Atmospheric Circulation and Atmospheric Circulation and WeatherWeather

Composition and Composition and Properties of the Properties of the AtmosphereAtmosphere

Lower atmosphere Lower atmosphere nearly homogenous nearly homogenous mixture of nitrogen mixture of nitrogen 78.1% and oxygen 78.1% and oxygen 20.9% (argon and 20.9% (argon and co2 = 1%).co2 = 1%).

Density of air is Density of air is influenced by its influenced by its temperaturetemperature and and water content.water content.

• Warm air is Warm air is less dense less dense than cold air, than cold air, (because of (because of molecular molecular movement) it movement) it occupies more occupies more space.space.

Humid air is less Humid air is less dense than dry air dense than dry air at the same at the same temperature (water temperature (water vapor less dense vapor less dense than nitrogen and than nitrogen and oxygen molecules oxygen molecules it displaces).it displaces).

Atmospheric PressureAtmospheric Pressure• Weight of the atmosphere – decreases as Weight of the atmosphere – decreases as

you ascend into the atmosphere.  Most you ascend into the atmosphere.  Most pressure is at the surface.  pressure is at the surface. 

• Air near the surface is packed densely by Air near the surface is packed densely by its own weight – when this air is lifted it its own weight – when this air is lifted it will expand – expanding air becomes will expand – expanding air becomes cooler because going from high to low cooler because going from high to low pressure.pressure.

• Opposite is also true, when air is Opposite is also true, when air is compressed it becomes warmer – compressed it becomes warmer – descending air warms because it is descending air warms because it is compressed by higher atmospheric compressed by higher atmospheric pressure near earth’s surface.pressure near earth’s surface.

• Warm air can hold more water vapor than Warm air can hold more water vapor than cold air.  When air is rising expanding and cold air.  When air is rising expanding and cooling water vapor condenses into cooling water vapor condenses into clouds.  The cooler air can no longer hold clouds.  The cooler air can no longer hold as much water vapor.as much water vapor.

Atmospheric Atmospheric CirculationCirculation

WeatherWeather =  the =  the state of the state of the atmosphere at a atmosphere at a specific place and specific place and time.time.

ClimateClimate =  a long =  a long term average of term average of weather in an weather in an area.area.

    Both are Both are influenced by the influenced by the amount of solar amount of solar radiation the area radiation the area receives.receives.

Air does not remain Air does not remain stationary over earth, stationary over earth, but flows in large but flows in large patterns shaped or patterns shaped or caused by unequal caused by unequal solar heating of the solar heating of the earth with earth with latitudelatitude and and seasonseason, and by the , and by the rotation of the earthrotation of the earth.  .  The mass movement of The mass movement of air is known as air is known as windwind..

Uneven solar Uneven solar heating and latitude heating and latitude

Poles have less Poles have less solar heating solar heating becausebecause

• sun reaches poles sun reaches poles at greater angle at greater angle energy spread energy spread over greater area.  over greater area.  (lower angle of (lower angle of incidence).incidence).

Equator has more Equator has more solar heating solar heating because because • equator has high equator has high

angle of angle of incidenceincidence

• sunlight sunlight subjected to less subjected to less filtration and filtration and surface reflectionsurface reflection

Uneven solar heating and the seasons:Uneven solar heating and the seasons:           Mid latitudes receive about 3 times as Mid latitudes receive about 3 times as

much solar energy per-day in June than much solar energy per-day in June than December – reason is the 23 1/2% tilt of December – reason is the 23 1/2% tilt of the earth, its the earth, its orbital inclinationorbital inclination.  The .  The inclination of the axis causes the change inclination of the axis causes the change of seasons.of seasons.

True color imagesTrue color images

June

December

March

September

http://www.nasa.gov/vision/earth/features/blue_marble.html

Uneven solar heating and atmospheric Uneven solar heating and atmospheric circulation circulation

most of suns heat is at the equator.most of suns heat is at the equator. We know that warm air rises and cold air We know that warm air rises and cold air

sinks (convection in room example).sinks (convection in room example).

