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Hot Arid and Semi-Arid Environments
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In-depth overview of A-level CIE Geography Hot Arid and Semi-Arid Environments content including case studies.
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Arid and Semi-Arid Environments Revision4.1: The distribution and climatic characteristics of hot arid and semi-arid environments
Keywords:
Aridity: a permanent water deficit. Based on water balance – inputs (precipitation) & outputs (evapotranspiration)
Drought: an unexpected short-term shortage of available moisture.
High pressure Systems: the anticyclonic systems which bring clear, dry conditions to latitudes 20°-30° for most of the year.
Rain shadow Effect: the dry, lee side of a mountain
Continentality: the effect on a climate of being far from the sea; having a high temperature range
Potential Evapotranspiration: a measure of how much water loss/evapotranspiration would take place if there was an unlimited supply of
water.
Evaporation: the amount of heat energy needed to change a substance from liquid to gas.
Diurnal Temperature Range: the differences in temperature between day and night.
Annual Temperature Range: the difference in temperature between the hottest and coldest months.
Albedo: the degree of reflection of the sun’s rays according to the ground surface colour.
Classification Annual Precipitation Aridity Index
Hyper-Arid <100 mm (12 months of no <0.05
precipitation recorded. No seasonal
precipitation).
Arid 100-250mm 0.05-0.2
Semi-Arid 250-500mm 0.2-0.5
An arid climate is defined as one that receives less than 25.4 cm of rainfall per annum, limiting its ability to sustain
ecosystems.
Rainfall influenced by…
• Evaporation Rate: affected by temperature, wind speed.
• Seasonality: winter rain → less evaporation.
• Rainfall Intensity: v. intense → runoff, little infiltration.
• Soil Type: impermeable clay soils → little capacity to absorb water. Sandy soils susceptible to drought.
Deserts have variable rainfall → as rainfall total decreases, variability increases.
Rainfall Variability Index
V = mean deviation from average / the average x 100 Eg: Sahara → 50%-80%
Global distribution of arid areas:
Australia is the most arid continent (75% land classified as arid/semi-arid).
Located in…
- Tropics (STHPB)
- Next to cold ocean currents (Namib [S. Africa], Atacama [S.
America/Chile])
- Lee of mountain ranges (Gobi in N.W China & S. Mongolia Altai
Mountains, Patagonian in Argentina)
- Continental interiors (Sahara, Australian).
Aridity is based on…
• Latitude, Altitude, Topography, Winds,
Continentality, Ocean Currents, Albedo
,Köppen Climate Classification System
Classifies worlds climates based on annual & monthly averages of temperature & precipitation.
‘B’ – initial given to dry climates
‘BW’ – arid climates
‘BS’ – semi-arid climates
‘h’ – suffex added where mean temperatures are over 18°
‘k’ – suffex added where temperatures are lower than 18°
‘BWh’ – low latitude, hot deserts, low rainfall. Eg: Sahara
‘BSk’ – semi-arid, high latitude, cold winter deserts. Eg: Patagonia
‘BShw’ – semi-arid outer tropical climate. Long dry winters, subsiding air, brief erratic rains. Hot temps and rapid evaporation → less effective
for plant growth. Eg: Sahel
Thornthwaite – used Köppen and introduced P/E index.
Total monthly precipitation/Total monthly evaporation, add 12 monthly values to get total.
Trewartha – based on Köppen. ‘B’ climates are those which have evaporation exceeding precipitation.
BWh: Hot deserts with annual mean temperatures over 18⁰C. Related to tropical continental stable air masses and dry tropical winds
(e.g. Sahara and Central Australia)
BWn: Similar to the above but where there is frequent offshore fog (e.g. the Namib and the Atacama deserts)
BWc: Mid-latitude interior deserts dominated by tropical continental air masses in summer, but by polar continental air masses in winter (e.g.
