ch. 8: circulation of the atmosphere

Ch. 8: Circulation of the Ch. 8: Circulation of the AtmosphereAtmosphere

Main ConceptsMain Concepts Different amounts of solar energy are absorbed at different latitudes. The Different amounts of solar energy are absorbed at different latitudes. The

tropics are warmer than the polar regions because of this difference.tropics are warmer than the polar regions because of this difference. The interaction of ocean and atmosphere moderates surface temperatures, The interaction of ocean and atmosphere moderates surface temperatures,

shapes Earth’s weather and climate, and creates most of the sea’s waves and shapes Earth’s weather and climate, and creates most of the sea’s waves and currents.currents.

To observers on the surface, Earth’s rotation causes moving air (or any moving To observers on the surface, Earth’s rotation causes moving air (or any moving mass) in the Northern Hemisphere to curve to the right of its initial path, and in mass) in the Northern Hemisphere to curve to the right of its initial path, and in the Southern Hemisphere to the left. The apparent curvature of path is known the Southern Hemisphere to the left. The apparent curvature of path is known as the Coriolis effect.as the Coriolis effect.

The Coriolis effect does not initiate the movement of air or water, but once they The Coriolis effect does not initiate the movement of air or water, but once they are moving, it influences the direction of their movement.are moving, it influences the direction of their movement.

The atmosphere responds to uneven solar heating by flowing in three great The atmosphere responds to uneven solar heating by flowing in three great circulating cells over each hemisphere: A Hadley cell, a Ferrel cell, and a polar circulating cells over each hemisphere: A Hadley cell, a Ferrel cell, and a polar cell. Air circulation within these cells is powered by uneven solar heating and cell. Air circulation within these cells is powered by uneven solar heating and influenced by the Coriolis effect.influenced by the Coriolis effect.

Circulation of air is responsible for about two-thirds of the heat transfer from Circulation of air is responsible for about two-thirds of the heat transfer from tropical to polar regions. (Ocean currents account for the other third.)tropical to polar regions. (Ocean currents account for the other third.)

Large storms are spinning areas of unstable air that develop between or within Large storms are spinning areas of unstable air that develop between or within air masses. Extratropical cyclones originate at the boundary between air air masses. Extratropical cyclones originate at the boundary between air masses. Tropical cyclones, the most powerful of Earth’s atmospheric storms, masses. Tropical cyclones, the most powerful of Earth’s atmospheric storms, occur within a single humid air mass.occur within a single humid air mass.

Riding the WindRiding the Wind

Steven Fossett’s 14-day balloon trip around the world in Steven Fossett’s 14-day balloon trip around the world in 2002 illustrates the basic west-to-east movement of winds 2002 illustrates the basic west-to-east movement of winds in the mid-latitudes of both hemispheres.in the mid-latitudes of both hemispheres.

He also crossed the Atlantic the previous year in a twin-He also crossed the Atlantic the previous year in a twin-hulled sailboat, establishing a new record for the crossing.hulled sailboat, establishing a new record for the crossing.

These, and other similar journeys are possible because of These, and other similar journeys are possible because of the the uneven heating of the Earth’s surfaceuneven heating of the Earth’s surface..– This phenomenon is responsible for the movement of winds, and This phenomenon is responsible for the movement of winds, and

the formation of weather and storms.the formation of weather and storms.– Air circulation, in turn, is largely responsible for the movement of Air circulation, in turn, is largely responsible for the movement of

ocean waters.ocean waters.

Atmosphere and OceanAtmosphere and Ocean

AtmosphereAtmosphere: the volume of gases, water vapor, and : the volume of gases, water vapor, and airborne particles enveloping the Earth.airborne particles enveloping the Earth.

Atmosphere and ocean exchange gases and Atmosphere and ocean exchange gases and waters freely.waters freely.

Gases entering atmosphere from ocean influence Gases entering atmosphere from ocean influence climate.climate.

Gases entering ocean from atmosphere can Gases entering ocean from atmosphere can influence sediment deposition, distribution of life, influence sediment deposition, distribution of life, and some physical characteristics of the ocean.and some physical characteristics of the ocean.

Atmosphere and Ocean (cont.)Atmosphere and Ocean (cont.)WindWind: the mass movement of air, helps : the mass movement of air, helps

minimize worldwide temperature extremes.minimize worldwide temperature extremes.WeatherWeather: the state of the atmosphere at a : the state of the atmosphere at a

specific time and place.specific time and place. Is influenced by the interface of air and water Is influenced by the interface of air and water

at ocean’s surface.at ocean’s surface. Flow of air influences oceanic circulation.Flow of air influences oceanic circulation.ClimateClimate: the long-term average of weather in : the long-term average of weather in

an area.an area.

Composition and Properties of the Composition and Properties of the AtmosphereAtmosphere

Lower atmosphere is a nearly homogeneous mix of gases Lower atmosphere is a nearly homogeneous mix of gases ((Fig. 8.1, p. 178Fig. 8.1, p. 178), primarily:), primarily:– Nitrogen: 78.1%Nitrogen: 78.1%– Oxygen: 20.9%Oxygen: 20.9%

Other components include:Other components include:– argon, carbon dioxide, neon, helium, methane, et al.argon, carbon dioxide, neon, helium, methane, et al.

There is always some water vapor in the atmosphere:There is always some water vapor in the atmosphere:– Liquid: clouds, fogLiquid: clouds, fog– Vapor: invisibleVapor: invisible

Residence time of water in lower Residence time of water in lower atmosphere is ~ 10 days.atmosphere is ~ 10 days.

Water leaves atmosphere by:Water leaves atmosphere by:– condensing into dew, rain, or snow = condensing into dew, rain, or snow =

precipitationprecipitation..

Residence time of water in lower Residence time of water in lower atmosphere is ~ 10 days.atmosphere is ~ 10 days.

Water leaves atmosphere by:Water leaves atmosphere by:– condensing into dew, rain, or snow = condensing into dew, rain, or snow =

precipitationprecipitation..

1.1. Air has mass.Air has mass.– One cmOne cm22 column of air rising to top of atmosphere column of air rising to top of atmosphere

weighs1.04 kg (2.3 lbs).weighs1.04 kg (2.3 lbs).2.2. Warm air is less dense than cold air.Warm air is less dense than cold air.3.3. Humid air is less dense than dry air at the same Humid air is less dense than dry air at the same

temperature.temperature.– Water vapor molecules weigh less than the oxygen Water vapor molecules weigh less than the oxygen

and nitrogen molecules displaced by water vapor.and nitrogen molecules displaced by water vapor.4.4. Air is packed more densely at sea level than at Air is packed more densely at sea level than at

elevation.elevation.– Greater mass of air above it weighing down on it Greater mass of air above it weighing down on it – Air rising from sea level to higher altitude will expand Air rising from sea level to higher altitude will expand

as it is subjected to less pressure from above.as it is subjected to less pressure from above.

