Aridity:
- Most arid areas are located in tropics, associated with
subtropical high-pressure belt.
- Some are located alongside cold ocean currents, e.g.
Namib desert.
- Some are located in the lee of mountain ranges, e.g.
Gobi desert.
- Others are located in continental interiors, e.g. Sahara
desert.
- Many definitions of aridity are based on water balance
(relationship between inputs of precipitation and losses
via evapotranspiration).
- Actual amount of evapotranspiration depends on
amount of water available, so PET used to measure how
much would take place if there was unlimited water
supply.
- Aridity is permanent water deficit, while drought is
unexpected short-term shortage of available moisture.
- Rainfall effectiveness is affected by:
o Rate of evaporation – dependent on temperature,
wind speed and moisture. Losses are high in hot,
dry areas.
o Seasonality – winter rainfall more effective than
summer rainfall as evaporation losses are lower.
o Rainfall intensity – heavy, intense rain produces
rapid runoff with little infiltration.
o Soil type – impermeable clay soils have little
capacity to absorb water, while porous + sandy
soils may be susceptible to drought.
,- Causes of aridity:
(1) Global atmospheric circulation – dry,
descending air associated with subtropical
high-pressure belt is main cause of aridity
around 20-30N (air descending at Tropics
warms up + decreases in rel. humidity, so
few/no clouds form allowing for uninterrupted
SWR).
(2) Continentality – amount of water carried
across land by winds is limited, since most
moisture lost via rainfall near the coast and air
has travelled for hundreds/thousands of km
over a relatively dry land surface, so not much
moisture picked up. SHC of land is 2.1 kJ/kg/C
(water is 4.2), so land surfaces heat up more
rapidly, warming overlying air masses.
(3) Cold offshore currents – cool overlying air
masses, increasing relative humidity and limit
amount of condensation so that it may
become saturated. Forms fog, which only
exists for short distance as it is rapidly heated
from below (conduction & convection). Rel.
humidity then plummets, leading to v. dry air.
Occurs in deserts like Atacama/Namib.
(4) Intense rain shadow effects – air rises, cools,
increases its rel. humidity and condenses as it
passes over mountains, leading to relief
rainfall on windward side. Air then descends
on leeward side, increasing in temperature,
, decreasing in rel. humidity and bringing v. dry
air with few clouds or rainfall.
E.g. Mojave Desert, which lies in rain
shadow of Sierra Nevada.
Climatic characteristics of hot arid areas:
1. Rainfall – low annual rainfall (>100mm = hyper-arid,
101-250mm = arid, 251-500mm = semi-arid). Can be v.
intense convectional rainfall triggered by intense
heating. Variable through time & space, with potential
for long periods of drought then sudden downpours.
Semi-arid environments are often associated with
movement of ITCZ (thermal equator), so are located at
10-15N/S.
2. Temperature – high max. daily temperatures (50C+).
Can be v. cold at night, e.g. Sonoran Desert in SW USA
has 100 nights per year below 0C. Linked to entrenched
high pressure systems (lack of clouds) and high intensity
insolation (sun is high in sky, travelling through shorter
distance of atmosphere). Large diurnal temperature
range of 30-40C (UK max. is around 20C) in more
continental deserts, whereas coastal deserts have low
seasonal/diurnal ranges.
3. Winds – persistent, consistent, desiccating, dry (v. low
relative humidity), strong (lack of vegetation so less
friction between wind and surface), dusty.
