Hot Arid and Semi-Arid Environments

Spot the difference

Describe the location of arid and semi-arid environments.

Characteristics Hot Arid – eg Sahara - less than 250 mm Rainfall / 13°C winter and 37°C summer (dry = 15 mm) Semi Arid eg. Kalahari – mm / 10°C winter ad 23°C summer (norm 363mm). Other semi-arids have distinct periods of aridity.

Factors influencing the climate Temperature – aridity is defined by the total amount of rainfall but is more importantly dependent on the relationship between rainfall and evaporation (rainfall effectiveness). The large diurnal (daily) range in temperature is due to lack of cloud cover. Intense solar radiation heats the ground and warms the air above it. At night heat escapes under clear skies. Precipitation – can vary from 10 to 500 mm/yr. It is also unreliable – may be convectional causing flash floods (Sahara), it may take the form of coastal fog (Atacama) or it may be seasonal due to global shifts in air pressure patterns and winds.

Explain the location of hot arid and semi arid environments.

Causes of Aridity (explain the location of arid and semi- arid environment) 1.Global circulation of air 1.Global circulation of air : Hot arid and semi arid deserts are found in the tropics and subtropics due to two large convective cells called Hadley Cells that control the air circulating between the equator and the tropics in the northern and southern hemispheres. Annotated diagram.

2.Rain shadows 2.Rain shadows : prevent moisture laden air arriving. The air is forced to rise over mountains and cools, condenses forming cloud and rain. eg the Atacama desert (Andes acts as barrier stopping SE trade winds from Argentina and Atlantic..

3.Cold Ocean Currents Cold ocean currents bring cool dense air that displaces warmer air over the land. Whilst there is no rain there is coastal fog which brings some moisture. (diagram).

4.Prevailing winds 4.Prevailing winds : In many hot arid areas prevailing winds are blowing from land to sea and carry little moisture (NE trade wind from N.Africa to Atlantic influence Sahara.

5.Continentality 5.Continentality Many deserts are in the middle of continents far away from rainbearing winds.

Question Homework – green activity box Draw graph and: Annotate with important climatic features (max/min, range, seasonality). Don’t do questions.

Why are hot-arid deserts fringed by semi-arid deserts?

Why are some deserts semi-arid? Tropical semi-arid deserts are found between the equator and hot deserts, they fringe hot deserts southwards and polewards and experience seasonal rainfall in summer. Reason for those near the equator – migration of hadley cells and rain at the equator due to migration of overhead sun between tropic of cancer and capricorn at the INTERTROPICAL CONVERGENGE ZONE (ITCZ). ITCZ normally at equator but migrates into tropical semi-arid areas bringing convectional rainfall (largely ineffective due to evaporation loss and intensity)

Why are some deserts semi-arid? Sub-tropical, semi-arid areas polewards of the hot-arid deserts get rain in winter due to converging winds at polar fronts. Depressions occur bringing rain.

Winds Winds are strong and gusty influencing landforms. At the centre of the high pressure cell winds are calm but at the edges of the system are the trade winds and westerlies. It can be very calm or very windy in a desert.

How does the climate affect the physical environment – produce a spider diagram. Impact of climate on the physical environment Diurnal range in temperature:

Describe and explain the location of hot arid and semi arid environments.

How does the climate affect the landscape in hot arid and semi arid environments? Key features of the climate: How do these key features of the climate affect the landscape and types of landforms in hot arid and semi-arid environments.

Prep Find pictures of: Inselberg Yardang Saltpan Dunes Mesa and butte Pedestal rock Wadi Canyon

Processes that give hot arid and semi- arid environments their distinctive characteristics. Weathering. Aeolian erosion Water erosion

Weathering

Weathering Processes HA and SA environments generally experience slow rates of weathering. This is largely due to the lack of water, which means that chemical breakdown of the rocks is slow. Rock breakdown is therefore mainly due to physical weathering, which can be quite rapid due to the absence of soil and plant cover exposing bedrock over large areas.

Three important physical weathering processes are; Freeze thaw Salt weathering Insolation weathering Freeze thaw – possible where temperatures fluctuate above and below freezing and where there is sufficient moisture. Generally found in semi arid mid-latitude environments.

Aeolian erosion Deflation Abrasion Attrition

Ventifacts in the Sahara Desert

Case Study – Arches National Park Where Characteristics Landforms and explanation of formation

Aeolian transportation and deposition. Application of knowledge What types of transportation are there? What landform of deposition exists in deserts? Draw one!

Formation of desert dunes

Case study The Namib

Water erosion and deposition Wadis, canyons and canyon landscape (pediments and alluvial fans) Salt pans and playas.

Case study Grand Canyon – mesa and buttes.