Air becomes cooler when it expands.Air becomes cooler when it expands.– Consider air rushing from a tire valve.Consider air rushing from a tire valve.– DemoDemo: blow on hands open-mouthed, then by pursing lips.: blow on hands open-mouthed, then by pursing lips.

Compressed air becomes warmer.Compressed air becomes warmer.– Air descending from high elevation to sea level warms as it is Air descending from high elevation to sea level warms as it is

compressed by increasingly higher pressure from above.compressed by increasingly higher pressure from above. Warm air can hold more water vapor than can cold air.Warm air can hold more water vapor than can cold air.

– Rising, humid air will expand, cool Rising, humid air will expand, cool water vapor to condense into water vapor to condense into clouds clouds rain or snow. rain or snow.

These dynamics (rising These dynamics (rising expanding expanding cooling and falling cooling and falling compressing compressing warming) influence atmospheric warming) influence atmospheric circulation, weather, and climate (circulation, weather, and climate (see Fig. 8.2, p. 179see Fig. 8.2, p. 179).).

Atmospheric CirculationAtmospheric Circulation Sunlight powers atmospheric circulation.Sunlight powers atmospheric circulation. On average, 7 million calories/mOn average, 7 million calories/m22/day strike the top of the /day strike the top of the

atmosphere, i.e. 17 trillion kw.atmosphere, i.e. 17 trillion kw. 51% of the sun’s incoming energy is absorbed by the 51% of the sun’s incoming energy is absorbed by the

Earth’s land and water surface (Earth’s land and water surface (seesee Fig. 8.3, p. 180Fig. 8.3, p. 180).).

Atmospheric CirculationAtmospheric Circulation Sunlight powers atmospheric circulation.Sunlight powers atmospheric circulation. On average, 7 million calories/mOn average, 7 million calories/m22/day strike the /day strike the top of the top of the

atmosphereatmosphere, i.e. 17 trillion kw., i.e. 17 trillion kw. 51% of the sun’s incoming energy is absorbed by the 51% of the sun’s incoming energy is absorbed by the

Atmospheric CirculationAtmospheric Circulation Sunlight powers atmospheric circulation.Sunlight powers atmospheric circulation. On average, 7 million calories/mOn average, 7 million calories/m22/day strike the top of the /day strike the top of the

atmosphere, i.e. 17 trillion kw.atmosphere, i.e. 17 trillion kw. 51% of the sun’s incoming energy is absorbed by the 51% of the sun’s incoming energy is absorbed by the

Atmospheric CirculationAtmospheric Circulation The amount of light that penetrates the ocean depends on:The amount of light that penetrates the ocean depends on:

– The angle at which the sun strikes the surfaceThe angle at which the sun strikes the surface– The surface turbulence of the oceanThe surface turbulence of the ocean– The presence of ice covering or light-colored foamThe presence of ice covering or light-colored foam– Other factorsOther factors

Atmospheric CirculationAtmospheric Circulation The short-wave light energy from the sun that The short-wave light energy from the sun that

strikes the land is converted to long-wave infrared strikes the land is converted to long-wave infrared radiation (heat) and transferred to the atmosphere radiation (heat) and transferred to the atmosphere via conduction, radiation and evaporation.via conduction, radiation and evaporation.

Atmospheric CirculationAtmospheric Circulation Eventually, this energy is radiated back to Eventually, this energy is radiated back to

space.space.

Atmospheric CirculationAtmospheric Circulation The Earth’s The Earth’s heat budgetheat budget is an expression of the total solar energy is an expression of the total solar energy

received on Earth during some period of time and the total heat lost received on Earth during some period of time and the total heat lost from Earth by reflection and radiation into space through the same from Earth by reflection and radiation into space through the same period.period.– Over the long-term, the Earth’s incoming heat equals the outgoing heat: Over the long-term, the Earth’s incoming heat equals the outgoing heat:

thermal equilibriumthermal equilibrium..– In other words, the Earth is neither growing significantly warmer nor colder.In other words, the Earth is neither growing significantly warmer nor colder.

Uneven Solar Heating and Uneven Solar Heating and LatitudeLatitude

DemoDemo: flashlight shining on desk at 90: flashlight shining on desk at 90 angle vs. acute angleangle vs. acute angle

Sunlight striking polar latitudes is spread out over larger area, thus less Sunlight striking polar latitudes is spread out over larger area, thus less radiation per unit area.radiation per unit area.

Sunlight also filters through thicker band of atmosphere before reaching polar Sunlight also filters through thicker band of atmosphere before reaching polar land and ocean surfaces.land and ocean surfaces.

The acute angle of incoming sunlight favors reflection more than 90The acute angle of incoming sunlight favors reflection more than 90 angle. angle. Tropical latitudes receive much more radiant energy per unit area than polar Tropical latitudes receive much more radiant energy per unit area than polar

regions.regions.

Uneven Solar Heating and the Uneven Solar Heating and the SeasonsSeasons

The Earth is tilted 23.5The Earth is tilted 23.5 on its axis with regard to the Sun. on its axis with regard to the Sun.– Causes the Northern Hemisphere to tilt Causes the Northern Hemisphere to tilt towardtoward the Sun in June, and the Sun in June, and awayaway

from it in December (from it in December (StudyStudy Fig. 8.6, p. 181Fig. 8.6, p. 181).).– Also causes days to lengthen as summer approaches, and to become Also causes days to lengthen as summer approaches, and to become

shorter during winter.shorter during winter.– Mid-latitude regions receive about three times as much light energy (and, Mid-latitude regions receive about three times as much light energy (and,

therefore, heat) in the summer than in winter.therefore, heat) in the summer than in winter. The tilt of the Earth The tilt of the Earth the seasons the seasons..

Uneven Solar Heating and Uneven Solar Heating and Atmospheric CirculationAtmospheric Circulation

Warm air rises; cool air sinks.Warm air rises; cool air sinks. Convection currentConvection current: the circular movement of : the circular movement of

rising warm material (in this case, air) and falling rising warm material (in this case, air) and falling cool material (cool material (see Fig. 8.7, p. 182see Fig. 8.7, p. 182).).

The changing density of the air causes the current The changing density of the air causes the current (warm air expands, becoming less dense; cool air (warm air expands, becoming less dense; cool air descends, becoming compressed).descends, becoming compressed).