The ideal model of atmospheric The ideal model of atmospheric circulation would be – air heated at circulation would be – air heated at tropics expands, rises, and goes tropics expands, rises, and goes poleward where it loses heat and sinks to poleward where it loses heat and sinks to the surface and moves equatorward.  the surface and moves equatorward.  This large circuit of air is called an This large circuit of air is called an atmospheric circulation cell.  But this is atmospheric circulation cell.  But this is not what happens.  Global circulation is not what happens.  Global circulation is governed by both governed by both unevenuneven solar heatingsolar heating andand the rotation of the earththe rotation of the earth..

The affects of the rotating earth on The affects of the rotating earth on atmospheric circulationatmospheric circulation

The eastward rotation of earth deflects moving air The eastward rotation of earth deflects moving air and water away from its initial course.  The and water away from its initial course.  The deflection is to the right in the northern hemisphere.  deflection is to the right in the northern hemisphere.  The apparent deflection is called The apparent deflection is called coriolis effectcoriolis effect..

With a rotating earth, wind and water get deflected.  With a rotating earth, wind and water get deflected.  Because of this coriolis effect a more complex Because of this coriolis effect a more complex circulation model is needed to explain atmospheric circulation model is needed to explain atmospheric circulation.circulation.

In this new model of In this new model of atmospheric atmospheric circulation, air still circulation, air still warms and expands warms and expands at the equator.  This at the equator.  This rising air loses rising air loses moisture and is now moisture and is now drier and more drier and more dense. dense. 

The air will travel The air will travel poleward and will poleward and will become dense become dense enough to fall back enough to fall back toward the surface at toward the surface at about 30 degrees about 30 degrees north and 30 degrees north and 30 degrees south latitude. south latitude. 

Most of this air near Most of this air near the surface will travel the surface will travel back towards the back towards the equator and it is then equator and it is then deflected to the right deflected to the right coming from the north coming from the north east (NE Trade Winds east (NE Trade Winds or Easterlies). or Easterlies). 

These winds warm as These winds warm as they approach the they approach the equator, evaporating equator, evaporating water and becoming water and becoming less dense, completing less dense, completing the circuit. This large the circuit. This large atmospheric atmospheric circulation cell is called circulation cell is called the the Hadley CellHadley Cell..

A more complex cell A more complex cell – – Ferrell CellFerrell Cell operates in the mid operates in the mid latitudes.  Air latitudes.  Air descending at 30 descending at 30 degrees N/S going degrees N/S going poleward is poleward is deflected to the right deflected to the right forming the forming the westerlieswesterlies..

The polar easterlies The polar easterlies are formed from air are formed from air returning from the returning from the poles.  This cold air poles.  This cold air is deflected is deflected westward.  This westward.  This circuit is called the circuit is called the Polar Cell. Polar Cell. 

Where the polar cell Where the polar cell meets the Ferrell Cell meets the Ferrell Cell is called is called polar frontpolar front – – which creates most which creates most weather in mid – weather in mid – latitudes.latitudes.

Wind PatternsWind Patterns At the bands between circulation cells air is At the bands between circulation cells air is

moving vertically, and the surface winds are moving vertically, and the surface winds are weak and erratic.weak and erratic.

At the equator where the two Hadley cells At the equator where the two Hadley cells converge is called the doldrums. Also called converge is called the doldrums. Also called Intertropical Convergence Zone (ITCZ).Intertropical Convergence Zone (ITCZ).

Air moving from equator loses precipitation Air moving from equator loses precipitation and cools.  This cooler dry air moves and cools.  This cooler dry air moves poleward but is dense enough to descend poleward but is dense enough to descend around 30 degrees latitude between Hadley around 30 degrees latitude between Hadley and Ferrell Cells.  This area of high pressure and Ferrell Cells.  This area of high pressure and little surface winds because of vertical and little surface winds because of vertical movement is called the movement is called the horse latitudeshorse latitudes..

Sailors are most interested in surface winds Sailors are most interested in surface winds between cells, surface winds of Hadley cells between cells, surface winds of Hadley cells are theare the trade winds or easterlies trade winds or easterlies 15 degrees 15 degrees N and S.  Surface winds of Ferrel Cells called N and S.  Surface winds of Ferrel Cells called westerlies westerlies 45 degrees N and S.45 degrees N and S.

Modifications of circulation due to Modifications of circulation due to unequal distribution of land.unequal distribution of land.

Local variations occur when land heats up Local variations occur when land heats up more in summer causing low pressure and more in summer causing low pressure and cools more in winter creating high cools more in winter creating high pressure.pressure.

