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Class #16: Friday, February 13. Review of the 3-cell model Pressure and vertical motions Surface winds and jet streams aloft. Surface pressure in the 3-cell model. High at both poles, called Polar Highs High in the subtropics, about 30 ºN and 30ºS, called Subtropical Highs

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class 16 friday february 13

Class #16: Friday, February 13

Review of the 3-cell model

Pressure and vertical motions

Surface winds and jet streams aloft

Class #16 Friday, February 13, 2009

surface pressure in the 3 cell model
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

Class #16 Friday, February 13, 2009

average vertical motions in the 3 cell model
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

Class #16 Friday, February 13, 2009

thermal circulations in the 3 cell model
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.

Class #16 Friday, February 13, 2009

the 3 cell model s circulation in middle latitudes
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.

Class #16 Friday, February 13, 2009

consequences of earth s rotation from west to east
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).

Class #16 Friday, February 13, 2009

more consequences of earth s rotation
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.

Class #16 Friday, February 13, 2009

complications of the real earth
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

Class #16 Friday, February 13, 2009

observed surface pressures
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.

Class #16 Friday, February 13, 2009

seasonal shifts
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.

Class #16 Friday, February 13, 2009