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HIGHER GEOGRAPHY PHYSICAL ENVIRONMENTS ATMOSPHERE 1 By the end of this topic you should be able to: explain with the aid of an annotated diagram, why Tropical latitudes receive more of the sun’s energy than Polar regions explain why there is a net gain of solar region in the Tropical latitudes and a net loss towards the poles describe the role of atmospheric circulation in the redistribution of energy over the globe describe and explain the earth’s energy exchanges shown on a diagram describe the factors which affect the amount of sunlight reflected from the earth’s surface describe and account for the generalised pattern of atmospheric circulation and global winds, or ocean currents shown on a world map describe the variations in world temperature for the last 100 years (shown eg. on a graph) and suggest both physical and human reasons for these variations describe and explain the origin, nature and weather characteristics of Tropical Maritime (mT) and Tropical Continental (cT) air masses which affect West Africa with reference to the Inter-Tropical Convergence Zone and the movement of air masses, describe and account for the variations in West African rainfall. 2 GMTs describe and interpret climate maps, diagrams and graphs construct and analyse climate graphs describe and explain climate graphs comment on the accuracy of statements which describe climate patterns shown on maps etc. 3 Troposphere = main zone of weather and climate. lapse rate = decrease in temperature with altitude = 6.4ºC for every 1000metres 4 Mt Everest (8800metres) Calculate the difference in temperature between sea level and the summit of the mountain. 5 ATMOSPHERIC GASES Nitrogen - 78% Oxygen - 21% Carbon dioxide - 0.036% …...and rising!! Water vapour - variable - up to 4% over tropical oceans. (as humidity increases the relative amounts of other gases decrease). 6 Global extremes of Temperature 58ºC San Luis Potosi, Mexico Al Aziziyah, Libya -88ºC Vostok Antarctica In the absence of an atmosphere the Earth would average about 30ºC less than it does at present. Life (as we now know it) could not exist. 7 SOLAR INSOLATION reflected by clouds and dust, water vapour and other gases in the atmosphere 100% 26% 18% absorbed by clouds and dust, water vapour and other gases in the atmosphere 56% reflected by surface 6% absorbed by surface 50% 8 SOLAR INSOLATION 100% solar insolation 26% reflected by atmosphere 18% absorbed by atmosphere TOTAL ALBEDO = 26 + 6 = 32% 56% reaches surface TOTAL ABSORPTION = 18 + 50 = 68% 6% reflected by surface 50% absorbed by surface 9 ENERGY SURPLUS and DEFICIT The Earth's atmosphere is put into motion because of the differential heating of the Earth’s surface by solar insolation. The Poles receive less heat than the Tropics because: 1. Insolation has to pass through more of the Earth’s atmosphere 2. the angle of incidence of insolation and 3. higher levels of surface albedo. 10 3 2 1 1 2 3 Insolation has to pass through more of the Earth’s atmosphere The angle of incidence of insolation energy is spread out over a larger area because the sun’s rays strike the surface at a lower angle. Higher levels of surface albedo - the ice-cap reflects more solar insolation, and vegetation at the equator 11 reflects far less. In theory an imbalance in energy receipt could result in lower latitudes becoming warmer and higher latitudes becoming even colder. In reality energy is transferred from lower latitudes (areas of surplus) to higher latitudes (areas of deficit) BY 1. ATMOSPHERIC CIRCULATION and 2. OCEAN CURRENTS 12 90º Pole DEFICIT 1. ATMOSPHERIC CIRCULATION 2. OCEAN CURRENTS SURPLUS 0º Equator 13 SINGLE CELL MODEL At the Equator the atmosphere is heated Air becomes less dense and rises. Rising air creates low pressure at the equator. Air cools as it rises because of the lapse rate. Air spreads. As air mass cools it increases in density and descends. Descending air creates high pressure at the Poles. Surface winds blow from HP to LP. 0º Equator LP 90º Pole HP 14 warm air is less dense therefore lighter air rises in the Tropics this creates a zone of LOW PRESSURE air spreads N and S of the Equator air cools and sinks over the Poles this is a zone of HIGH PRESSURE air returns as surface WINDS to the Tropics 15 surplus 0º Equator deficit 90º Pole 16 TRANSFER of ENERGY by ATMOSPHERIC CIRCULATION 0º Equator 90º Pole 17 THREE CELL MODEL Hadley Cell 0º Equator LP Polar Cell Ferrel Cell 30º HP 60º LP 90º Pole HP 18 ENERGY TRANSFER Hadley Cell Ferrel Cell Polar Cell Heat energy is transferred from the Hadley Cell to the Ferrel Cell and from the Ferrel Cell to the Polar Cell. In this way heat is transferred from the Equator where there is an energy surplus to the Poles where there is an energy deficit. 