Circulation of the Atmosphere
A. Scales of Atmospheric Motion
Winds are classified according to
the size of the system and the
time frame in which they occur.
1. Large- and Small-Scale
Circulation
a.
Macroscale Winds: The largest wind
patterns
(1) __________Scale
Planetary
Winds:
(a) Last _________________
weeks or longer
(b) Can extend the entire globe (1,000 to 40,000 km
(c) Examples are the westerlies and trade winds.
Trade Winds
Synoptic
(2) ___________
Scale Winds (Also called
weather-map winds)
(a)Last ______________
days to weeks
(b)Have horizontal dimensions of 100 to
5,000 km
(c) Well-known examples include:
Wave Cyclones
Hurricane Nora
Sept. 22, 1997
Hurricanes
b. Mesoscale Winds
(1) Have a strong ________component
vertical
several minutes and may exist
(2) Last for ___________________________
__________.
for hours.
(3) Are usually less than 100 km across.
(4) Include:
Thunderstorms
Tornadoes
Greensburg, KS on May 5, 2007
Land Breezes and Sea Breezes
c. Microscale Winds
(1) Smallest
________scale air
motion (less than 1 km)
(2) Last for seconds or at
most a few minutes
(3) Include:
(a) Simple Gusts
(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.
Dust Devils on Mars
05/15/2005
Credit: Mars rover Spirit
• 10 times larger than any tornado on Earth
– Kilometers high and 100’s of meters wide
• Wind speeds: 30 m/sec (70 mi/hr)
2. Structure of Wind Patterns
a. Global winds are a composite of all ________________.
scales of motion
b. This is analogous to a meandering river whose current
one
flows consistently in _____direction
but contains many
eddies (swirling currents) which may contain
large _______
even smaller eddies.
c. Flow associated with hurricanes is
an example.
(1)
From space hurricanes appear as a large
whirling cloud moving slowly across the ocean.
(2)
Hurricanes often have a net motion from east
__________
to west
indicating that they are larger eddies embedded in a
larger macroscale flow.
(3)
Hurricanes have several scales of motion.
(a) Mesoscale thunderstorms and tornadoes
(b) Microscale disturbances are imbedded in the mesoscale
motions.
(c) The counterclockwise synoptic scale flow is imbedded in the
larger-scale planetary winds (_________
and ____________).
trades
westerlies
B. Local Winds
1. Land and Sea Breezes:
Caused by daily temperature
contrast between land and water
a. Sea Breeze During the Day
By Mid-Afternoon
H
Sea Breeze (Develops during the day)
L
Cooler, Denser Air
Land
Water
Land heats faster and is warmer
Water heats slower than the land and is cooler
Sea Breeze Showing Horizontal and
Vertical Airflow
b. Land Breeze at Night
The reverse of the sea breeze forms after sunset
L
Land Breeze (Develops at night)
H
Warmer, Less dense Air
Land
Water
Land cools faster and is cooler
Water cools slower than the land and is warmer
Land Breeze Showing Horizontal and
Vertical Airflow
2. Mountain and Valley Breezes
Valley
a. _____________
Breeze
(1) Heating during the day causes air______.
rise
thermals
(2) Also referred to as ________.
(3) Often recognized cloud development on mountain
peaks.
Valley Breeze
• Cloud development on mountain peaks from a daytime
upslope (valley) breeze
• Can develop into mid-afternoon thunderstorms
b. ________
Mountain Breeze
Cooling
(1) __________at
night
(2) ________air
drains into the valley
Denser
3.
Chinook (Foehn) Winds
a. Strong downslope winds from mountains.
b. Caused by a significant difference in pressure on the windward side vs. the
leeward side.
c. Air rises, and cools on the windward side and then heats due to
compression as it descends on the leeward side
d. Can cause a temperature increase of 10 to 20 degrees Celsius in a matter
of minutes.
e. Common in the Rockies (where they are called chinooks meaning snoweater) and the Alps (where they are called (foehns).
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.
5.
Katabatic or Fall Winds
a. Cold and dense air cascades over a highland area.
b. The air does heat as it sinks but it’s still colder than the
air it displaces due to its very cold original temperature.
c. Occurs on ice sheets of Greenland and Antarctica.
Called a mistral from the French
Alps to the Mediteranean Sea
Called a bora from the mountains of
Yugoslavia to the Adriatic Sea
C. The General Circulation of
the Atmosphere
1. Large Scale Air Flow - Caused by:
a. ___________________by
the Sun resulting
Unequal heating
in pressure differences.
b. Earth’s _________________________
rotation (the Coriolis Effect)
2. A Nonrotating Earth
a. A simple convection system produced by unequal
heating.
b. Greatest heating in________________
region
Equatorial
c. Polar regions __________________
coldest
d. Convection cell model first proposed by George
Hadley in 1735
rotating
3. The Three Cell Model for the__________Earth
a. Accounts for the maintenance of Earth’s
heat balance and conservation of angular
momentum
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
5. Doldrums
At the equator there is a weak
pressure gradient with light
winds and humid conditions.
H
L
Hadley Cells
6. Intertropical Convergence Zone (ITCZ)
Z
ITCZ
• The ITCZ is the equatorial region where the trade winds converge.
• This region has rising, hot air with abundant precipitation
Satellite Image of the ITCZ
ICTZ
The ITCZ is seen as the band of clouds across the
equatorial ocean and Central America
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.)
d. Polar Cell
Sinking
Air
L
(1) Polar regions (from about 60o north and south) and extending to each pole.
