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REMINDERS: Problem Set 3: Due Today Midterm 2: Friday, February 28 - lecture material covering chapters 6, 7, and 15 (since first midterm and through Wed lecture) - same Format as first midterm UPCOMING REVIEW SESSIONS: - Thursday, Feb 27, 6:30-8:00pm in HSS 1330 Thermal Circulations: • Circulation set up by change in air temperature Two columns with equal mass (same surface pressure) - one warm and the other is cool - warm column expands (air is less dense) - cold column contracts (air is more dense) warm air atmospheric columns of air cold air PA PB Psurf=1000mb Psurf=1000mb More mass above level Z on warm side compared to cold side Since Pressure ∝ mass above: at level Z, PB > PA height = Z THERMAL CIRCULATION L H H L surface COLD WARM THERMAL CIRCULATIONS Sea/Land Breezes - Daily timescale Monsoons - Seasonal timescale Clicker Question Set Frequency to "AD" 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 "AD" 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 Polar Cell NP Ferrel Cell 60 30 Hadley Cell EQ SP 3-‐Cell Circula,on System 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 Tradewinds Island of Hawai'i NE Tradewinds Most rainfall on east side of island Satellite Image 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 "AD" 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 "AD" 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 Simulated Water Planet Reality with Land Masses Because of land masses, subtropical highs and polar lows are not con,nuous bands Simulated Water Planet Reality with Land Masses par,cularly true in N. Hem where there is more land mass Because of land masses, subtropical highs and polar lows are not con,nuous bands Average Pressure Patterns in July Average Pressure Patterns in January In Northern Hemisphere: Highs Pacific High Bermuda High Lows Aleu,an Low Icelandic Low In Northern Hemisphere: Highs Lows Pacific High Aleu,an Low Bermuda High Icelandic Low weaken and strengthen and WINTER shiJ south shiJ south In Northern Hemisphere: Highs Lows Pacific High Aleu,an Low Bermuda High Icelandic Low weaken and strengthen and WINTER shiJ south shiJ south strengthen and weaken and SUMMER shiJ north shiJ north Semi-permanent Subtropical Highs Pacific High - Pacific High is an important factor for California's climate Bermuda High U.S. West vs. East Coast Weather/Precipitation 34°N 34°N U.S. West vs. East Coast Weather/Precipitation Why so little precipitation during summer in So Cal? 34°N 34°N Clicker Question Set Frequency to "AD" When are the semi-permanent high pressure systems known as the "Pacific High" and "Bermuda High" at their maximum intensity? (A) Summer (B) Winter (C) Equal at all times of the year Clicker Question Set Frequency to "AD" When are the semi-permanent high pressure systems known as the "Pacific High" and "Bermuda High" at their maximum intensity? (A) Summer (B) Winter (C) Equal at all times of the year West US: Prevailing wind is from the Northwest bringing cool and moist air onto land. Because air is cool, still not much actual water vapor in air, so over land RH is low. => inhibits thunderstorm development East US: Prevailing wind is from the Southeast bringing warm and moist air onto land. Because air is warm there is large amount of water vapor in air, so over land RH is high. => good for thunderstorm development West US: Prevailing wind is from the Northwest bringing cool and moist air onto land. Because air is cool, still not much actual water vapor in air, so over land RH is low. => inhibits thunderstorm development East US: Prevailing wind is from the Southeast bringing warm and moist air onto land. Because air is warm there is large amount of water vapor in air, so over land RH is high. => good for thunderstorm development Also important: in summer ocean water off West coast (50-70°F) much colder than water off East coast (70-85°F)