Turkestan and Patagonian deserts)
BSh: Semi-arid tropical or subtropical areas. Tropical continental air masses are dominant, but there is a short rainy season (e.g. the Sahel region
and the Tell area of the Maghreb)
BSk: Semi-arid mid latitude deserts with low summer rainfall (often from thunderstorms) and cold winters (e.g. Gobi and the Great Basin
deserts)
UN’s index of Aridity – relationship between precipitation and evapotranspiration
AI = Aridity Index
AI = P/PET P = Avg. annual precipitation
PET = annual potential evapotranspiration
Causes of Aridity
1. Global Atmospheric Circulation (HADLEY CELL) eg: Sahara
• Direct heating over equator → warm air rises (trade winds converge to form ITCZ, air converges at the surface & diverges at the
tropopause).
• Warm unstable air rises & expands, uses energy → loses heat → cools (holds less water → rains at equator). Air reaches stratosphere
(acts like ceiling) → too thin to hold air parcel.
• Convection currents (circulatory movements) mean hot air is constantly rising → air parcel can’t sink → forces air to diverge N & S.
• Dryer air (as it rains) heads polewards becoming cooler & denser → sinks 30° N & S. Sinks = high pressure system (STHPB).
• HP results in anticyclonic conditions for 90% of year (clear skies, low rainfall, high rates of insolation & evaporation). Absence of
cloud cover allows great build-up of heat during day and lots of heat loss during night → diurnal temp range.
2. Continentality (distance from sea) eg: Turkestan Desert, Gobi Desert, up to 2500km from nearest oceans.
• Sun heats up water (insolation). Water heats up slower than land (land has lower SHC - heats and loses heat quicker). High
evaporation in coastal areas (lots of rain → clouds).
• Air moves inland → Depleted of moisture (doesn’t pick up moisture). No clouds → more sunshine reaching ground.
3. Rain-Shadow eg: Patagonian Desert
• Moist air being brought inland by prevailing winds (prevailing north westerlies) will reach mountain range (Andes).
• Wind contains moisture due to evaporation of ocean → warm moist air rises up → inc. in altitude → air expands and cools (can’t hold
as much moisture, adiabatic cooling) → reaches dew point → condenses → clouds → precipitation.
, • Rains on the windward side of mountain, moisture is now lost → warm dry air descends on leeward side (no rain) → deprived of
moisture.
Another example;
S.E California = rain-shadow desert → Death Valley
High mountains on the west block movement of wet winter storms.
Costal storms moving east collide with Sierra Nevada Mountains dropping
most of the moisture on the west sides of the mountains. Land on the east side
receives much less rain → desert.
4. Cold Ocean Currents eg: Namib desert S.W Africa
• During earths heat exchange process, there are large transfers of energy within the oceans → warm water moves towards colder
latitudes & colder water transported to warmer latitudes. Circulatory cells ‘gyres’.
• Cold ocean currents eg: Peruvian Current (S.W America, along Atacama), Benguela Current (S.W Africa, Namib coast), Californian
Current (S.W coast USA), Canaries Current (Atlantic coast of Sahara); usually found along western shorelines of deserts.
• Prevailing winds from east pass over cold ocean currents & cools → R.H increases → any moisture (water vapour) in air parcel
condenses → DPT → rain/fog/mist. Air v. cold → can’t hold moisture → little evaporation → little precipitation → dry.
5. Human – desertification
Temperature Variation in Deserts
Continental interiors show extremes of temperature, both seasonally and diurnally.
Coastal areas have low seasonal and diurnal ranges.
Characteristics of Deserts
Hyper-Arid Zones Arid-Zones Semi-Arid Zones
No rain at all for several years (Northern Infrequent and unreliable precipitation; Receives higher and more reliable rainfall
edge of the Sahara Desert – 0.4mm of rain rarely exceeds 250mm/year
annual average and aridity index 0.02).
Water deficit (evapotranspiration > P)
Very sparse vegetation – shrubs and low Vegetation is still very sparse, but small Vegetation consists of a variety of grasses
grass trees might occur and also annual grasses and soil formation is possible.
(dependent on seasonal rainfall).