A similar phenomenon A similar phenomenon occurs to atmospheric occurs to atmospheric air on a global scale air on a global scale ((see Fig. 8.8, p. 182see Fig. 8.8, p. 182).).

This pattern is affected This pattern is affected by two factors:by two factors:– Uneven solar heatingUneven solar heating– The Earth’s eastward The Earth’s eastward

rotationrotation

The Coriolis EffectThe Coriolis Effect

The Earth’s eastward rotation causes moving The Earth’s eastward rotation causes moving objects (including air and water) to be deflected objects (including air and water) to be deflected away from their initial course: the away from their initial course: the Coriolis effectCoriolis effect, , named after:named after:

Gaspard Gustave de CoriolisGaspard Gustave de Coriolis, who worked out , who worked out its mathematics in 1835.its mathematics in 1835.– In In Northern HemisphereNorthern Hemisphere: deflection is : deflection is to the rightto the right

((clockwiseclockwise).).– In In Southern HemisphereSouthern Hemisphere: deflection is : deflection is to the leftto the left

((counterclockwisecounterclockwise).). Observed deflection is caused by the observer’s Observed deflection is caused by the observer’s

moving frame of reference on the spinning Earth.moving frame of reference on the spinning Earth.

The Coriolis EffectThe Coriolis Effect Consider two cities: Quito, Ecuador and Consider two cities: Quito, Ecuador and

Buffalo, NY (both on 79Buffalo, NY (both on 79W line of longitude).W line of longitude).

– Both cities make one complete Both cities make one complete rotation in 24 hours, i.e. both rotation in 24 hours, i.e. both cities move eastward at an cities move eastward at an angular rate of 15angular rate of 15 per hour per hour (360(360/24 hrs)./24 hrs).

– Buffalo’s circumferential path is Buffalo’s circumferential path is much shorter than Quito’s, so much shorter than Quito’s, so which city is traveling at a which city is traveling at a higher rate of speed?higher rate of speed?

– Buffalo’s circumferential path is Buffalo’s circumferential path is much shorter than Quito’s, so much shorter than Quito’s, so which city is traveling at a which city is traveling at a higher rate of speed?higher rate of speed? Quito. Quito.

– Buffalo’s circumferential path is Buffalo’s circumferential path is much shorter than Quito’s, so much shorter than Quito’s, so which city is traveling at a which city is traveling at a higher rate of speed? Quito.higher rate of speed? Quito.

– Buffalo: 1,260 km/hr (783 mi/hr)Buffalo: 1,260 km/hr (783 mi/hr)– Quito: 1,668 km/hr (1,036 mi/hr)Quito: 1,668 km/hr (1,036 mi/hr)

– Consider a cannonball Consider a cannonball shot from Quito shot from Quito northnorth toward Buffalo (toward Buffalo (Fig.8.12, Fig.8.12, p. 184p. 184)) Cannonball is initially Cannonball is initially

traveling eastward at 1,668 traveling eastward at 1,668 km/hr, while Buffalo is only km/hr, while Buffalo is only traveling at 1,260 km/hr.traveling at 1,260 km/hr.

Cannonball’s path will cause Cannonball’s path will cause it to it to deflect to the rightdeflect to the right and and it it will fall to the east of its will fall to the east of its targettarget..

– A cannonball launched A cannonball launched from Buffalo from Buffalo southsouth toward toward Quito:Quito: Starts out at an initial Starts out at an initial

eastward speed of 1,260 eastward speed of 1,260 km/hr, while Quito is moving km/hr, while Quito is moving to the east at 1,668 km/hr.to the east at 1,668 km/hr.

Cannonball’s path again will Cannonball’s path again will deflect to the rightdeflect to the right, causing , causing it to it to fall to the west of its fall to the west of its targettarget..

– Observing the Coriolis Observing the Coriolis effect depends on our effect depends on our frame of reference and the frame of reference and the direction from which we direction from which we view the problem.view the problem. Looking north at cannonball Looking north at cannonball

1’s flight from Quito toward 1’s flight from Quito toward Buffalo, the ball veers Buffalo, the ball veers clockwise (to the right).clockwise (to the right).

Looking south at cannonball Looking south at cannonball 2’s flight from Buffalo toward 2’s flight from Buffalo toward Quito, the ball again veers Quito, the ball again veers clockwise (to the right).clockwise (to the right).

– The The Coriolis effectCoriolis effect influences any moving influences any moving object with mass, including air and waterobject with mass, including air and water on on Earth.Earth.Most apparent in mid-latitude situations involving almost Most apparent in mid-latitude situations involving almost

frictionless flow of fluids: frictionless flow of fluids: – Between layers of water in the oceanBetween layers of water in the ocean– In the circulation of windsIn the circulation of winds

The Coriolis Effect and The Coriolis Effect and Atmospheric Circulation CellsAtmospheric Circulation Cells

See Fig. 8.13, p. 185See Fig. 8.13, p. 185 There are six large There are six large

convection cells convection cells ((atmospheric atmospheric circulation cellscirculation cells) ) spanning the globe spanning the globe (3 in the Northern (3 in the Northern Hemisphere and 3 in Hemisphere and 3 in the Southern the Southern Hemisphere)Hemisphere)

1.1. Warm air rises at the Warm air rises at the Equator and then Equator and then moves poleward at moves poleward at high altitude (some high altitude (some air toward each pole)air toward each pole)

2.2. This parcel of air is This parcel of air is rich in water, but as it rich in water, but as it rises, it cools; water rises, it cools; water vapor vapor clouds clouds rain.rain. This explains the This explains the

tropical rain forests.tropical rain forests.

3.3. Depleted of most of Depleted of most of its water, and now its water, and now cool, this air mass cool, this air mass becomes denser and becomes denser and sinks back to the sinks back to the surface of the Earth surface of the Earth at ~ 30at ~ 30N and 30N and 30S S latitude, becoming latitude, becoming increasingly warmer increasingly warmer as it compresses.as it compresses.

This explains the This explains the presence of the presence of the world’s great deserts world’s great deserts at ~ 30at ~ 30N and 30N and 30S S latitudelatitude

4.4. Air that rose at the Air that rose at the Equator must be Equator must be replaced at the surface, replaced at the surface, and so, the air from 30and so, the air from 30 N and 30N and 30S travels back S travels back toward the Equator along toward the Equator along the surface of the Earth, the surface of the Earth, completing the completing the convection cell.convection cell.

