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Circulation of the Atmosphere

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Title: Circulation of the Atmosphere


1
Circulation of the Atmosphere
2
A. Scales of Atmospheric Motion
  • Winds are classified according to the size of the
    system and the time frame in which they occur.

3
Large- and Small-Scale Circulation
4
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
5
(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
6
Wave Cyclones
7
Hurricanes
Hurricane Nora Sept. 22, 1997
8
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.
9
Thunderstorms
10
Tornadoes
Greensburg, KS on May 5, 2007
11
Land Breezes and Sea Breezes
12
(No Transcript)
13
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
14
(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.

15
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)

16
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
17
c. Flow associated with hurricanes is an example.
  • (1) From space hurricanes appear as a large
    whirling cloud moving slowly across the ocean.

18
(2) Hurricanes often have a net motion from
__________ indicating that they are larger eddies
embedded in a larger macroscale flow.
east to west
19
(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
20
B. Local Winds
  • 1. Land and Sea Breezes
  • Caused by daily temperature contrast between
    land and water

21
a. Sea Breeze During the Day
  • By Mid-Afternoon

H
L
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
22
Sea Breeze Showing Horizontal and Vertical Airflow
23
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
24
Land Breeze Showing Horizontal and Vertical
Airflow
25
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
26
Valley Breeze
  • Cloud development on mountain peaks from a
    daytime upslope (valley) breeze
  • Can develop into mid-afternoon thunderstorms

27
b. ________ Breeze
Mountain
Cooling
  1. __________at night
  2. ________air drains into the valley

Denser
28
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).

29
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.

30
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.

31
Called a mistral from the French Alps to the
Mediteranean Sea
32
Called a bora from the mountains of Yugoslavia to
the Adriatic Sea
33
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)
34
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
35
3. The Three Cell Model for the__________Earth
rotating
  • a. Accounts for the maintenance of Earths heat
    balance and conservation of angular momentum

36

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

H
L
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
37
  • 5. Doldrums

At the equator there is a weak pressure gradient
with light winds and humid conditions.
H
L
Hadley Cells
38
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

39
Satellite Image of the ITCZ
ICTZ
  • The ITCZ is seen as the band of clouds across the
    equatorial ocean and Central America

40
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.)

41
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.

42
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.

43
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
44
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

45
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

46
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.

47
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.

48
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

49
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.

50
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
51
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.

52
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
53
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
54
  • 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
55
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
56
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.

57
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.

58
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
59
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
60
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
61
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
62
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
63
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
64
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.

65
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

66
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 .
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