Soils are poorly developed Sedentary agriculture is possible where Sedentary agriculture is possible, but
aquifers or perennial streams are available irrigation in low rain season may be
for irrigation. required.
Strong winds are frequent (dust and sand
storms). High levels of solar radiation (large
diurnal temp range)
Pastoralism
4.2: Processes producing desert landforms
Weathering: decomposition & disintegration of rocks in situ (no transport involved).
Weathering is greatest in shady sites & areas within reach of soil moisture.
Chemical weathering is enhanced in areas experiencing dew or coastal fog.
As rainfall increases, weathering increases, soils have more clay, less salt & more distinct horizons.
Factors affecting the rate of weathering:
1) Rock characteristics (mineral composition & solubility, physical features like joints & bedding planes)
2) Climate (temperature & moisture, chem weathering most affective in areas with high temp & abundant moisture). Van’t Hoff’s law
states that the rate of chemical weathering increases 2-3 times for every 10o increase in temperature up to a max of 60o.
Differential weathering: disintegration of rocks caused by variations in composition.
Mechanical Weathering: (disintegration), tearing apart & mechanical breakdown of rocks physically destroying them (smaller, angular
fragments) due to temp change, moisture, frost, pressure. Chemical composition remains unaltered.
Salt Crystallisation (Salt crystal growth) – causes decomposition of rock by solutions of salt.
1) Areas where temps fluctuate (26°-28°), sodium sulphate (NA2SO4) & sodium carbonate (NA2CO3) expand by 300%. This creates
pressure on joints forcing them to crack. Growth of salt crystals beneath the surface results in surface scaling.
2) Salt is dissolved in water (brine), evaporation leaves behind salt crystals. As temp rises, salts expand & exert pressure/stress on rock.
Both are frequent in hot desert regions (low rainfall, high temps) → cause salts to accumulate below the surface. Desert rocks have soluble salts.
,Chalk decomposes fastest, then limestone, sandstone, shale.
The more porous the rock, the more susceptible to salt crystallisation.
Effectiveness:
• Most effective salts: sodium sulphate (causes 100g block of stone to break down about 30g, loss of 70%), magnesium sulphate,
calcium chloride
• Rate of disintegration related to porosity & permeability.
• Surface texture & grain size control rate of rock breakdown.
Freeze-Thaw
• Water enters a crack in the rock (joints & bedding planes)
• Water in cracks/joints freezes at 0°; when it freezes, water expands by 9% & exerts pressure (max 2100kg/cm² at -22°). Avg. pressure
14kg/cm²).
• Repeated freezing & thawing causes rock to split (bigger crack), causing rock to deepen & widen.
• Most effective in periglacial & mountainous alpine regions, high altitude deserts & continental interiors (frequent fluctuations of temp.
above & below freezing). Also need water/moisture availability → can’t occur in extreme cold temps as it will permanently be frozen.
Thermal Fracture – the break-up of rock as a result of repeated changes in temperature over a prolonged period of time. Diurnal temp change.
Results in block disintegration, granular disintegration, shattering and exfoliation.
Shattering: rocks may shatter into irregular fragments with sharp edges eg: basalt is black and highly metallic → expands & contacts rapidly.
Disintegration/Insolation Weathering/Exfoliation/Heating & Cooling
• Found in hot desert areas where there is a large diurnal temp range (40° day, 0° night)
• Absence of clouds in tropical deserts & overhead sun produces high daytime temps (lots of SWR).
• Rocks are poor conductors of heat (low SHC) → heat up, (surface/outer layers of rock expand)
• At night, there’s no clouds → rocks cool quickly due to radiation cooling (causes outer layers to contract)
• Repeated heating and cooling causes peeling/exfoliation/onion skin weathering. Rounded profile.
• Also need moisture for this to happen. Minerals expand & contract at different rates causing stresses along mineral boundaries →
creates weaknesses within the rock.
• Quartz inc. in size 3x more than feldspar.