19 ENERGY TRANSFER Warm air rises at the Equator Inter-Tropical Convergence Zone (ITCZ). Equatorial air flows to ~30º N then sinks to the surface and returns as a surface flow to the tropics. This is the Hadley cell. Cold air sinks at the North Pole. It flows S at the surface and is warmed by contact with land/ocean, by ~60º N it rises into the atmosphere. This the Polar cell. Between 60º N and 30º N there is another circulation cell. This is the Ferrel cell. The Hadley cell and the Polar cell are thermally direct cells. The Ferrel cell is a thermally indirect cell. 20 TRANSFER of ENERGY by OCEAN CURRENTS 90º Pole 0º Equator 21 SINGLE CELL MODEL The single cell model of atmospheric circulation was developed to explain the transfer of energy from the Tropics to the Poles. This was later improved and a three cell model was developed. Today the three cell model is also considered to be an oversimplification of reality. 22 HADLEY CELL ITCZ ITCZ = Inter-tropical convergence Zone (Low Pressure) STH = Sub-tropical High (High Pressure) 23 convergence divergence convergence divergence WINDS 0º Equator LP 30º HP 60º LP 90º Pole HP winds blow from high pressure zones to low pressure zones 24 CONVERGENCE and …………DIVERGENCE 25 26 Coriolis occurs because the Earth rotates. Earth rotates about its axis every 24 hours. Distance around the equator is ~25,000 miles the earth is travelling east at ~ 1,000 miles per hour. Distance around the Earth at 40ºN ~19,000 miles the earth is travelling east at ~800mph. The Coriolis effect results from this difference in velocity. In the Northern hemisphere the Coriolis effect deflects movement to the right. In the Southern hemisphere Coriolis effect deflects movement to the left. The combination of atmospheric cells and Coriolis effect lead to the wind belts. Wind belts drive surface ocean circulation 27 PLANETARY WINDS High Pressure Coriolis effect WIND pressure gradient force Low Pressure Winds are named by the direction from which they blow. 28 90ºN WIND BELTS Polar easterlies Temperate Low LP 60ºN South westerlies Sub-tropical High - Horse Latitudes HP 30ºN NE Trades Equatorial Low - Doldrums LP 0º SE Trades Sub-tropical High - Horse Latitudes HP 30ºS North westerlies Temperate Low LP 60ºS Polar easterlies 29 90ºS WIND BELTS Polar easterlies convergence LP 60ºN South westerlies divergence Sub-tropical High HP 30ºN NE Trades convergence Inter-tropical convergence zone LP 0º SE Trades divergence HP 30ºS Sub-tropical High North westerlies convergence LP 60ºS Polar easterlies 30 90ºS WIND BELTS Northern Hemisphere Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºN Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºN Northeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone. Southern Hemisphere Southeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone. Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºS Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºS 31 Series of High and Low pressure centres approx. every ? latitude ? pressure zones associated with descending air ( ? ) Low pressure zones associated with ? air (convergence) SLIDE 37 ? circulation cells in each hemisphere: ? ? direct ? thermally indirect Polar Cell thermally direct Wind is the horizontal movement of air arising from differences in ? . Very little wind at the Equator ( ? ) because air is being convected ? . Little wind at 30ºN and S (Horse Latitudes) because direction of air movement is down. Winds always blow from an area of ? Pressure to ? Pressure. Winds are affected by the ? Effect. Coriolis is a consequence of motion on a rotating sphere. Acts to the ? of direction of motion in Northern Hemisphere Acts to the ? of direction of motion in the Southern Hemisphere Major wind belts of the Earth surface 0 to 30ºN ? ? ? Southeast Trades 30 to 60ºN/S ? 60 to 90ºN/S Polar ? 