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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 .

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Presentation Transcript
slide2

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

atmospheric pressure
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 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
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
slide7

Cold air is more dense than warm and exerts more

Pressure than warm air at a given altitude

isobars lines of equal pressure
Isobars-Lines of Equal Pressure

Drawn 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 pressure10
Isobars-Lines of Equal Pressure

Wind 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.

slide13
As these winds get pushed over the mountains, they undergo adiabatic compression, becoming hot and dry
global wind patterns and the coriolis effect
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 effect15
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
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
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 wind18
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 pressure19
Isobars-Lines of Equal Pressure

Wind 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 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
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
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
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.

global circulation in the mid latitudes
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.

global circulation the polar cell
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.
upper level circulation winds aloft 30 40 000 ft agl
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 atmosphere jet streams
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
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
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
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 fronts39
Warm Fronts

Commonly, warm fronts are slow moving, producing stable conditions

stratus type clouds (no vertical development)

steady precipitation

warm fronts40
Warm Fronts

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

slide41
Map symbol for Warm Front – half circles on the side of rising air – move generally north in n. hemisphere
cold front
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 front43
Cold Front

Result is cumulus and cumulonimbus cloud formation (clouds with significant vertical development)

Showery precipitation/ thunderstorms

Cold clear weather after the front passes.

slide44
Map symbol for cold front – triangle on the side of descending air – move south and east in N. hemisphere
occluded front cutoff low
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 low46
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.

slide48

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.

slide49

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.

slide50

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.

slide51

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.

slide52

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).

slide53

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.

slide54

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.

animation about fronts
Animation about fronts

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

fronts57
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 data
Precipitation Data

The 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.

boiling water at altitude
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
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
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
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
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
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 atmosphere rossby waves and the jet stream
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.