Hot Deserts

• Desert area

  Dry or arid area characterised by little to no rainfall and high daytime temperatures

  
• Degree of aridity

  • Semi-arid (250-500 mm mean annual rainfall, sparse vegetation such as short grasses and scattered trees)
  • Arid (25-250 mm mean annual rainfall, plants only appear along river valleys and in oases)
  • Extremely arid (<25 mm mean annual rainfall, bare dry surfaces, few plants grow soon after rainstorm)

  
• Most deserts are located in the 15° to 30° either side of the equator
• Apart from the Sahara Desert which extends across Africa, most deserts are on the western side of continents
• Examples of deserts

  • Atacama in South America
  • Kalahari in Botswana
  • Namib in Namibia
  • Australian deserts
  • Mexican deserts
  • Mojave
  • Iranian
  • Thar
• Climate graph

  Shows the average temperature and rainfall for a city or region over the year, with temperature shown on a line graph and rainfall on a bar graph

  
• Interpreting climate graphs

  1. Look for patterns in temperature data (e.g. number of seasons, warmest/coolest months, annual temperature range)

  2. Look for patterns in rainfall data (e.g. year-round, seasonal, total annual rainfall)

  3. Describe the patterns in temperature and rainfall, including how they relate to each other

  
• Temperature in hot deserts

  • Very high daytime temperatures often over 50°C
  • Low night temperatures below 20°C
  • Clear skies and sometimes ground frost
  • Diurnal (daily) temperature range is always high
  • Coastal areas are much cooler due to the moderate influence of the sea and cold offshore currents
  • Mean monthly temperatures range from 29°C in the hot season to 10°C in the cooler season, varying considerably from desert to desert

  
• Precipitation in hot deserts

  • Average annual precipitation total less than 250mm a year
  • Some places have no recorded precipitation at all
  • Rainfall occurs in the form of high-intense and short-lived storms (torrential convectional downpours)
  • Storms are extremely rare and erratic, with places sometimes having no more rainfall for years after a storm

  
• Reasons why rainfall is low in hot deserts

  1. Influence of pressure on precipitation (descending air causes warming and decreased relative humidity)

  2. Influence of cold ocean currents on precipitation (onshore winds are cooled, leading to fog formation and reduced inland precipitation)

  3. Influence of relief on precipitation (rain shadow effect where descending air on the leeward side becomes dry)

  4. Influence of temperature on precipitation (occasional torrential convectional storms, with summer being the wettest season near the equator and winter being the wettest season on the poleward side of deserts)

  
• Trade winds are strong and constant because they derive from sinking air, and they do not contain much moisture

  
• The combination of sinking air and offshore trade winds leads to very low precipitation in hot deserts

  
• Winds blow out of high-pressure systems into low pressure systems, and they are deflected by the Earth's rotation (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere)

  
• When onshore winds blow to the west coasts of deserts, they can be cooled enough by the cold ocean currents, leading to the formation of fog which reduces visibility and removes moisture from the air

  
• As the foggy air moves inland, the water droplets quickly evaporate, and daytime warming further reduces the relative humidity and chance of precipitation

  
• Rain shadow effect

  1. Air containing moisture from the sea reaches a physical barrier like a mountain, forcing the air to rise on the windward side and creating relief or orographic rainfall

  2. On the leeward side, the descending air is compressed and warmed, reducing the relative humidity and preventing rainfall

  
• Rain does not fall often in deserts but, when it does, it is usually torrential and often causes flash floods
• Areas nearest to the equator have occasional convectional storms in the summer heat, while areas on the poleward side of deserts have winter rain

  
• Vegetation adaptation in hot desert areas

  • Must be able to withstand exceptionally high temperatures, high evaporation rates, long periods without any rainfall and often ground salinity
  • Plants with adaptations which allow them to live in hot and dry conditions are called xerophytic

  
• Characteristics of desert vegetation

  • Sparse and characterised by low shrubs/bushes/scrubs
  • Palm trees usually found where there is an oasis
  • Some plants have narrow leaves/spines/spiky and waxy
  • Some plants have fleshy stems/succulents
  • Some plants have long roots and lateral roots

  
• Sparse vegetation

  Due to low precipitation (aridity) which reduces competition for water
• Adaptations of desert plants

  • Thick waxy skins to reduce moisture loss through transpiration
  • Narrow (spiky), waxy leaves to reduce transpiration
  • Long taproots to reach deep underground water supplies
  • Shallow lateral/horizontal roots to make use of surface water
  • Ability to lie dormant during dry spells and germinate quickly after rain
  • Succulents that store water in stems, leaves, roots or fruits
  • Short life cycles to take advantage of short wet seasons
  • Spines instead of leaves to reduce water loss
  • Capable of growing in saline depressions (halophytes)
• Desert soil

  Mostly sandy soil (90–95%) found in low-rainfall regions, with low nitrogen and organic matter, high calcium carbonate and phosphate, making it infertile

  
• Salinisation

  Accumulation of water-soluble salts in the soil to a level that impacts agricultural production, environmental health, and economic welfare
• Excess salt has the same deadly effect on plants as drought: it reduces the ability of plants to take up water, interfering with their growth and reducing their vitality</b>
• Animal adaptations in deserts

  • Zebras can detect water below the surface and dig holes to reach it
  • Fennec foxes can go for long periods without water and reduce water loss by being nocturnal
  • Fennec foxes have thick fur to insulate their bodies
  • Desert tortoises feed on plants in spring and store moisture to last until next spring
  • Light-coloured fur or feathers to reflect sun and camouflage
  • Small animals that shelter from heat in burrows, rocks or leaves, or are nocturnal
  • Horned vipers bury in sand for protection and camouflage
  • Seasonal coat changes to stabilise body temperature
• Camel adaptations

  • Long eyelashes and ability to close nostrils for protection in sandstorms
  • Large water holding capacity in stomach
  • Can go 6-8 days without water
  • Store fat in humps which is digested to form water

  
• Camels have more characteristics that enable them to survive than any other desert animals
• Further reading: Read notes on opportunities for human activities in deserts page 225-230 Complete Geography