32 Series of High and Low pressure centres approx. every 30º latitude High pressure zones associated with descending air (divergence) Low pressure zones associated with rising air (convergence) Three circulation cells in each hemisphere: Hadley Cell thermally direct Ferrel Cell thermally indirect Polar Cell thermally direct Wind is the horizontal movement of air arising from differences in pressure. Very little wind at the Equator (Doldrums) because air is being convected upward. Little wind at 30ºN and S (Horse Latitudes) because direction of air movement is down. Winds always blow from an area of High Pressure to Low Pressure. Winds are affected by the Coriolis Effect. Coriolis is a consequence of motion on a rotating sphere. Acts to the Right of direction of motion in Northern Hemisphere Acts to the Left of direction of motion in the Southern Hemisphere Major wind belts of the Earth surface 0 to 30ºN Northeast Trades 0 to 30ºS Southeast Trades 30 to 60ºN/S Westerlies 60 to 90ºN/S Polar easterlies 33 34 35 23º The most intense heating of the sun, occurring at the so-called thermal equator, annually moves between the tropics. On or around June 20th each year the sun is overhead at 23½ºN, the Tropic of Cancer. On or around December 20th the the sun is at overhead at 23½ºS, the Tropic of Capricorn. These two dates are the solstices. Twice a year, at the equinoxes, on or around March 20th and September 20th the overhead sun crosses the equator. This annual north to south and back again "shift" of the thermal equator shifts the belts of planetary winds and pressure systems to the north and to the south as the year turns. 36 June Summer Solstice 23½ºN TROPIC of CANCER September Autumn Equinox March Spring Equinox 0º EQUATOR December Winter Solstice 23½ºS TROPIC of CAPRICORN 37 The location of the ITCZ varies throughout the year The ITCZ over land moves farther north or south than the ITCZ over the oceans due to the variation in land temperatures. ITCZ JULY ITCZ JANUARY 38 39 The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal precipitation patterns across that continent. 40 THE ITCZ AND PRECIPITATION IN SUMMER South North Stable, dry, cT air from the Sahara moves southwards. It causes the dry season Occasional across this area rainfall Heaviest rainfall ITCZ Moist unstable mT air now reaches far inland, bringing rain to about 17ºN 20ºN 15ºN 10ºN GULF OF GUINEA 5ºN Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham 41 THE ITCZ AND PRECIPITATION IN WINTER North South Stable, dry, cT air from the Sahara moves southwards. It causes the dry season across this area mT air right at the ITCZ is prevented from rising by stable cT air ITCZ Warm land surface makes the mT air unstable. It rises and rain fallsGULF a OF GUINEA short way behind the ITCZ 20ºN 15ºN 10ºN Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham 5ºN 42 DESERT SAVANNA dry all year dry ‘winter’ wet ‘summer’ RAINFOREST wet all year ITCZ moves north in summer 43 The further North of the Equator in tropical Africa:- the lower the annual rainfall the more the rainfall is concentrated in the summer months the more variable the rainfall. 44 RAINFOREST 0º GUINEA SAVANNA SAHEL SAVANNA 10ºN rainfall decreases DESERT 20ºN seasonality increases variability increases 45 savanna ‘parkland’ 46 47 3 forces: solar heating surface winds Coriolis effect result in a clockwise circulation of water in the Northern hemisphere and an anticlockwise circulation in the Southern hemisphere. This circulation is known as a GYRE. 48 OCEAN CURRENTS IN THE NORTH ATLANTIC 90º Pole 1 NORTH EQUATORIAL CURRENT 4 2 GULF STREAM 3 NORTH ATLANTIC DRIFT 4 NORTH ATLANTIC DRIFT 5 LABRADOR CURRENT 6 CANARIES CURRENT 5 3 6 2 1 0º Equator 49 50 51 52 The greenhouse effect is the name applied to the process which causes the surface of the Earth to be warmer than it would have been in the absence of an atmosphere. Global warming or the enhanced greenhouse effect is the name given to an expected increase in the magnitude of the greenhouse effect, whereby the surface of the Earth will amost inevitably become hotter than it is now. 53 About 70% of the sun's energy is radiated back into space. But some of the infrared radiation is trapped by greenhouse gases and warms the atmosphere, 54 55 Water vapour accounts for 98% of the natural Greenhouse effect. Water vapour has lower ‘radiative forcing’ properties than some other atmospheric gases such as carbon dioxide, methane and nitrous oxide which are naturally present in the atmosphere in small quantities. Since the Industrial Revolution the proportion of these gases has increased significantly. 56 Enhanced Greenhouse Effect 3 11% 4 6% CFCs N2O CH4 2 19% CO2 1 64% 1 Carbon Dioxide > fossil fuels, vehicle emissions, forest clearance 2 Methane > rice cultivation, biomass burning, digestive fermentation, termites, sewage, landfill, natural gas production 3 CFCs > aerosol propellants, refrigerants, foaming agents 4 Nitrous oxide > nitrogen fertilisers, industrial pollution 57 CONCENTRATION CHANGES SINCE 1750 Carbon Dioxide: 280 ppm Methane: 0.70 ppm 360 ppm (+30%) 1.80 ppm (+145%) Methane c25 x effect of CO2 CFCS (chlorofluorocarbons) recent significant decrease due to concern about OZONE LAYER BUT CFCs c10,000 x effect of CO2 58 59 60 61 °C Temperature anomalies from the period 1961-1990 62