(2) Polar Easterlies: Prevailing winds are from the northeast in the Arctic and
the southeast in the Antarctic.
(3) Caused by the subsidence of cold dense air at the poles.
(4) Eventually this cold polar air collides with the warmer westerly flow from
the mid-latitudes resulting in the polar front.
Ideal Pressure Belts vs. The “Real World”
(a) An imaginary uniform Earth with idealized, continuous pressure
belts.
(b) The real Earth with disruption of the zonal pattern caused by large
landmasses. This causes the formation of semipermanent high
and low pressure cells.
Semipermanent Pressure and Wind Systems
Average Surface Pressure and Global Winds
for January and July
Note the change in the position of the ITCZ the semipermanent Highs
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
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
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.
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.
D. Monsoons
Seasonal
(1) ____________
change in Earth’s global wind
circulation.
(2) Monsoon refers to a wind system that exhibits
reversal in direction
a pronounced seasonal __________________
not just a “rainy season.” A monsoon could
result in a dry season
3. Summer Monsoon
ITCZ migrates northward and draws warm
Moist air onto the continent
a.
b.
c.
Cherrapunji, India
from the sea toward the land
Warm moist air blows ____________________________
Results in abundant precipitation.
One of the world’s rainiest regions is found on the slopes of the Himalayas.
(1) Rising moist air from the Indian Ocean cools.
(2) Cherrapunji, India once had 25 m (82.5 ft.) of rain during a four-month period
during the summer monsoon.
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
a. Winds ________________________.
dry
b. Results in a _____winter
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.
E. The Westerlies:
Upper level air flow in the middle latitudes has a strong west-to-east component.
1.
2.
The temperature difference between the poles and the equator
drives these winds.
Air pressure decreases more rapidly in a column of cold air
( denser and more compact
) than in a column of warm air.
3. At the Equator:
a.
b.
c.
Air pressure decreases more gradually than over the cold polar regions
At the same altitude, equatorial regions have higher
pressure than over
the poles.
higher
Aloft, the pressure gradient is directed from the equatorial region of ________
pressure towards the polar region of lower pressure
4.
5.
As air from the tropics move poleward the Coriolis force changes the
direction of airflow to the right .
Eventually the pressure gradient force and the Coriolis force are
balanced
and winds flow geostrophically from west to east.
F.
Jet Streams
1. Narrow and meandering belts of air found near
tropopause
the ____________.
a. Width varies from less than 100 km to over
500 km; 60 mi. to 300 miles
b. Altitude is 7500 to 12,000 meters; 25,000 to
40,000 feet.
2. ____________winds
speeds that range from
High velocity
200 km/hour to 400 km/hour (120 mi/hour to
240 mi/hr)
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.
3.
Origin
a. Large surface temperature contrasts produce large
temperature gradients aloft (and higher wind speeds).
b. In winter it can be warm in Florida and near-freezing a
short distance away in Georgia.
c. Polar Jet: Occurs along the polar front where large
temp. contrasts are found.
4. Polar and Subtropical Jet Streams
a. The Polar Jet Stream.
Polar Jet Stream
(1) Mainly occurs in the middle
latitudes.
polar front and migrates with the seasons.
(2) Occurs along
(3) Usually has a meandering path, sometimes flowing almost due
north-south.
b. Subtropical Jet Streams
(1) Semipermanent jet stream
wintertime
(2) Mostly a ______________
phenomenon
(3)
Slower than the polar jet
(4) Due to the small temperature
gradient in summer, it’s
weak
during the warm
season.
(1) Centered at 25 degrees
latitude at an altitude of
about 13 km (8 miles)
(2) None have been studied in
great detail.
Subtropical
Jet Stream
G. Waves in the Westerlies
1.
2.
3.
Westerlies follow wavy paths
long
with ________wavelengths.
The longest wave patterns
have wavelengths of 4000 to
6000 km and are called
Rossby
________waves,
after C.G.
Rossby who first explained
them.
Shorter Waves:
troposphere
a.
Occur in the middle and upper _________________
b.
Associated with surface cyclones
4. Upper-level waves undergo seasonal changes.
Simplified 500 mb height-contour chart for January
a.
Wind speeds vary from summer to winter.
(1) Higher wind speeds shown by _______spaced
contour lines in
closer
winter.
(2) Caused by a ________
higher temperature gradient across the middle
latitudes.
b. The position of the polar Jet Stream changes from
summer to winter
equator
(1) Migrates towards the __________in
winter and moves back towards the poles in summer.
(2) In winter it may extend as far as central Florida which can bring severe weather to the
southern states.
(a) Influences tracks of cyclones
(b) Generates more cyclones in winter.
(3) In summer the storm track is across the northern U.S. and some cyclones never leave
Canada.
(4) Integral part of the westerlies and is associated with outbreaks of severe thunderstorms and
tornadoes when it shifts northward.
H. Westerlies and the Heat Budget
1. The equator has excess heat and the poles
experience a deficit .
south
2. West-to-east winds can transfer heat from ________
to north
.
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.
b. Flow aloft may begin to meander without warning.
(1) Large -amplitude waves result.
(2) A general north-south flow develops allowing cold air to advance
southward.
(3) This results in a stronger temperature gradient, strengthened upper-air flow
that forms into rotating cyclonic systems.
(4) )Cyclonic activity dominates the weather.
(a) Storms move cold air equatorward and warm air poleward.
(b) This redistributes large amounts of heat across the middle latitudes
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 .