Download Global Winds 19.2 Pressure Centers and Winds

Chapter
19
Air Pressure and
Wind
19.2 Pressure Centers and Winds
Highs and Lows
 Cyclones are centers of low pressure.
 Anticyclones are centers of high pressure.
 In cyclones, the pressure decreases from
the outer isobars toward the center. In
anticyclones, just the opposite is the case—
the values of the isobars increase from the
outside toward the center.
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
19.2 Pressure Centers and Winds
Highs and Lows
 Cyclonic and Anticyclonic Winds
• When the pressure gradient and the Coriolis
effect are applied to pressure centers in the
Northern Hemisphere, winds blow
counterclockwise around a low. Around a high,
they blow clockwise.
• In either hemisphere, friction causes a net flow
of air inward around a cyclone and a net flow of
air outward around an anticyclone.
Cyclonic and Anticyclonic Winds
19.2 Pressure Centers and Winds
Highs and Lows
 Weather and Air Pressure
• Rising air is associated with cloud formation and
precipitation, whereas sinking air produces clear
skies.
 Weather Forecasting
• Weather reports emphasize the locations and
possible paths of cyclones and anticyclones.
• Low-pressure centers can produce bad weather
in any season.
Airflow Patterns, Surface and Aloft
19.2 Pressure Centers and Winds
Global Winds
 The atmosphere balances these differences
by acting as a giant heat-transfer system.
This system moves warm air toward high
latitudes and cool air toward the equator.
 Non-Rotating Earth Model
• On a hypothetical non-rotating planet with a
smooth surface of either all land or all water, two
large thermally produced cells would form.
Circulation on a Non-Rotating Earth
19.2 Pressure Centers and Winds
Global Winds
 Rotating Earth Model
• If the effect of rotation were added to the global
circulation model, the two-cell convection system
would break down into smaller cells.
• Trade winds are two belts of winds that blow
almost constantly from easterly directions and are
located on the north and south sides of the
subtropical highs.
• Westerlies are the dominant west-to-east motion
of the atmosphere that characterizes the regions
on the poleward side of the subtropical highs.
19.2 Pressure Centers and Winds
Global Winds
 Rotating Earth Model
• Polar easterlies are winds that blow from the
polar high toward the subpolar low. These winds
are not constant like the trade winds.
• A polar front is a stormy frontal zone separating
cold air masses of polar origin from warm air
masses of tropical origin.
Circulation on a Rotating Earth
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.
Deserts of the World
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
• 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.
19.2 Pressure Centers and Winds
Global Winds
 Influence of Continents
• The only truly continuous pressure belt is the
subpolar low in the Southern Hemisphere. In the
Northern Hemisphere, where land masses break
up the ocean surface, large seasonal temperature
differences disrupt the pressure pattern.
• Monsoons are the seasonal reversal of wind
direction associated with large continents,
especially Asia. In winter, the wind blows from land
to sea. In summer, the wind blows from sea to
land.
Surface Pressure