Land and Sea Land and Sea breezes (small breezes (small daily monsoons)daily monsoons)

coastal areas coastal areas during the day in during the day in warm months, warm months, land is heated land is heated more than water.  more than water.  This warm low This warm low pressure air rises pressure air rises and the cooler air and the cooler air above the sea above the sea replaces it replaces it creating a creating a sea – sea – breeze.breeze.

At night the land cools At night the land cools more rapidly than the more rapidly than the water creating high water creating high pressure over land.  pressure over land.  Air goes from high to Air goes from high to low pressure forming low pressure forming land breezeland breeze..

MonsoonsMonsoons Similar to land and sea Similar to land and sea

breezes but are breezes but are seasonal and occur on seasonal and occur on a large land mass like a large land mass like Asia.Asia.

Land heated in summer Land heated in summer creates low pressure.  creates low pressure.  Cooler air over ocean Cooler air over ocean (high pressure) moves in (high pressure) moves in to replace the lifting low to replace the lifting low pressure air over land pressure air over land (remember air goes from (remember air goes from high to low pressure).  The high to low pressure).  The moist air from the ocean moist air from the ocean warms over land rises and warms over land rises and condenses to form condenses to form precipitation.  The rainy precipitation.  The rainy seasons.seasons.

The opposite occurs in autumn, as the The opposite occurs in autumn, as the land cools more rapidly than adjacent land cools more rapidly than adjacent ocean.  Air cools and sinks over land ocean.  Air cools and sinks over land (high pressure), and dry surface winds (high pressure), and dry surface winds move seaward.  The dry season.  move seaward.  The dry season.  MonsoonMonsoon = is a pattern of wind = is a pattern of wind circulation that changes with the circulation that changes with the seasons.seasons.

StormsStorms Storms form between or within air masses.  Storms form between or within air masses. 

AirAir MassMass = large body of air with nearly = large body of air with nearly uniform temperature, humidity, and uniform temperature, humidity, and therefore density throughout.  These are therefore density throughout.  These are determined by the land which the air determined by the land which the air passes over.passes over.

• These air masses move within or between These air masses move within or between circulation cells. circulation cells.

• Different air masses do not mix well because of Different air masses do not mix well because of different densities.different densities.

The lighter air mass The lighter air mass is wedged upward is wedged upward causing it to expand, causing it to expand, cool, and condense, cool, and condense, which contributes to which contributes to the turbulence the turbulence between boundaries.between boundaries.

The boundary The boundary between air masses between air masses of different density is of different density is called called a fronta front..

Two main types of storms – Extratropical Cyclone and Two main types of storms – Extratropical Cyclone and tropical cyclone.tropical cyclone.

CycloneCyclone = huge rotating mass of low – pressure air in = huge rotating mass of low – pressure air in which winds converge and ascend.which winds converge and ascend.

• Extratropical cyclonesExtratropical cyclones = form at the boundary = form at the boundary between each hemispheres polar cell and its between each hemispheres polar cell and its Ferrel cell – the Ferrel cell – the polar frontpolar front.  (Occur mainly in .  (Occur mainly in winter where temperature and density differences winter where temperature and density differences across front are most pronounced).  Principle across front are most pronounced).  Principle cause of weather in mid – latitudes regions.cause of weather in mid – latitudes regions.

Tropical CyclonesTropical Cyclones These form from disturbances within one These form from disturbances within one

air mass.  These are great masses of air mass.  These are great masses of warm, humid rotating air.warm, humid rotating air.

Large ones called hurricanes (winds of Large ones called hurricanes (winds of 74 mph)74 mph)• typhoons in Pacific, typhoons in Pacific, • Tropical cyclone in Indian and Tropical cyclone in Indian and • willi - willis in Australian waters.willi - willis in Australian waters.

Winds less than 74 mph called tropical Winds less than 74 mph called tropical storms and tropical depressions.storms and tropical depressions.

Develops from a small tropical Develops from a small tropical depression which forms over a large depression which forms over a large warm air mass (Africa).  The air mass warm air mass (Africa).  The air mass goes over ocean waters that must be goes over ocean waters that must be 79 degrees F or higher and warm 79 degrees F or higher and warm humid air is forced up.  The storm humid air is forced up.  The storm begins to develop.  begins to develop. 

The energy is from The energy is from latent heat oflatent heat of evaporationevaporation..