Ch15 Global Circulation and Weather

Weather patterns are caused by differential heating of the Earth’s surface, both on a local and a global scale

Atmospheric Pressure

• Defined as the force per unit area exerted on any surface by the weight of the overlying air column. At sea level, the atmosphere exerts an average of 14.7 pounds of force on a one-by-one inch area (slightly bigger than a postage stamp).

• Atmospheric pressure decreases with altitude, since gravity pulls air molecules towards the earth.

Atmospheric pressure units• Atmospheric pressure is often

measured in millibars (1000 mb = 1 bar).

• Standard pressure at sea level is 1.0132 bar or 1013.2 mb, often rounded to 1000 mb.

• American pilots and TV weatherman express atmospheric pressure in “inches mercury” where 29.9 inches = standard pressure at sea level.

• Atmospheric pressure is often called barometric pressure, and it is measured with a barometer.

Pressure and wind• Wind – movement of air from high to low

pressure areas.• Wind is caused by pressure differences due to

unequal heating of Earth’s atmosphere

Air pressure increases with density

Cold air is more dense than warm and exerts more Pressure than warm air at a given altitude

Isobars-Lines of Equal PressureDrawn at the

earth’s surface (there are upper-level charts as well)

Corrected to “sea level” so only effects of weather shown, not elevation

Units are millibars (mb) where 1013 mb is standard sea-level pressure.

Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.

Isobars-Lines of Equal PressureWind flags: Winds

blow away from the flags and towards the dots

Winds blow from high pressure to low pressure

Winds blow towards the L, away from the H

Although temperature infuences pressure, no real correlation

Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.

Sea and Land Breezes, a local phenomenon

Santa Ana Conditions High pressure to our east and northeast drive winds from east to west

As these winds get pushed over the mountains, they undergo adiabatic

compression, becoming hot and dry

Global Wind Patterns and the Coriolis Effect

• Wind blows from high to low pressure

• Wind blows straight in one direction, but earth turns underneath

• We experience the wind as curving, not us moving!

• This apparent “turning force” is the Coriolis Effect.

Global Wind Patterns and the Coriolis Effect

• If you stand behind the wind and watch it move away from you in the N. Hemisphere, it appears to curve to the right

• If you stand behind the wind and watch it move away from you in the S. Hemisphere, it appears to curve to the left.

Cyclones and Anticyclones

• Regions of local pressure highs and lows have characteristic circulation patterns– Cyclones: local low pressure centers: Air spirals

inward and upward– Anticyclones: local high pressure centers: Air spirals

outward and downward

Air Movement at a Cyclone (Low)• Warm (or humid) air is less dense than cold air and

therefore exerts lower pressure• Rising warm air undergoes expansional (adiabatic)

cooling which causes clouds and rain.• Low surface pressure associated with unsettled weather

and rain.

Pressure and wind• Cool (or dry) air is denser than warm

– This air exerts a higher pressure than warm air and will tend to sink,

– Compressional (adiabatic) warming prevents saturation and cloud formation

– High pressure often associated with good weather

Isobars-Lines of Equal PressureWind flags: Winds

blow away from the flags and towards the dots.

Wind spirals in a counterclockwise direction around the L.

Wind spirals in a clockwise direction around the H.

Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.

Global Circulation and the 3-cell model

• Global circulation patterns are created by differential heating and modified by the Coriolis Effect.

• Idealized atmospheric model: 3 convection cells in each hemisphere:– Hadley Cell

(tropical)– Ferrel Cell (mid-

latitude)– Polar Cell

• Note warmer air at surface for all cells

Global atmospheric circulation ITCZ• The equatorial low

pressure is due to rising warm equatorial air

• Adiabatic expansions causes the frequent rainfall. It rains a lot in the tropics!

• Returning air from the Hadley Cell converges at the Intertropical Convergence Zone (ITCZ).

• Since air is rising up after converging at the ITCZ, there is little wind, hence the sailor’s term: “the “doldrums”.

Global atmospheric circulation –Trade Winds

• Leg of Hadley Cell closest to Earth’s surface is pushed west by Coriolis Effect.

• results are winds that curve in from the east and converge at the ITCZ.

