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Hot arid and Semi-arid Environments by hcj


									Hot Arid and Semi-arid
                 Simple one
What is a desert (arid area)?

• <250mm precipitation per year
(Semi arid area = 250 – 500 mm per year)

More advanced……
                WHAT IS ARIDITY?

      Ratio of average precipitation (P) to potential
        evapotranspiration (PET*) = aridity index

                       Arid areas: <0.20
                    Semi-arid areas 0.20 -0.50

                       Koppen's classification
*See page 134 for def of pet
Semi-   Arid
   Types of arid environments
• See page 134 for characteristics

        • Where are they?
                     Hot Arid Areas
• BWh – hot deserts (annual mean > 18°C)

Hot dry with winter dry season   e.g Sahara
Bwn - hot deserts with Coastal fogs
                       • E.g.
          Semi arid areas
• BShw – Semi arid between Equator and
  hot deserts (rain in summer)
  e.g. sub Sahara
• BShs – semi-arid pole-wards of hot
  deserts (rain in winter) e.g. Iraq
         Five main areas of Hot arid regions
           Each with various sub-divisions

SW USA                 Sahara and
                       Middle East



                     Not studied for A level
     General locational points
• Latitude : 20° – 35° N and S of equator
• West coast of Continents
• Offshore cold ocean currents

• Complete Activity page 136
  based on Alice Springs fig 4.2
Other points to note
• High Diurnal range
• Why?
 Very high temperatures due to
• Low latitudes – high incidence of angle of
  sun’s rays
• Little cloud cover to reflect absorb or
  scatter solar radiation
     Variations caused by Local
         conditions such as
1.   Latitude
2.   Altitude
3.   Distance from the sea
4.   Presence of cold offshore ocean currents
5.   Albedo of surface

  Defining factor of areas BUT
• High inter-annual variability 50% - 100%
• E.g. Death Valley 1887-1994 average
  98mm: Range 54mm to 171mm
 Why are these areas so DRY?
1. Global circulation (i.e. their latitudinal
   position in relation to it)
This sinking air is warming
as it descends due to
compression, making the
atmosphere cloud free –
causing a permanent high
pressure air (sub-tropical
high) and little likelihood of
• On or near the equator, where average solar radiation is
  greatest, air is warmed at the surface and rises. This
  creates a band of low air pressure, centered on the
  equator known as the intertropical convergence zone
• The Intertropical Convergence Zone draws in surface air
  from the subtropics. When this subtropical air reaches
  the equator, it rises into the upper atmosphere because
  of convergence and convection. It attains a maximum
  vertical altitude of about 14 kilometers (top of the
  troposphere), and then begins flowing horizontally to the
  North and South Poles.
• This rising air comprises one segment of a circulation
  pattern called the Hadley Cell (see diagram below). The
  Hadley cell eventually returns air to the surface of the
  earth, near 30 deg N and S. -
            I.e where the world ‘s arid regions are
• The convection cell is completed by the
  winds which return to the equator at
  ground level called the trade winds
Offshore winds
Trade winds

         2. Prevailing winds
• Offshore winds carry very little moisture
  e.g. Ne Trades blowing over North Africa
             3. Topography
• Mountain ranges block moisture laden air
  from entering some regions (rain shadow
  effect) e. g. Mojave desert in California by
  coastal ranges
          4. Continentality
• Distance from the sea
• Some areas are remote from rainbearing
5. Cold offshore ocean currents
• Cold ocean currents can cause local
  winds to blow onshore bringing cold air
  with them
• This displaces warmer air which rises
  causing a temperature inversion
• Convection is unable to take place and
  therefore rain unlikely
• However…..
• This can cause fog which covers the land
  below the cool air
• Very important in Atacama and Namib
  deserts where plants and animals rely on it
  for their survival
• Complete Activity page 137
               Seasonal rain
• Found in Semi-arid areas
• Due to the seasonal shift of the global wind belts
  with apparent migration of the sun
• In Semi arid areas on the equator side of hot
  deserts it occurs as summer rain – convectional
  –least effective for plant growth
• On pole ward edges it occurs as winter rain due
  to depressions migrating from polar front
• Activity page 139

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