5.5. As this air mass travels As this air mass travels across the surface, it across the surface, it causes evaporation of causes evaporation of surface water, which surface water, which adds humidity to the air adds humidity to the air as it travels toward the as it travels toward the EquatorEquator

This is called a This is called a Hadley Hadley cellcell, named for George , named for George Hadley, a London Hadley, a London lawyer and philosopher lawyer and philosopher who worked out the who worked out the pattern of wind pattern of wind circulation in 1735.circulation in 1735.

Two more circulation Two more circulation cells exist between 30cells exist between 30 and 60and 60 N and S: N and S: Ferrel Ferrel cellscells..

Named for William Named for William Ferrel, an American who Ferrel, an American who described their action in described their action in the mid-1800’s.the mid-1800’s.

1.1. Some of the air descending at Some of the air descending at 3030N and S latitude turns N and S latitude turns poleward when it reaches the poleward when it reaches the Earth’s surface (rather than Earth’s surface (rather than back toward the Equator).back toward the Equator).

2.2. High altitude air from the north High altitude air from the north merges with this air as it merges with this air as it descends.descends.

3.3. Surface air travels northward to Surface air travels northward to about 60about 60N (in the Northern N (in the Northern Hemisphere), picking up Hemisphere), picking up surface heat and moisture as it surface heat and moisture as it does so.does so.

4.4. At ~ 60At ~ 60N (and 60N (and 60S) this air S) this air mass begins to rise once again.mass begins to rise once again.

A third pair of cells A third pair of cells exists between the exists between the poles and 60poles and 60N and N and 6060S in each S in each hemisphere.hemisphere.

1.1. Air that has been chilled as it Air that has been chilled as it approaches the poles becomes approaches the poles becomes denser and sinks at the poles.denser and sinks at the poles.

2.2. Once it reaches the ground it turns Once it reaches the ground it turns equatorward, skimming the surface equatorward, skimming the surface of Earth until it meets the rising air of Earth until it meets the rising air from its neighboring Ferrel cell, from its neighboring Ferrel cell, where it rises once more: a where it rises once more: a polar polar cellcell..

3.3. Here (60Here (60N and 60N and 60S latitude) the S latitude) the cool air from the north and warm air cool air from the north and warm air from the south converge and create from the south converge and create unstable weather conditions unstable weather conditions mid- mid-latitude storms.latitude storms.

4.4. The frigid air descending at the The frigid air descending at the poles, has been drained of much of poles, has been drained of much of its water in these storms. Thus, the its water in these storms. Thus, the polar regions are very dry (rather polar regions are very dry (rather like arctic deserts).like arctic deserts).

These global atmospheric circulation cells are all These global atmospheric circulation cells are all powered by the uneven solar heating of the powered by the uneven solar heating of the Earth’s surfaceEarth’s surface..

Wind PatternsWind Patterns The Coriolis Effect The Coriolis Effect

influences all these influences all these circulation cells, causing circulation cells, causing winds to veer to the right of winds to veer to the right of their initial path in the their initial path in the Northern Hemisphere, and Northern Hemisphere, and to the left of their initial path to the left of their initial path in the Southern Hemisphere. in the Southern Hemisphere. Thus . . . Thus . . .

– Surface winds between Surface winds between 3030N and the Equator blow N and the Equator blow from NE to SW (from NE to SW (NE trade NE trade windswinds).).

– Surface winds between Surface winds between 3030S and the Equator blow S and the Equator blow from SE to NW (from SE to NW (SE trade SE trade windswinds).).

Wind PatternsWind Patterns** ** NoteNote: Winds are : Winds are

named for the named for the direction from which direction from which they comethey come..

Surface winds between Surface winds between 3030N and 60N and 60N blow N blow from SW to NE from SW to NE ((westerlieswesterlies).).

Surface winds between Surface winds between 3030S and 60S and 60S blow S blow from NW to SE from NW to SE ((westerlieswesterlies).).

Wind PatternsWind Patterns** ** NoteNote: Winds are : Winds are

Wind PatternsWind Patterns** Note** Note: Winds are : Winds are

Wind PatternsWind Patterns Surface winds between Surface winds between

6060N and 90N and 90N (North N (North Pole) blow from NE to Pole) blow from NE to SW (SW (polar easterliespolar easterlies).).

Surface winds between Surface winds between 6060S and 90S and 90S (South S (South Pole) blow from SE to Pole) blow from SE to NW (NW (polar easterliespolar easterlies).).

Wind PatternsWind Patterns Surface winds between Surface winds between

6060N and 90N and 90N (North N (North Pole) blow from NE to Pole) blow from NE to SW (SW (polar easterliespolar easterlies).).

Surface winds between Surface winds between 6060S and 90S and 90S (South S (South Pole) blow from SE to Pole) blow from SE to NW (NW (polar easterliespolar easterlies).).

Wind PatternsWind Patterns Notice the places on the Notice the places on the

globe where the air is globe where the air is moving moving verticallyvertically: Equator, : Equator, 3030N & 30N & 30S, 60S, 60N & 60N & 60S, S, at the poles.at the poles.

Where air is moving upward Where air is moving upward low pressure systemlow pressure system (Equator, 60(Equator, 60N & S).N & S).

Where air is moving Where air is moving downward downward high high pressure systempressure system (30(30 N & S, the Poles). N & S, the Poles).

Wind PatternsWind Patterns At these latitudes, winds are At these latitudes, winds are

weak and erratic.weak and erratic.

Equatorial doldrumsEquatorial doldrums: low : low pressure band encircling the pressure band encircling the Earth at the Equator; AKA Earth at the Equator; AKA intertropical convergence intertropical convergence zone (ITCZ)zone (ITCZ)..

Horse latitudes Horse latitudes (subtropical high)(subtropical high): high : high pressure band located at pressure band located at 3030N & S latitude. N & S latitude.

Wind PatternsWind Patterns Places within the circulation Places within the circulation

cells where air travels cells where air travels horizontally are horizontally are characterized by strong, characterized by strong, reliable winds. Mariners reliable winds. Mariners long ago learned how to long ago learned how to take advantage of these take advantage of these wind conditions.wind conditions.

Sailors outbound from Sailors outbound from Europe for the New World Europe for the New World would take a more southerly would take a more southerly route, utilizing the trade route, utilizing the trade winds.winds.

For the return trip to Europe, For the return trip to Europe, they would sail north and they would sail north and take advantage of the take advantage of the westerlies.westerlies.