• These are the easterly trade winds (coming from NE in the northern hemisphere; from SE in the southern hemisphere).

• Trade winds drive surface equatorial ocean currents in the tropics.

Global atmospheric circulation – subtropical high pressure

• ~30o N/S latitude, air from the Hadley Cell lost some heat and much moisture, so falls

• Adiabatic compression causes hot dry air and high pressure at the surface – a subtropical high pressure zone in both northern and southern hemispheres.

• Much of the world’s deserts are located in this part of the world.

• San Diego’s latitude is approximately 33°N. It’s climate is influenced by the high pressure belt.

We’re here!

Global Circulation in the Mid-latitudes• The boundary between

Hadley and Ferrel Cells is located at about 30° N/S latitude.

• Warm, dry air descending at this junction diverges

• NE/SE trade winds go towards the Equator.

• Other branch goes towards the N/S pole and is deflected to the East by the Coriolis Force.

• Since they blow from the west, these winds are called the Westerlies.

Westerlies

Global Circulation– The Polar Cell

• The cold pole creates permanent high pressure at the N/S Pole)

• Polar easterlies from descending polar air

• Rising air from the junction of the Ferrel and Polar Cells create a region of stormy, unsettled weather at about 60° N/S

• Polar jet stream– forms along the polar front.

We’re here!

Polar Cell Circulation

Upper Level Circulation: Winds aloft – 30-40,000 ft agl

• Upper-level wind patterns:– Weak equatorial easterlies– Tropical high pressure

belts– Upper-air westerlies– Polar low

• Polar front – boundary between upper-air westerlies and the polar easterlies (low pressure)

• Rossby Waves – refer to the meandering waves made by the upper-air westerlies along the polar front

Winds in the Upper AtmosphereRossby Waves and the Jet Stream

Winds in the Upper AtmosphereRossby Waves and the Jet Stream

Winds in the Upper AtmosphereRossby Waves and the Jet Stream

Winds in the Upper AtmosphereRossby Waves and the Jet Stream

Winds in the Upper AtmosphereJet Streams

• Wind streams– At high altitude– In a narrow corridor– Speeds are maximum

toward the center– The jet stream located

closest to each pole is the polar-front jet stream

Air masses• large body of air

with uniform temperature and moisture characteristics over a large area.

• retain integrity for several days before mixing.

• Source region– origin of the air

mass– give air mass its

characteristics

Fronts• Sharply-defined

boundary between a 2 air masses with different characteristics

• Fronts may be warm, cold, occluded (closed) or stationary

• Cause weather systems – cyclonic, or frontal precipitation

Warm Fronts• Moving warm air mass overtakes a stationary or

slow-moving cold air mass.• Warm air rises over the cooler air and cools

adiabatically• Lifting process is called frontal wedging.

Warm FrontsCommonly, warm fronts are slow moving, producing stable

conditions– stratus type clouds (no vertical development)– steady precipitation

Warm Fronts

• if the warm air mass is unstable (pushed up too quickly), cumulonimbus clouds and thunderstorms result (not shown here).

Map symbol for Warm Front – half circles on the side of rising air – move generally north in

n. hemisphere

Cold Front

• Cold front – fast-moving cold air overtakes warm air and shoves underneath it, creating a steep contact (frontal wedging, again)– Warm air rises rapidly, causing unstable conditions

Cold Front• Result is cumulus and cumulonimbus cloud

formation (clouds with significant vertical development)

• Showery precipitation/ thunderstorms• Cold clear weather after the front passes.

Map symbol for cold front – triangle on the side of descending air – move south and east

in N. hemisphere

Occluded Front (Cutoff Low)• Faster moving cold air mass traps a warm air mass

against a second cold (or at least cool) air mass.• Note the more gently-sloping warm front compared to the

more steeply-sloping cold front and the different weather patterns resulting from them.

Occluded Front (Cutoff Low)• Warm air is completely cut off from the surface.• Precipitation occurs along both frontal boundaries

– narrow band of heavy, possible convective precipitation along former cold front

– wider band of steady precipitation at warm front• Net result is large zone of inclement weather.• Continues until “cutoff” warm air mass runs out of moisture.

