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 Pressure
Drawn at the
earth’s surface
(there are upperlevel 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 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.
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 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
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 (midlatitude)
– 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.
 Polar Cell Circulation
We’re here!

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
Rossby Waves and the Jet Stream
Winds in the Upper Atmosphere
Rossby Waves and the Jet Stream
Winds in the Upper Atmosphere
Rossby Waves and the Jet Stream
Winds in the Upper Atmosphere
Rossby Waves and the Jet Stream
Winds in the Upper Atmosphere
Jet 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 Fronts
Commonly, 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_found
ations_6/140/36041/9226506.cw/index.ht
ml
• 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?
Fronts
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.
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 sealevel 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 Atmosphere
Rossby 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.