Transcript Class #14: Monday, February 11

Class #13: Monday,
September 27
Chapter 7
Global Winds
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What a description of global
winds should explain
• Seasonal patterns of precipitation around
the world
• Seasonal patterns of cloudiness around the
world
• The relationships between average wind
patterns and pressure patterns and upward
and downward air motions
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Fig. 7-4a, p. 191
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Fig. 7-4b, p. 191
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Fig. 7-5a, p. 192
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Fig. 7-5b, p. 192
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The surface winds over Earth
• Are very complicated because of the
changing seasons, differences between land
and water, and differences in latitude.
• Can be simplified using a conceptual model.
• Have been described using a 3-cell model
with no land and no seasons. Only
temperature differences from equator to
pole are included.
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The 3-cell conceptual model of
the general circulation
• Has 3 wind belts in each hemisphere
• NH and SH
– Polar easterlies
– Prevailing or mid-latitude westerlies
– Trade winds
• Trade winds blow equatorward
– Northeasterly trade winds in NH
– Southeasterly trade winds in SH
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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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More on the thermal circulation
• The thermal circulation begins aloft.
• In diagrams of the thermal circulation, “H”
and “L” refer to the horizontal pressure
gradient, not to the vertical pressure
gradient.
• The thermal circulation comes about
because hydrostatic balance requires that
the warmer air column expands compared
to the cooler air column.
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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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Fig. 7-12a, p. 197
Fig. 7-12a (1), p. 197
Fig. 7-12a (2), p. 197
Fig. 7-12b, p. 198
Fig. 7-12b (1), p. 198
Fig. 7-12b (2), p. 198
Fig. 7-12b (2), p. 198
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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The thermal circulation
• The sea breeze is a thermal circulation.
• A thermal circulation has both horizontal
and vertical air motions.
• The horizontal pressure gradient force is
most important in a thermal circulation.
• Upward air motions occur in the warmer air
column of the circulation; downward air
motions occur in the cooler air column.
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The sea breeze
• Is a daytime circulation.
• Depends on differential heating at the
surface between land and water.
• Has the warmer air column over the land,
which absorbs more incoming solar
radiation.
• Has the cooler air column over the water,
which absorbs less radiation.
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The sea breeze and the
land breeze
• As solar heating diminishes in the late
afternoon, the sea breeze weakens.
• At night, differential cooling occurs.
• The cooler air column is over land, where
radiational cooling is more rapid than over
the water.
• The warmer air column is over the water.
• The land breeze develops at night.
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Scales of motion in the
atmosphere
• Describe the size and lifetime of wind
patterns in the atmosphere.
• Determine which forces are most important
to forming the wind patterns.
• Are largest when the lifetimes are longest.
• Are smaller when the lifetime is shorter.
• Have a variety of names and definitions.
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More on scales of motion
• The horizontal pressure gradient force is
important for all scales of motion.
• The Coriolis Force is important for the
planetary scale, the synoptic scale, and for
the larger mesoscale wind patterns.
• The vertical pressure gradient force is
important for small mesoscale winds.
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The surface winds over Earth
• Are very complicated because of the
changing seasons, differences between land
and water, and differences in latitude.
• Can be simplified using a conceptual model.
• Have been described using a 3-cell model
with no land and no seasons. Only
temperature differences from equator to
pole are included.
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2010
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The surface winds over Earth
• Are very complicated because of the
changing seasons, differences between land
and water, and differences in latitude.
• Can be simplified using a conceptual model.
• Have been described using a 3-cell model
with no land and no seasons. Only
temperature differences from equator to
pole are included.
Class #13 Monday, September 27,
2010
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Class #13 Monday, September 27,
2010
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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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2010
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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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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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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 subtropical highs.
• Sometimes these jet streams merge into one.
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