Pressure Release/Dilatation
• Heavy overlying rocks are removed by erosion
• The release of this pressure (unloading) → removal of overburden & weight, causes exposed/underlying rocks to expand and fracture
parallel to the rock surface (bedding planes)
• Unloading of pressure by removal of overlying rocks causes cracks/joints to form at 90 o to unloading surface (cracks are lines of
weakness to rocks.)
Block Disintegration: (limestone, sandstones) rock splits in two, larger blocks of rock are detached from the main body of rock.
Granular Disintegration: (granite, coarse sandstones) surface breaks up into small grains of rock due to certain grains [black mica & white
quartz] being more prone to expansion & contraction from heating and cooling than others → exerts pressure → forces them to break off.
Chemical Weathering: (decomposition), breakdown (dissolving) of rocks through chemical changes/reactions → changes chemical
composition.
Hydrolysis → block disintegration
• Occurs on rocks with orthoclase feldspar (granite).
• Feldspar reacts with acid water & forms kaolin (kaolinite) or china clay (weakens rocks), silicic acid & potassium hydroxyl are
removed in the solution leaving the kaolin behind.
• Other minerals in granite (quartz, mica) remain in the kaolin.
Spheroidal weathering is the rounding of rocks by chemical weathering, causes the corners & edges of the rock to become more rounded.
Hydration
• Process whereby minerals absorb water, expand (swell) and change chemical composition.
• This makes them weaker and less resistant to erosion.
• Eg: Anhydrite absorbs water to become gypsum (expands by 0.5%).
• Extreme cases: increase in volume up to 1600% by shale & mudstones when clay minerals absorb water.
Features formed by weathering:
Crust Formation:
• Chotts/salt lakes/playas – found in lowest part of desert surface where ephemeral streams flow into
inland depressions. High evaporation rates on surface & within groundwater leave salt deposits
(sodium chloride) on the surface & thick crust. During the dry season the surface of the lake is
usually hard and rough, where as in the rainy season it gets wet and soft. The water usually creates a
small hole meaning there is a very shallow lake in the desert. Eg: Chott el Djerid, Tunisia
,Pitting of the rocks: Common in Sinai Peninsula, Egypt
• Alveoles: small scale hollows in a honeycomb surface formed by solution and/or salt crystallisation.
• Tafoni: larger cavities in the rock, cave line features, often in sedimentary rock. Formed by salt weathering, wetting-drying, freeze-
thaw.
Alveoles Tafoni
Desert Varnish: deposition of a thin, dark red layer of iron & magnesium oxides when evaporated through soil or rock – silica rich oxides are
also common.
Wind Action
Aeolian processes: winds that blow across deserts often produce an effect similar to fluid in motion.
Aerial photography & satellites show major features aligned with prevailing wind systems.
The lack of vegetation reduces surface roughness permitting smoother wind/land contact. The wind produces particulate sand, which is
transported or deposited. All types of Aeolian processes require there to be loose sand and other particles so that wind can pick them up and use
them to alter the form of the landscape.
Wind Erosion
• Abrasion/Corrosion: wind causes sand particles to scrape along rock surfaces → sand blast affect
• Attrition: rock particles rub against each other/collide and wear away.
• Deflation: the lifting and removal of loose particles (unconsolidated
material) such as clay and silt → gradually lowering surface of desert.
Deflation Basins/hollows: some basins have deflated so much; their bases reach the water table & form
an oasis eg: Bahariya & Farafra oases (W. Egypt).
Eg: Qattara Depression (134m below sea level); wind erosion carves out a pit* (deflation hollow, range
in size from few-100s meters in diameter & develop over days or seasons). Responsible for creating
stony surfaces.
Oasis
*Fine grained particles from rocks that are easily weathered are carried away creating a hollow (strong
wind eddies). As the hollow deepens they collect water during the rainy season. This water helps speed up the weathering of the rock creating
more particles to be blown away by the wind. Due to this, the hollow is able to deepen faster than the rest of the land surrounding it, creating a
large hole in the ground.