Cell Circulation: Ideal vs. ActualCell Circulation: Ideal vs. Actual The above model is accurate in representing the The above model is accurate in representing the overall overall

average air flowaverage air flow over the globe over many years. over the globe over many years. Certain irregularities occur from differences in topography Certain irregularities occur from differences in topography

from one place to another.from one place to another. The ocean’s thermostatic effect minimizes the irregularities The ocean’s thermostatic effect minimizes the irregularities

over water.over water. ITCZ is much narrower and more consistent over the ITCZ is much narrower and more consistent over the

ocean than over land.ocean than over land.

Cell Circulation: Ideal vs. ActualCell Circulation: Ideal vs. Actual

There is more land (with attendant lower There is more land (with attendant lower heat capacity than water) and less ocean heat capacity than water) and less ocean surface in the Northern Hemisphere.surface in the Northern Hemisphere.Seasonal differences in temperature and cell Seasonal differences in temperature and cell

circulation are more extreme in the Northern circulation are more extreme in the Northern Hemisphere.Hemisphere.

Cell circulation is more symmetrical in the Cell circulation is more symmetrical in the Southern Hemisphere.Southern Hemisphere.

Cell Circulation: Ideal vs. ActualCell Circulation: Ideal vs. Actual There is more land (with attendant lower heat capacity than There is more land (with attendant lower heat capacity than

water) and less ocean surface in the Northern Hemisphere.water) and less ocean surface in the Northern Hemisphere. The ITCZ lies not along the geographical equator, but The ITCZ lies not along the geographical equator, but

along the along the meteorologicalmeteorological (or (or thermalthermal) ) equatorequator: the : the irregular, imaginary line of thermal equilibrium between irregular, imaginary line of thermal equilibrium between the two hemispheres (the two hemispheres (see Fig. 8.14, p. 186see Fig. 8.14, p. 186).).

Cell Circulation: Ideal vs. ActualCell Circulation: Ideal vs. Actual – This line shifts with the seasons, moving somewhat farther to the This line shifts with the seasons, moving somewhat farther to the

north during the northern summer and returning to the equator in north during the northern summer and returning to the equator in the northern winter.the northern winter.

– Atmospheric and oceanic circulation in the two hemispheres are Atmospheric and oceanic circulation in the two hemispheres are roughly symmetrical about the meteorological equator roughly symmetrical about the meteorological equator

– The tradewinds, westerlies, doldrums, etc. move north in the The tradewinds, westerlies, doldrums, etc. move north in the northern summer and south in the northern winter.northern summer and south in the northern winter.

Cell Circulation: Ideal vs. ActualCell Circulation: Ideal vs. Actual There are also east-west variations in the There are also east-west variations in the

expected patterns.expected patterns. In the northern winter, cold air settles over the In the northern winter, cold air settles over the

large land masses of N.A. and Siberia, creating large land masses of N.A. and Siberia, creating high pressure systems.high pressure systems.

Meanwhile, low pressure systems form over the Meanwhile, low pressure systems form over the relatively warmer ocean waters near the Aleutians relatively warmer ocean waters near the Aleutians and Iceland.and Iceland.

Air moves from high pressure to low pressure, Air moves from high pressure to low pressure, altering the flow of air in the affected cells.altering the flow of air in the affected cells.

In summer, the situation is reversed.In summer, the situation is reversed.

See Table 8.1, p. 188See Table 8.1, p. 188– Summarizes the major surface wind and Summarizes the major surface wind and

pressure systems of the world, along with pressure systems of the world, along with prevailing weather conditions.prevailing weather conditions.

– These winds are responsible for two-thirds of These winds are responsible for two-thirds of the heat transfer from the tropics to the poles.the heat transfer from the tropics to the poles.

MonsoonsMonsoons

MonsoonMonsoon (from Arabic (from Arabic mausimmausim = “season”): = “season”): a pattern of wind circulation that changes with a pattern of wind circulation that changes with the seasons; usually wet summers and dry the seasons; usually wet summers and dry winters. Related to:winters. Related to:

1.1. The different heat capacities of land and waterThe different heat capacities of land and water2.2. The seasonal north-south movement of the The seasonal north-south movement of the

ITCZITCZ

MonsoonsMonsoons Spring Spring Summer: Summer:

– Land heats up faster than ocean Land heats up faster than ocean warm air rises (low pressure warm air rises (low pressure zone)zone)

– Relatively cool air descending over ocean (high pressure zone) Relatively cool air descending over ocean (high pressure zone) moves onshore, laden with moisture moves onshore, laden with moisture summer monsoonal rains summer monsoonal rains over land.over land.

MonsoonsMonsoons Fall Fall Winter: Winter:

– Land cools down faster than ocean Land cools down faster than ocean creates high pressure zone creates high pressure zone of descending air over land of descending air over land air flows from land to ocean, where air flows from land to ocean, where the relatively warm water forms a low pressure zone of rising air the relatively warm water forms a low pressure zone of rising air dry winter season.dry winter season.

MonsoonsMonsoons Most intense summer monsoons occur in Most intense summer monsoons occur in

Asia.Asia.

Sea Breezes and Land BreezesSea Breezes and Land Breezes Small, daily mini-monsoons.Small, daily mini-monsoons. During the day:During the day:

– Land heats up faster than ocean Land heats up faster than ocean air above heats up and rises air above heats up and rises low pressure system.low pressure system.

– Relatively Relatively cooler aircooler air of high pressure system over ocean flows of high pressure system over ocean flows onshore: onshore: sea breezesea breeze..

Sea Breezes and Land BreezesSea Breezes and Land Breezes At night:At night:

– Land cools down faster than ocean Land cools down faster than ocean air over ocean is relatively air over ocean is relatively warmer and rises, forming a low pressure zone.warmer and rises, forming a low pressure zone.

– Cooler air over land descends in a high pressure zone.Cooler air over land descends in a high pressure zone.– Air flows from land to sea: Air flows from land to sea: land breezeland breeze..

StormsStorms StormStorm: regional atmospheric disturbances : regional atmospheric disturbances

characterized by strong winds and, frequently, characterized by strong winds and, frequently, precipitation.precipitation.– Powered by stored sunlight.Powered by stored sunlight.

StormsStorms CyclonesCyclones (Gk: (Gk: kyklonkyklon = “object traveling in a = “object traveling in a

circle”): huge, rotating masses of low-pressure air circle”): huge, rotating masses of low-pressure air in which winds converge and ascend.in which winds converge and ascend.

StormsStorms

Demo:Demo: Tracing a cyclone on paper Tracing a cyclone on paper

StormsStorms Tropical cyclonesTropical cyclones: occur in tropical regions: occur in tropical regions

– 13 November, 197013 November, 1970: Bangladesh tropical : Bangladesh tropical cyclone killed 300,000.cyclone killed 300,000.

