007_definitions and distribution

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Information about 007_definitions and distribution

Published on January 8, 2009

Author: aSGuest9677

Source: authorstream.com

Hot deserts : Hot deserts Definitions and distribution A) What is a desert? : A) What is a desert? The world’s deserts : The world’s deserts B) Climatic definitions of aridity : B) Climatic definitions of aridity put simply: Hyper-arid <100 mm per year Arid 100-250 mm per year Semi-arid 250-500 mm per year Slide 5: ....or Köppen’s (1918) climate classification (deserts as ‘B’ type climates) ....or Trewartha’s (1970+) amplification of Köppen’s system (BWh, BWn, BWk, BSh, BSk climate types – refer to your atlas!) Slide 6: ....or Meigs’ (1953) more complex definition (based on Thornthwaite’s (1948) index of aridity): Im = (100S – 60D) / PE where: Im is the Index of Moisture Availability S & D are annually aggregated monthly moisture surplus and deficit figures respectively PE is potential (not actual) evapotranspiration. Slide 7: Hyper-arid Im < -56 Arid -56 < Im < -40 Semi-arid -40 < Im < -20 so.....it’s a continuum fromsemi-arid to hyper-arid. Semi-arid (Botswana) : Semi-arid (Botswana) Arid (southern Morocco) : Arid (southern Morocco) Hyper-arid(Peru) : Hyper-arid(Peru) C) How much of theworld is desert? : C) How much of theworld is desert? Estimates vary by definition, but it’s about 1/3 of the land area. Hyper-arid c.10% Arid c.10% Semi-arid c.15% About 500 million people live in lands that are at least semi-arid..... The world’s deserts : The world’s deserts D) How long have therebeen deserts? : D) How long have therebeen deserts? fossil dunes in Canada:1.8 billion BP Namib: 80 million+ Sahara: 38 million+ Atacama: 15 million+ ...& deserts on Mars (it’s 90%+ desert) and various solar system satellites E) Where are theworld’s deserts? : E) Where are theworld’s deserts? Five major provinces: North Africa/Arabia/Central Asia (Sahara, Arabian, Thar, Gobi) Southern Africa (Kalahari, Namib) North America (Mojave, Sonoran, Chihuahuan) South America (Atacama, Patagonia) Australia (Western, Simpson) The world’s deserts : The world’s deserts F) What causes an areato be a desert? : F) What causes an areato be a desert? 1) Global atmospheric circulation : 1) Global atmospheric circulation the 20°-30° belt in each hemisphere Hadley/Ferrel Cells – the subtropical high semi-permanent high pressure ? descending air ? little condensation ? arid conditions examples: Sahara, SW USA, Australia The world’s deserts : The world’s deserts 2) Rain shadow effect : 2) Rain shadow effect mountain ranges can ‘soak up’ moisture from rain-bearing air masses (e.g. eastern UK) at extremes, can trigger aridity examples: Atacama (Andes), Great Plains USA (Rockies), Australian outback (Snowy Mountains, SE Australia) The world’s deserts : The world’s deserts 3) Continentality : 3) Continentality centres of land masses are very distant from the moisture of the oceans rain-bearing air masses simply cannot penetrate examples: Sahara, Central Asia, Australia The world’s deserts : The world’s deserts 4) Cold ocean currents : 4) Cold ocean currents cold water offshore can suppress evaporation so precipitation along the coast is very limited (opposite of warm currents, e.g. NW Europe) examples: Atacama (Humboldt), western Sahara (Canaries), Namib (Benguela) The world’s deserts : The world’s deserts Slide 25: No single factor is usually responsible for aridity on its own: it’s generally a combination of factors.

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