Deflation causes various types of desert landscape:
Categories of deserts:
• Hamada: (barren rock highlands). All lose material is blown
away leaving large areas of bare rock, often strewn with large,
Figure 2: Hamada
immovable weathered rocks. Figure 4: Reg
• Reg: (A vast stony plain). The finest material has been deflated
leaving a gravely or stony desert.
• Erg: (sand seas). Classic sandy desert.
• Desert Pavement: pebbles are concentrated eg: by a flash flood &
packed together into a mosaic; the tops are then worn flat by wind
erosion and become shiny as a coat of desert varnish develops.
Figure 5: Erg Figure 2: Desert Pavement
, Ventifacts: formed when large rock fragments, too heavy to be transported by the wind, are worn down on the
windward side, correlate with wind direction. Formed from hard, fine-grained rocks such as quartz, chert and obsidian.
Undercutting (abrasion 1m above ground) produces these landforms:
Ventifact
Rock Pedestals: Resembles a mushroom where the base has been undercut, and bands of hard & soft rock have
been differentially sand blasted. Weaker parts of the rock are worn away by rock particles carried by the wind
(abrasion). This occurs when there is soft rock underneath that is easily eroded and harder rock on top that can’t be
eroded away (differential erosion). Wind action strongest at base of the rock – narrow bottom, wide top. Eventually the
top will collapse as base gets too thin to support it. Due to wind-borne sand grains following bouncing trajectories that
carry sand as high as 1m above the ground.
Rock Pedestool
Zeugens:
➢ Created by aeolean wind erosion – abrasion.
➢ Zeugen form in rocks that have alternating horizontal bands
of hard and soft rock.
➢ Rocks have joints & cracks in them. Joints are made wider by
different forms of weathering (dew, heating & cooling).
➢ Once these joints are widened, they are further enlarged by
abrasion.
➢ The furrows/hollows develop in the less resistant rock.
➢ The remaining hard rock is left standing and is called a zeuge.
➢ Zeugen can be up to 30m high. Creates a “ridge and furrow” landscape.
A zeugen is a tabular mass of resistant rock, standing prominently in the desert with ridges and furrows. It is composed of alternating layers of
hard and soft rocks that lie horizontally on top of one another. The hard rock layer usually lies above a layer of less resistant rock. Joints in the
resistant top rock are widened by weathering. Abrasion also acts on the hard rock as sand being transported hits the masses of rock and wear it
away- similar to sand blasting. Once the less resistant rock is exposed it is eroded more readily and deep furrows form. The hard rock forms a
protetive cap and undercutting may occur.
Yardangs
• Created by aeolean wind erosion – abrasion
• Yardangs lie parallel to one another and to the direction of the prevailing wind.
• Yardangs form in rocks that have alternating vertical bands of hard and soft rock.
• Softer/weaker rocks are worn down (abrasion cuts through resistant rock,
undercutting) & develops deep furrows/trough in softer rock underneath.
• Harder rocks are left behind.
Yardangs are narrow, streamlined ridges that are usually three to four times longer
than they are wide. They are made up of long vertical ridges of hard resistant rocks
alternating with narrow furrows of soft rocks. Here, both the bands of hard and soft
rocks aligned parallel to the direction of the blowing prevailing winds. They are
eroded predominately by abrasion, where sand being transported hits the masses of
rock and wears it away- similar to sand blasting. This effect is most effective within
1.5m of the surface. However deflation also contributes to their formation. Eventually,
the bands of hard rocks remain standing high above the soft bands that have been worn
into narrow corridors.
Factors affecting Aeolian erosion (wind):
• Strength/velocity & duration/frequency of the wind
• Wind direction
• Particle size of the material carried
• Moisture content of the soil
• Lithology of rock → structure and composition
• Vegetation cover
Aeolian Transport: wind speed > 20km/hr
• Suspension: smaller grains (sand, silt, clay, 0.05-0.14mm diameter), picked up & carried by the wind. High velocity winds →
sandstorm.
• Saltation: grains/coarser sands temporarily lifted, bounce/jump along surface in direction of the wind (0.15-0.25mm). Few cm above
ground.
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