– Storm surge 12 m (40 ft) highStorm surge 12 m (40 ft) high– May 1991May 1991: Another cyclone killed 200,000: Another cyclone killed 200,000

StormsStorms Extratropical cycloneExtratropical cyclone ( (extraextra = “outside”): = “outside”):

occur mostly in Ferrel cellsoccur mostly in Ferrel cells March, 1993: U.S. East coast storm killed ~ March, 1993: U.S. East coast storm killed ~

290.290.– 175 km/hr (109 mi/hr) winds in Florida175 km/hr (109 mi/hr) winds in Florida– Record cold in Alabama.Record cold in Alabama.– $1 billion in damage.$1 billion in damage.

Tropical storm damage: Galveston, TX, 1900

Air MassesAir Masses Air massAir mass: large body of air with nearly uniform : large body of air with nearly uniform

temperature, humidity and density throughout.temperature, humidity and density throughout. Air takes on the characteristics of the surface below: Air takes on the characteristics of the surface below:

ocean or land.ocean or land. Six general categoriesSix general categories based on type of land or based on type of land or

water over which they form and the latitude at which water over which they form and the latitude at which they form:they form:– a. a. Surface typeSurface type is designated by lower case letter: is designated by lower case letter:

mm (maritime); (maritime); cc (continental) (continental)– b. b. Source regionSource region is designated by upper case letter: is designated by upper case letter:

AA (arctic); (arctic); PP (polar); (polar); TT (tropical) (tropical)

Air MassesAir Masses Air massAir mass: large body of air with nearly uniform : large body of air with nearly uniform

temperature, humidity and density throughout.temperature, humidity and density throughout. Air takes on the characteristics of the surface below: Air takes on the characteristics of the surface below:

ocean or land.ocean or land. Six general categoriesSix general categories based on type of land or based on type of land or

water over which they form and the latitude at which water over which they form and the latitude at which they form:they form:– a. a. Surface typeSurface type is designated by lower case letter: is designated by lower case letter:

mm (maritime); (maritime); cc (continental) (continental)– b. b. Source regionSource region is designated by upper case letter: is designated by upper case letter:

AA (arctic); (arctic); PP (polar); (polar); TT (tropical) (tropical)

Air MassesAir Masses

Continental polarContinental polar ( (cP) and ) and continental arctic continental arctic (cA)(cA): usually produce: : usually produce:

a. a. winter: cold, dry weatherb. b. summer: cool, pleasant weather

Maritime polarMaritime polar and and maritime arcticmaritime arctic: pick up : pick up moisture as they travel across oceans and tend to moisture as they travel across oceans and tend to produce cool, moist weather.produce cool, moist weather.

Continental tropicalContinental tropical: hot, dry weather of summer.: hot, dry weather of summer. Maritime tropicalMaritime tropical: warm, humid weather.: warm, humid weather.

Continental polarContinental polar and and continental arcticcontinental arctic: usually : usually produce: produce:

a. winter: cold, dry weathera. winter: cold, dry weatherb. summer: cool, pleasant weatherb. summer: cool, pleasant weather

Maritime polarMaritime polar ( (mP) and ) and maritime arctic (mA)maritime arctic (mA): : pick up moisture as they travel across oceans and pick up moisture as they travel across oceans and tend to produce tend to produce cool, moist weather. weather.

Continental tropicalContinental tropical: hot, dry weather of summer.: hot, dry weather of summer. Maritime tropicalMaritime tropical: warm, humid weather.: warm, humid weather.

Air MassesAir Masses Continental polarContinental polar and and continental arcticcontinental arctic: usually : usually

produce: produce: a. winter: cold, dry weathera. winter: cold, dry weatherb. summer: cool, pleasant weatherb. summer: cool, pleasant weather

Continental tropical (cT)Continental tropical (cT): : hot, dry weather of weather of summer.summer.

Maritime tropicalMaritime tropical: warm, humid weather.: warm, humid weather.

Continental polarContinental polar and and continental arcticcontinental arctic: usually : usually produce: produce:

a. winter: cold, dry weathera. winter: cold, dry weatherb. summer: cool, pleasant weatherb. summer: cool, pleasant weather

Continental tropicalContinental tropical: hot, dry weather of summer.: hot, dry weather of summer. Maritime tropical (mT)Maritime tropical (mT): : warm, humid weather. weather.

Cold, dry weather Cold, dry weather increased density increased density high pressure zonehigh pressure zone

Warm, humid weather Warm, humid weather lighter lighter low low pressure zone pressure zone air rises, expands, cools air rises, expands, cools water vapor condenses. water vapor condenses.

At boundaries, where different air masses At boundaries, where different air masses meet, turbulent weather is fostered.meet, turbulent weather is fostered.

Extratropical CyclonesExtratropical Cyclones AKA “mid-latitude cyclones”AKA “mid-latitude cyclones” Form at the Form at the boundary between the polar and Ferrel

cells in each hemisphere: the in each hemisphere: the polar frontpolar front.... Mainly in winter hemisphere when temperature and density Mainly in winter hemisphere when temperature and density

differences between the cells are most pronounced.differences between the cells are most pronounced.

Extratropical CyclonesExtratropical Cyclones

1.1. Cold polar wind is Cold polar wind is generally moving generally moving from east to west.from east to west.

2.2. Warmer air to the Warmer air to the south of the front south of the front is moving from is moving from west to east.west to east.

1.1. Cold polar wind is Cold polar wind is generally moving generally moving from east to west.from east to west.

2.2. Warmer air to the Warmer air to the south of the front south of the front is moving from is moving from west to east.west to east.

3.3. High and low High and low pressure zones pressure zones along the front along the front may bend it into a may bend it into a series of waves.series of waves.

Extratropical CyclonesExtratropical Cyclones 4.4. As the winds move in As the winds move in

different directions different directions along the front, the along the front, the wave shape may twist.wave shape may twist.

5.5. Eventually, the cold air Eventually, the cold air mass may catch up to mass may catch up to the warm air.the warm air.

6.6. Cold, dense air will Cold, dense air will slip below the warmer slip below the warmer air, pushing it upward, air, pushing it upward, but mixing will but mixing will generally not occur.generally not occur.

7.7. This twisting mass of air becomes an This twisting mass of air becomes an extratropical cyclone.extratropical cyclone. Counterclockwise in Northern Hemisphere.Counterclockwise in Northern Hemisphere. Clockwise in Southern Hemisphere.Clockwise in Southern Hemisphere.

