Class #17: Monday, February 18

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Transcript Class #17: Monday, February 18

Class #17: Monday, February 16
Surface pressure and winds
Vertical motions
Jet streams aloft
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Surface pressure in the
3-cell model
• High at both poles, called Polar Highs
• High in the subtropics, about 30ºN and
30ºS, called Subtropical Highs
• Low near the equator, called the Equatorial
Low, or the Intertropical Convergence Zone
(ITCZ)
• Generally light winds at the Polar and
Subtropical Highs, and in the ITCZ
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Average vertical motions in the
3-cell model
• Downward at the poles where surface
pressure is high and the troposphere has low
temperatures over ice
• Downward at the subtropical highs
• Upward in the ITCZ
• Upward at about 60°N and S near the polar
front
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Thermal circulations in the 3-cell
model
• The Hadley cells have their rising branch in
the ITCZ and their sinking branch in the
subtropics.
• The Hadley cells cover half of the surface
area of Earth.
• The polar cells have a rising branch near the
polar front and sinking at the pole.
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The 3-cell model’s circulation in
middle latitudes
• Is thermally indirect, because the air nearer the
pole is rising, and the air nearer the equator is
sinking.
• Is an average based on smaller wind patterns in
extratropical cyclones, in which the warmer air
does rise, and the colder air sinks.
• Has the motions required by the polar and Hadley
cells.
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Consequences of Earth’s rotation
from west to east
• The trade winds in the NH do not blow
from the north, but are deflected to the right
in the NH, so blow from the northeast.
• If Earth rotated much more slowly, there
would be only the Hadley cell.
• If Earth rotated much more quickly, there
would be more wind belts (like on Jupiter).
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More consequences of Earth’s
rotation
• If it were not for the Midlatitude westerlies,
Earth’s speed of rotation would slow. Easterlies
alone would everywhere act to slow the rotation.
• The polar easterlies blow from the pole and curve,
blowing from the northeast in the NH and from
the southeast in the SH.
• The westerlies blow away from the equator and
curve in both hemispheres, that is, they blow from
the southwest in the NH, and from the northwest
in the NH.
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Complications of the real Earth
• Earth has seasons
– The ITCZ (sometimes called the thermal
equator) shifts latitude with the seasons.
– The ITCZ shifts north of the equator in NH
summer, and south of the equator in SH
summer (NH winter)
• Earth has large land masses
– Continents and oceans set up thermal
circulations
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Observed surface pressures
• Vary with the seasons, requiring both a January
and a July depiction
• Are on average high in the sub-tropics (near 30°)
and near the pole
• Are on average low in the ITCZ and along the
polar front (near 60°)
• In summer are high over the oceans and low over
the continents (thermal lows).
• In winter are high over the continents and low
over the oceans.
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Seasonal shifts
• The ITCZ, the subtropical highs, and the
polar front all shift southward in NH winter
and northward in NH summer.
• Seasonal shifts are most intense over Asia,
which has the largest continental air mass.
• The summer monsoon is wet, with low
pressure over land; the winter monsoon is
dry, with high pressure over land.
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Other monsoons
• Africa, North America, and Australia have
monsoon-like wind patterns, particularly in
the warm season.
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Review of temperature and surface
pressure patterns
• High at surface
– Poles (low temperature)
– Subtropical highs, especially over oceans
– Land in winter, ocean in summer (low temp.)
• Low at surface (thermal lows have high T)
– Subpolar low at about 60º with polar front
– Intertropical Convergence Zone (ITCZ)
– Land in summer, ocean in winter (high temp.)
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Winds and pressures (heights)
well above the surface
• Pressures and heights are on average high in
the tropics and decrease to lows close to the
poles.
• Upper-level (500mb and above) winds are
generally easterlies (blowing east to west)
in the tropics and westerlies (blowing west
to east) in higher latitudes.
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Review of upper-level height
(pressure) patterns and temperature
• High heights (pressure) in warm air
columns with high temperature
• Low heights (pressure) in cold air columns
with lower temperature
• Look for ridges over land in summer and
oceans in winter
• Look for troughs over land in winter and
oceans in summer
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Important reminder!!!!
• Pressure and height patterns aloft are
generally opposite to those at the surface
• This opposition is necessary for thermal
circulations, because the horizontal pressure
gradient force must reverse aloft for a
complete circulation
• High at surface/low aloft;
• Low at surface/high aloft
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Jet Streams
• Jet streams are regions of especially high
wind speed in the atmosphere.
• In the upper-level westerlies, there can be
two jet streams, the Polar front jet stream,
above the polar front, and the Subtropical
jet stream above the polar highs.
• Sometimes these jet streams merge into one.
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Patterns in the upper-level
westerlies
• The pattern of upper-level westerlies has
waves, with axes of high height (ridges and
highs) and axes of low height (troughs and
lows). These appear on day to day weather
maps.
• These wavelike patterns help transport
energy poleward (advection) to balance the
energy budget of the Earth and atmosphere.
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