Occurs along the polar front where
   large temp. contrasts are found.

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4. Polar and Subtropical Jet Streams
a. The Polar Jet Stream.
Polar Jet Stream
middle

1. Mainly occurs in the
   latitudes.
2. Occurs along and
   migrates with the seasons.
3. Usually has a meandering path, sometimes flowing
   almost due north-south.

polar front
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b. Subtropical Jet Streams
Semipermanent

• jet stream
• Mostly a ______________ phenomenon
• than the polar jet
• Due to the small temperature gradient in summer,
  its
• during the warm season.
• Centered at 25 degrees latitude at an altitude of
  about 13 km (8 miles)
• None have been studied in great detail.

wintertime
Slower
weak
Subtropical Jet Stream
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G. Waves in the Westerlies

1. Westerlies follow wavy paths with
   ________wavelengths.
2. The longest wave patterns have wavelengths of
   4000 to 6000 km and are called ________waves,
   after C.G. Rossby who first explained them.

long
Rossby
3. Shorter Waves a. Occur in the middle and
upper _________________ b. Associated with
surface cyclones
troposphere
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4. Upper-level waves undergo seasonal changes.
Simplified 500 mb height-contour chart for January

• Wind speeds vary from summer to winter.
• (1) Higher wind speeds shown by _______spaced
  contour lines in winter.
• (2) Caused by a ________ temperature gradient
  across the middle latitudes.

closer
higher
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b. The position of the polar Jet Stream changes
from summer to winter
equator

• Migrates towards the __________in winter and
  moves back towards the poles in summer.
• In winter it may extend as far as
  which can bring severe weather
  to the southern states.
• Influences tracks of cyclones
• Generates more cyclones in winter.
• In summer the storm track is across the northern
  U.S. and some cyclones never leave Canada.
• Integral part of the westerlies and is associated
  with outbreaks of severe thunderstorms and
  tornadoes when it shifts northward.

central Florida
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H. Westerlies and the Heat Budget
excess

• The equator has heat and the
  poles experience a .
• West-to-east winds can transfer heat from
  ________
• to .

deficit
south
north
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a. West to east flow can persist for several
weeks or more.

• (1) Results in mild temperatures.
• (2) There are few disturbances in the region
  south of the jet stream.

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b. Flow aloft may begin to meander without
warning.
Large

• -amplitude waves result.
• A general north-south flow develops allowing cold
  air to advance southward.
• This results in a stronger temperature gradient,
  strengthened upper-air flow that forms into
  rotating cyclonic systems.
• )Cyclonic activity dominates the weather.
• Storms move cold air equatorward and warm air
  poleward.
• This redistributes large amounts of heat across
  the middle latitudes

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c. Ultimately the redistribution of heat weakens
the temperature gradient.

1. Flatter flow returns to the upper air
2. At the surface there is less intense weather .