Extratropical CyclonesExtratropical Cyclones Why is the direction opposite movement caused by the Coriolis Why is the direction opposite movement caused by the Coriolis

Effect?Effect?

Extratropical CyclonesExtratropical Cyclones Why is the direction opposite movement caused by the Coriolis Why is the direction opposite movement caused by the Coriolis

Effect?Effect? Winds approaching the low pressure zone from the surrounding Winds approaching the low pressure zone from the surrounding

areas of high pressure all converge and turn right (in the Northern areas of high pressure all converge and turn right (in the Northern Hemisphere), forming the counterclockwise rotation.Hemisphere), forming the counterclockwise rotation.

Extratropical CyclonesExtratropical Cyclones As air spins toward the center of the low pressure As air spins toward the center of the low pressure

zone, it speeds up (like a spinning skater).zone, it speeds up (like a spinning skater). Demo: Demo: student in rotating chairstudent in rotating chair

Extratropical CyclonesExtratropical Cyclones Extratropical cyclones occur in Ferrel cells where westerlies Extratropical cyclones occur in Ferrel cells where westerlies

predominate predominate – Travel eastward.Travel eastward.– May be 1,000 – 2,500 km (620 – 1,600 mi) in diameter.May be 1,000 – 2,500 km (620 – 1,600 mi) in diameter.

Rising air causes precipitation (as air cools and releases its Rising air causes precipitation (as air cools and releases its water vapor).water vapor).

Extratropical CyclonesExtratropical Cyclones Extratropical cyclones occur in Ferrel cells where westerlies Extratropical cyclones occur in Ferrel cells where westerlies

predominate predominate – Travel eastward.Travel eastward.– May be 1,000 – 2,500 km (620 – 1,600 mi) in diameter.May be 1,000 – 2,500 km (620 – 1,600 mi) in diameter.

Rising air causes precipitation (as air cools and releases its Rising air causes precipitation (as air cools and releases its water vapor).water vapor).

Extratropical CyclonesExtratropical Cyclones Cold front: Cold front: occurs when cold air approaches warm air occurs when cold air approaches warm air

and pushes the warm air upward.and pushes the warm air upward. Warm front: Warm front: occurs when warm air approaches the edge occurs when warm air approaches the edge

of a retreating cold air mass and flows over the top of it.of a retreating cold air mass and flows over the top of it.

Frontal storms: Frontal storms: the winds and and precipitation the winds and and precipitation associated with these fronts.associated with these fronts.– These are the principal cause of weather in the mid-latitudes.These are the principal cause of weather in the mid-latitudes.

Frontal storms: Frontal storms: the winds and precipitation associated the winds and precipitation associated with these fronts.with these fronts.– These are the principal cause of weather in the mid-latitudes.These are the principal cause of weather in the mid-latitudes.

Extratropical CyclonesExtratropical Cyclones Nor’easters Nor’easters

(northeasters): N.A.’s (northeasters): N.A.’s most violent most violent extratropical cyclones.extratropical cyclones.– Occur along Eastern Occur along Eastern

Seaboard in winter.Seaboard in winter.– Name indicates the Name indicates the

direction from which the direction from which the storm originates.storm originates.

– ~ 30/yr~ 30/yr– Cause great deal of Cause great deal of

property and economic property and economic damage.damage.

– Generate wind and waves Generate wind and waves strong enough to erode strong enough to erode beaches and barrier beaches and barrier islands.islands.

Tropical CyclonesTropical Cyclones Tropical cycloneTropical cyclone: a weather system of low : a weather system of low

atmospheric pressure around which winds atmospheric pressure around which winds blow.blow.– Originates in the tropics within a single air Originates in the tropics within a single air

mass, but may move to temperate waters if the mass, but may move to temperate waters if the water temperature is high enough to sustain it.water temperature is high enough to sustain it.

Tropical CyclonesTropical Cyclones Tropical cycloneTropical cyclone: a weather system of low : a weather system of low

atmospheric pressure around which winds atmospheric pressure around which winds blow.blow.– Originates in the tropics within a single air Originates in the tropics within a single air

mass, but may move to temperate waters if the mass, but may move to temperate waters if the water temperature is high enough to sustain it.water temperature is high enough to sustain it.

Tropical CyclonesTropical Cyclones

Three types of tropical cyclone:Three types of tropical cyclone:1.1. Tropical depressionsTropical depressions: small tropical : small tropical

cyclonescyclones2.2. Tropical stormsTropical storms: medium-sized tropical : medium-sized tropical

cyclonescyclones

3.3. HurricanesHurricanes (“ (“typhoons”typhoons” in western in western Pacific, “Pacific, “willi-willis” willi-willis” near Australia): large-near Australia): large-scale tropical cyclonesscale tropical cyclones

– Winds at least 119 km/hr (74 mi/hr)Winds at least 119 km/hr (74 mi/hr)– ~ 100/yr~ 100/yr

Hurricane Wilma, Oct. 15 – 25, 2005

Hurricane Katrina, Aug. 29, 2005

Tropical CyclonesTropical Cyclones HurricanesHurricanes (“ (“typhoons”typhoons” in western Pacific, “ in western Pacific, “willi-willis” willi-willis”

near Australia): large-scale tropical cyclonesnear Australia): large-scale tropical cyclones– May be 1,000 km (620 mi) acrossMay be 1,000 km (620 mi) across– 15 km (9.3 mi; 50,000 ft) high15 km (9.3 mi; 50,000 ft) high

Tropical CyclonesTropical Cyclones HurricanesHurricanes (“ (“typhoons”typhoons” in western Pacific, “ in western Pacific, “willi-willis” willi-willis”

near Australia): large-scale tropical cyclonesnear Australia): large-scale tropical cyclones– Calm center, or “eye” of the storm averages 13 – 16 km (8 – 10 mi) Calm center, or “eye” of the storm averages 13 – 16 km (8 – 10 mi)

in diameterin diameter– Eye is surrounded by bands of clouds and furious windsEye is surrounded by bands of clouds and furious winds

Tropical CyclonesTropical Cyclones The greatest of these tropical storms form in humid areas The greatest of these tropical storms form in humid areas

between 10between 10 and 25 and 25 latitude (either hemisphere). latitude (either hemisphere). Tropical depressionTropical depression: initial stage of a tropical storm; : initial stage of a tropical storm;

rotating winds forming in areas of low pressure within the rotating winds forming in areas of low pressure within the easterly trade winds. easterly trade winds. – If these depressions move over the ocean in areas with If these depressions move over the ocean in areas with

temperatures > 26temperatures > 26C (79C (79F), circular winds grow stronger and F), circular winds grow stronger and humid air is forced upward.humid air is forced upward.

