Circulation of the AtmosphereCirculation of the Atmosphere

A. Scales of Atmospheric MotionA. Scales of Atmospheric Motion

Winds are classified according to the size of the system and the

time frame in which they occur.

1.1. Large- and Small-Scale Large- and Small-Scale CirculationCirculation

a.a. Macroscale WindsMacroscale Winds: The largest wind : The largest wind patternspatterns

(1) __________Scale Winds:(a) Last _________________

(b) Can extend the entire globe (1,000 to 40,000 km

(c) Examples are the westerlies and trade winds.

Planetaryweeks or longer

Trade WindsTrade Winds

(2) ___________ Scale Winds (Also called weather-map winds)

(a)Last ______________

(b)Have horizontal dimensions of 100 to 5,000 km

(c)Well-known examples include:

Synoptic

days to weeks

Wave CyclonesWave Cyclones

HurricanesHurricanes

Hu

rric

ane

No

raS

ept.

22,

199

7

b. b. Mesoscale WindsMesoscale Winds

(1) Have a strong ________component

(2) Last for _____________________________________.

(3) Are usually less than 100 km across.

(4) Include:

vertical

several minutes and may exist

for hours.

ThunderstormsThunderstorms

TornadoesTornadoes

Greensburg, KS on May 5, 2007

Land Breezes and Sea BreezesLand Breezes and Sea Breezes

c. Microscale Windsc. Microscale Winds

(1) ________scale air motion (less than 1 km)

(2) Last for seconds or at most a few minutes

(3) Include:(a) Simple Gusts

Smallest

(b) Dust Devils(b) Dust Devils

• 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.

Arizona

Dust Devils on MarsDust Devils on Mars

• 10 times larger than any tornado on Earth– Kilometers high and 100’s of meters wide

• Wind speeds: 30 m/sec (70 mi/hr)

05/15/2005

Credit: Mars rover Spirit

2. Structure of Wind Patterns2. 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

oneeddies

c. Flow associated with hurricanes is c. Flow associated with hurricanes is an example.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 __________indicating that they are larger eddies embedded in a larger macroscale flow.

east to west

(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

Trades

westerlies

B. Local WindsB. Local Winds

1. Land and Sea Breezes:

Caused by daily temperature contrast between land and water

a. Sea Breeze During the Daya. Sea Breeze During the Day

By Mid-Afternoon

LandCooler, Denser Air

H

Warmer, Less dense air

LSea Breeze (Develops during the day)

Water

Water heats slower than the land and is cooler

Land heats faster and is warmer

Sea Breeze Showing Horizontal and Sea Breeze Showing Horizontal and Vertical AirflowVertical Airflow

b. Land Breeze at Nightb. Land Breeze at Night

The reverse of the sea breeze forms after sunset

LandWarmer, Less dense Air

L

Cooler, denser air

HLand Breeze (Develops at night)

Water

Water cools slower than the land and is warmer

Land cools faster and is cooler

Land Breeze Showing Horizontal and Land Breeze Showing Horizontal and Vertical AirflowVertical Airflow

2. Mountain and Valley Breezes2. Mountain and Valley Breezes

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

peaks.

Valley

risethermals

Valley BreezeValley Breeze

• Cloud development on mountain peaks from a daytime upslope (valley) breeze

• Can develop into mid-afternoon thunderstorms

b. ________ Breeze

(1) __________at night

(2) ________air drains into the valley

Mountain

Cooling

Denser

3.3. Chinook (Foehn) WindsChinook (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 sided. 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 snow-

eater) and the Alps (where they are called (foehns).

4. Santa Ana Wind4. 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.

5.5. Katabatic or Fall WindsKatabatic 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 Called a mistralmistral from the French from the French Alps to the Mediteranean SeaAlps to the Mediteranean Sea

Called a Called a borabora from the mountains of from the mountains of Yugoslavia to the Adriatic SeaYugoslavia to the Adriatic Sea

C.C. The General Circulation of The General Circulation of the the AtmosphereAtmosphere

1. Large Scale Air Flow - Caused by:

a. ___________________by the Sun resulting in pressure differences.

b. Earth’s _________________________

Unequal heating

rotation (the Coriolis Effect)

2. A Nonrotating Earth

a. A simple convection system produced by unequal heating.

b. Greatest heating in________________ regionc. Polar regions __________________d. Convection cell model first proposed by George

Hadley in 1735

Equatorial coldest

3. The Three Cell Model for the__________Earth

a. Accounts for the maintenance of Earth’s heat balance and conservation of angular momentum

rotating

(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

(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

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

Hadley Cells

L

H

5. Doldrums 5. Doldrums

Hadley Cells

L

H

At the equator there is a weak pressure gradient with light winds and humid conditions.