Map symbol for occluded front – alternating half circles and triangles on the same side

Mid-latitude Wave Cyclones

Initial conditions: Along the polar front, cold polar air mass from the north (cP or mP) meets warm humid subtropical air mass from the south (mT).

No relative movement between the air masses – yet.

Mid-latitude Wave Cyclones (1)

An undulation or disturbance causes cold air to push southeast and warm air to push north. This results in 2 fronts and a counter-clockwise circulation pattern.

Mid-latitude Wave Cyclones (1)An undulation or disturbance causes cold air to push southeast and warm air to push north. This results in 2 fronts and a counter-clockwise circulation pattern. This is the case for the location marked #1.

Mid-latitude Wave Cyclones (2)

A low pressure region (warm air rising) and cyclonic circulation develops. Cold front usually moves faster.

Characteristic precipitation zones along the both fronts.

Mid-latitude Wave Cyclones (2)The cold front is moving east (mP and/or cP air mass, north and west of frontal boundary), while the warm front is still moving north (mT air mass, south from Gulf of Mexico).

Mid-latitude Wave Cyclones (3)

The cold front catches up to the warm front, “pinching” the warm air and pushing it up above the colder air. An occluded front forms and precipitation continues over a large area.

Mid-latitude Wave Cyclones (3)

The northern part of the cold front catches up to the western part of the warm front, cutting off a portion of the warm air from the surface. Location 3 is an occluded front with widespread precipitation.

Mid-latitude Wave Cyclones

Animation about fronts

http://wps.prenhall.com/esm_lutgens_foundations_6/140/36041/9226506.cw/index.html

Fronts• Identify and label the cold front in blue.

• Identify and label the warm front in red.

• What air mass causes the cold front? Where is it on the map?

• What air mass causes the warm front? Where is it on the map?

Precipitation DataThe colored section

shows us where it’s raining. The red boxes are thunderstorm activity. It may help to sketch the fronts in.

Note most of the precip is north of the fronts, but the T-storm activity is a narrow band to the east.

Barometric Pressure Map – Any questions?

Boiling water at altitude

Why does water boil at a lower temperature at a higher elevation?

• Less air pressure allows the water to change state (from liquid to gas) without being so “energetic”.

• Since boiling water at altitude is not as hot, cooking times must be altered.

Atmospheric Pressure - force exerted by atmospheric gas molecules on a given area

• When air masses move around the earth due to differential heating, this value can change.

• Elevation also changes the value of atmospheric pressure.

• Atmospheric pressure is often called barometric pressure, as it is measured with a barometer.

Mean sea-level pressure

• When atmospheric pressure is corrected for elevation effects, it is reported as mean sea-level (msl) pressure.

• When using msl pressure, any change from 1013 mb can be attributed to weather systems, and not just elevation.

• The weatherman reports msl pressure, whether San Diego or on Mt. Everest.

Measuring atmospheric pressure – the barometer

• If you evacuate a tube (i.e. remove all the air) and put it in a dish of liquid, the liquid will fill the tube as the air pressure pushes on the liquid in the dish.

• If you tried this with a dish of water, the water would rise up to about 33 feet in the tube!

Measuring atmospheric pressure• Using mercury, a very heavy liquid,

we find that at normal sea-level barometric pressure, the liquid in the tube rises to a height of 760 mm (or 29.92 inches).

• This apparatus is the original form of the barometer, a device used for measuring barometric pressure.

• American TV weather reports are usually given in terms of inches of mercury

• Overseas reports use millibars.

Measuring barometric pressure – the modern way

• Mercury barometers are dangerous and difficult to use.

• Modern aneroid barometers use changes within a partially evacuated chamber to move the pointer to the correct value.

Winds in the Upper AtmosphereRossby Waves and the Jet Stream

• geostrophic wind: theoretical wind that would result from an exact balance between the Coriolis Effect and the pressure gradient.

• The true winds at upper levels of the atmosphere (30-40 thousand feet) outside the tropics can be approximated as geostrophic winds. Upper air winds are westerlies.

• Rossby Waves: Wave like undulations in the circulation patterns where warm tropical air interacts with the cold polar air.

• Jet Stream: narrow bands of high-speed air flow within upper air westerlies, usually at the boundaries of warm and cold air.