– Condensation begins and the storm develops.Condensation begins and the storm develops.

rotating winds forming in areas of low pressure within the rotating winds forming in areas of low pressure within the easterly trade winds.easterly trade winds. – If these depressions move over the ocean in areas with If these depressions move over the ocean in areas with

Tropical CyclonesTropical Cyclones If conditions are ideal, wind speed can increase to 119 If conditions are ideal, wind speed can increase to 119

km/hr (74 mi/hr) km/hr (74 mi/hr) hurricanehurricane..– Typical horizontal movement is westward toward the poles, due to Typical horizontal movement is westward toward the poles, due to

prevailing trade winds (prevailing trade winds (see Fig. 8.25b, p. 194see Fig. 8.25b, p. 194).).– The energy for such storms comes from the condensation of the The energy for such storms comes from the condensation of the

water vapor, which releases energy in the form of wind movement.water vapor, which releases energy in the form of wind movement.– Warm, humid air forms over the tropical ocean Warm, humid air forms over the tropical ocean the air rises, the air rises,

expands, cools expands, cools condensation of water vapor in the air condensation of water vapor in the air rain. rain.– This condensation releases tremendous amounts of energy.This condensation releases tremendous amounts of energy.

km/hr (74 mi/hr) km/hr (74 mi/hr) hurricanehurricane..– Typical horizontal movement is westward toward the Typical horizontal movement is westward toward the

poles, due to prevailing trade winds poles, due to prevailing trade winds – ((see Fig. 8.25b, p. 194see Fig. 8.25b, p. 194).).

Hurricane Georges, Sept. 18 – 28, 1998

Hurricane Katrina, Aug. 23 - 31, 2005

Tropical CyclonesTropical Cyclones Solar energy powers tropical stormsSolar energy powers tropical storms::

– Incoming solar energy Incoming solar energy heat absorption by the water heat absorption by the water evaporation evaporation condensation condensation conversion to kinetic conversion to kinetic energy (wind movement).energy (wind movement).

– This energy is available as long as the storm remains This energy is available as long as the storm remains over warm water and has an immediate source of warm, over warm water and has an immediate source of warm, humid air.humid air.

Tropical CyclonesTropical Cyclones Property damage and loss of life are caused by wind, Property damage and loss of life are caused by wind,

rains, and storm surges.rains, and storm surges. Storm surgeStorm surge: a dome of water produced by the low : a dome of water produced by the low

pressure system; a mass of water driven by a storm.pressure system; a mass of water driven by a storm. If a storm surge occurs during a high tide, the combined If a storm surge occurs during a high tide, the combined

results can be devastating, e.g.:results can be devastating, e.g.:1.1. 1970: 12 m (40 ft) storm surge in Bangladesh.1970: 12 m (40 ft) storm surge in Bangladesh.2.2. 1989: $3 billion of damage done by 5 m (16 ft) storm surge from 1989: $3 billion of damage done by 5 m (16 ft) storm surge from

Hurricane Hugo in South Carolina.Hurricane Hugo in South Carolina.

Tropical CyclonesTropical Cyclones Property damage and loss of life are caused by wind, Property damage and loss of life are caused by wind,

rains, and storm surges.rains, and storm surges. Storm surgeStorm surge: a dome of water produced by the low : a dome of water produced by the low

pressure system; a mass of water driven by a storm.pressure system; a mass of water driven by a storm. If a storm surge occurs during a high tide, the combined If a storm surge occurs during a high tide, the combined

results can be devastating, e.g.:results can be devastating, e.g.:1.1. 1970: 12 m (40 ft) storm surge in Bangladesh.1970: 12 m (40 ft) storm surge in Bangladesh.2.2. 1989: $3 billion of damage done by 5 m (16 ft) storm surge from 1989: $3 billion of damage done by 5 m (16 ft) storm surge from

Hurricane Hugo in South Carolina.Hurricane Hugo in South Carolina.

Tropical CyclonesTropical Cyclones Property damage and loss of life are caused by wind, rains, and storm Property damage and loss of life are caused by wind, rains, and storm

surges.surges. Storm surgeStorm surge: a dome of water produced by the low pressure system; : a dome of water produced by the low pressure system;

a mass of water driven by a storm.a mass of water driven by a storm. If a storm surge occurs during a high tide, the combined results can If a storm surge occurs during a high tide, the combined results can

be devastating, e.g.:be devastating, e.g.:1.1. 1970: 12 m (40 ft) storm surge in Bangladesh.1970: 12 m (40 ft) storm surge in Bangladesh.2.2. 1989: $3 billion of damage done by 5 m (16 ft) storm surge from Hurricane 1989: $3 billion of damage done by 5 m (16 ft) storm surge from Hurricane

Hugo in South Carolina.Hugo in South Carolina.3.3. 2005: $125 billion damage by 9 m (28 ft) storm surge from Hurricane 2005: $125 billion damage by 9 m (28 ft) storm surge from Hurricane

KatrinaKatrina

Tropical CyclonesTropical Cyclones Cyclones usually last 5 – 10 days (range: 3 hrs – Cyclones usually last 5 – 10 days (range: 3 hrs –

3 wks).3 wks).– Usually run down when they move over land, or cool Usually run down when they move over land, or cool

water, becoming drained of their energy from warm, water, becoming drained of their energy from warm, humid water.humid water.

– As the cyclone dissipates, it can still release torrential As the cyclone dissipates, it can still release torrential rains and do severe damage.rains and do severe damage.

Tropical cyclones are manifestations of water’s Tropical cyclones are manifestations of water’s latent heat of vaporization.latent heat of vaporization.

Tropical cyclones are nature’s escape valves, Tropical cyclones are nature’s escape valves, carrying solar energy poleward from the tropics.carrying solar energy poleward from the tropics.

Tropical cyclones are manifestations of water’s Tropical cyclones are manifestations of water’s latent heat of vaporization.latent heat of vaporization.

Tropical cyclones are nature’s escape valves, Tropical cyclones are nature’s escape valves, carrying solar energy poleward from the tropics.carrying solar energy poleward from the tropics.

Box 8.1: The Galveston Disaster, 1900Box 8.1: The Galveston Disaster, 1900– Read on your ownRead on your own

Hmwk:Hmwk:– ““Reviewing What You’ve Learned” 1 – 10Reviewing What You’ve Learned” 1 – 10– ““Thinking Critically” 1, 4, 7Thinking Critically” 1, 4, 7– (pp. 198 – 199)(pp. 198 – 199)

ch. 8: circulation of the atmosphere

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