6. Intertropical Convergence Zone (ITCZ)6. Intertropical Convergence Zone (ITCZ)

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

ITCZZ

Satellite Image of the ITCZSatellite Image of the ITCZ

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

ICTZ

c. Ferrel Cell (mid-latitude indirect cell)

(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.)

Ferrel Cells

d. Polar Cell

(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.

SinkingAir

L

Ideal Pressure Belts vs. The “Real World”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 SystemsSemipermanent Pressure and Wind SystemsAverage Surface Pressure and Global Winds Average Surface Pressure and Global Winds

for January and Julyfor January and July

Note the change in the position of the ITCZ the semipermanent Highs

Average Surface Pressure and Global Winds for JanuaryAverage 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 JanuaryAverage 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 JulyAverage 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 JulyAverage 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. D. MonsoonsMonsoons

(1) ____________ change in Earth’s global wind circulation.

(2) 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

Seasonal

3. Summer Monsoon

a. Warm moist air blows ____________________________b. Results in abundant precipitation.c. 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.

Cherrapunji, India

from the sea toward the land

ITCZ migrates northward and draws warmMoist air onto the continent

4. Winter Monsoon

a. Winds ________________________.

b. Results in a _____winter

blow off the continentdry

In January a strong high pressure develops over Asia and cool, dry continental air causes the winter monsoon.

The North American MonsoonThe 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.E. The WesterliesThe Westerlies::

1. The between the poles and the equator drives these winds.

2. Air pressure decreases more rapidly in a column of cold air

( ) than in a column of warm air.

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

temperature difference

denser and more compact

3. At the Equator:3. At the Equator:

a. Air pressure decreases more gradually than over the cold polar regions

b. At the same altitude, equatorial regions have pressure than over the poles.

c. Aloft, the pressure gradient is directed from the equatorial region of ________ pressure towards the polar region of pressure

higher

higherlower

4. As air from the tropics move poleward the force changes the direction of airflow to the .

5. Eventually the pressure gradient force and the Coriolis force are

and winds flow geostrophically from west to east.

Coriolisright

balanced

F.F. Jet StreamsJet Streams

1. Narrow and meandering belts of air found near 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 200 km/hour to 400 km/hour (120 mi/hour to 240 mi/hr)

tropopause

High velocity

DiscoveryDiscovery

• 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 Streams4. Polar and Subtropical Jet Streams

(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 Jet Stream

a. The Polar Jet Stream.

middlepolar front

b. Subtropical Jet Streamsb. Subtropical Jet Streams

(1) jet stream

(2) Mostly a ______________ phenomenon

(3) than the polar jet

(4) Due to the small temperature gradient in summer, it’s

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

Semipermanent

wintertime

Slower

weak

G. G. Waves in the WesterliesWaves in the Westerlies1. 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.

3. Shorter Waves: a. Occur in the middle and upper _________________

b. Associated with surface cyclones

Rossby

long

troposphere

4. Upper-level waves undergo seasonal changes.4. Upper-level waves undergo seasonal changes.

a. 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

Simplified 500 mb height-contour chart for January

b. The position of the polar Jet Stream changes from b. The position of the polar Jet Stream changes from summer to wintersummer to winter

(1) Migrates towards the __________in winter and moves back towards the poles in summer.(2) In winter it may extend as far as 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.

equatorcentral Florida

H. H. Westerlies and the Heat BudgetWesterlies and the Heat Budget

1. The equator has heat and the poles experience a .

2. West-to-east winds can transfer heat from ________

to .north

excessdeficit

south

a.a. West to east flow can persist for West to east flow can persist for several weeks or more.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.b. Flow aloft may begin to meander without warning.

(1) -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

Large

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

(1) Flatter flow returns to the upper air

(2) At the surface there is less intense weather .