Hot Arid and Semi-arid
Environments
CLASSIFICATION AND
DISTRIBUTION OF HOT ARID
AND SEMI ARID ZONES
Definitions
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
Semiarid
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.
Atacama
Chile
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
LEARN
SW USA
Sahara and
Middle East
Atacama
Kalahari
Not studied for A level
Australia
General locational points
• Latitude : 20° – 35° N and S of equator
• West coast of Continents
• Offshore cold ocean currents
Climate
Temperatures
• 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.
2.
3.
4.
5.
Latitude
Altitude
Distance from the sea
Presence of cold offshore ocean currents
Albedo of surface
Precipitation
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
rain
• 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
(ITCZ).
• 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
Hadley
cells
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
winds
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…..
FOG
• 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