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Thermal Circulations warm air atmospheric columns of air cold air PGF PA PB Psurf=1000mb Psurf=1000mb Since Pressure ∝ mass above: at level Z, PB > PA ALOFT, get PGF towards the cold air column height = Z THERMAL CIRCULATION L H H L surface COLD WARM WINTER Clear skies and winds blow from land to sea SUMMER Warm humid air blows up from equator bringing rainy weather. Clicker Question Set Frequency to "BB" We know that the Earth is warmer at the equator and colder at the poles. Assume for the moment, there is no Coriolis force. Which way would you expect the surface wind to blow at X? A (A) Southerly (poleward) X D B (B) Westerly Equator C (C) Northerly (equatorward) (D) Easterly Clicker Question Set Frequency to "BB" We know that the Earth is warmer at the equator and colder at the poles. Assume for the moment, there is no Coriolis force. Which way would you expect the surface wind to blow at X? A (A) Southerly (poleward) X D B (B) Westerly Equator C (C) Northerly (equatorward) (D) Easterly Single Cell Model Originally proposed by G. Hadley "Hadley Cell" - warm air at equator rises - goes poleward at higher levels - sinks at poles - moves equatorward at the surface A (Very Large) Thermal Circulation HOWEVER, Due to the Earth's rotation, the single cell model is too simplified and would result in extremely strong winds due to Coriolis force and become unstable. A more realistic view of the global circulation is provided by the "3-Cell Model" - Certainly not perfect, but does explain many features of the global circulation system NP EQ SP 3-Cell Model - no land mass - time averaged - no seasons NP 60 30 Hadley Cell EQ SP NP Ferrel Cell 60 30 Hadley Cell EQ SP Polar Cell NP Ferrel Cell 60 30 Hadley Cell EQ SP Polar Cell SURFACE WINDS NP Ferrel Cell 60 30 Hadley Cell EQ SP Polar Cell SURFACE WINDS NP Ferrel Cell 60 30 Hadley Cell EQ SP Polar Cell SURFACE WINDS NP Ferrel Cell 60 30 Hadley Cell EQ SP Polar Cell NP Ferrel Cell 60 30 Hadley Cell EQ TRADEWINDS -Northeasterly in NH SP Polar Cell NP Westerlies Ferrel Cell 60 - 30-60 Mid-latitudes 30 Hadley Cell EQ SP Polar Cell Polar Easterlies NP Ferrel Cell 60 30 Hadley Cell EQ SP Similar System of Cells in Southern Hemisphere Polar Cell NP Ferrel Cell 60 30 Hadley Cell EQ Hadley Cell 30 Ferrel Cell 60 SP Polar Cell Surface LOWS where air is RISING 60 L Polar Cell NP L RISING AIR Ferrel Cell L 30 Hadley Cell L L EQ L RISING AIR Hadley Cell 30 60 L L Ferrel Cell L RISING AIR SP Polar Cell SINKING AIR Surface HIGHS where air is SINKING NP H L 60 Polar Cell L Ferrel Cell L SINKING AIR H 30 H H Hadley Cell L L EQ L Hadley Cell H 30 H H SINKING AIR 60 L L Ferrel Cell L H SINKING AIR SP Polar Cell Clicker Question Set Frequency to "BB" In general, what sort of surface conditions would you expect at about 30° where the Hadley cell meets the Ferrell cell? (A) stormy, cool and moist with much precipitation (B) few storms, warm and dry with little precipitation Clicker Question Set Frequency to "BB" In general, what sort of surface conditions would you expect at about 30° where the Hadley cell meets the Ferrell cell? (A) stormy, cool and moist with much precipitation (B) few storms, warm and dry with little precipitation Desert Regions Desert Regions 30°˜N 30°˜S ITCZ Intertropical Convergence Zone (ITCZ): Region of NP Cumulus Towers and H Precipitation 60 L L H 30 Polar Cell Ferrel Cell L H H Hadley Cell ITCZ L L EQ L Hadley Cell H 30 60 H L L H Ferrel Cell L H SP Polar Cell ITCZ ITCZ Polar Front: Where Westerlies meet Polar Easterlies L 60 Polar Front H 30 Polar Cell NP H L Ferrel Cell L H H Hadley Cell ITCZ L L EQ L Hadley Cell H 30 H H Polar Front 60 L L Ferrel Cell L H SP Polar Cell Idealized Water Planet Real Earth Average Pressure Patterns in July Average Pressure Patterns in January Semi-permanent Subtropical Highs Pacific High - Pacific High is an important factor for California's climate Bermuda High