Deserts — Types, Formation, and Distribution

5.1 Definition and Classification

A desert is defined as an area receiving less than 250 mm (25 cm) of annual precipitation. Deserts cover approximately 33% of Earth's land area — about 50 million km².

Key misconception: Deserts are not only hot — the defining criterion is aridity (lack of moisture), not temperature.

5.2 Types of Deserts

Located in the subtropical high-pressure belt (20°–30° N and S latitude) where the descending limb of the Hadley Cell creates dry, sinking air.

Desert	Location	Area	Key Feature
Sahara	North Africa (11 countries)	9.2 million km²	World's largest hot desert; 25% sand (erg), 70% rock (reg/hamada), 5% salt
Arabian Desert	Arabian Peninsula	2.3 million km²	World's largest continuous sand desert (Rub' al Khali — "Empty Quarter")
Thar Desert	India-Pakistan (NW Rajasthan, Sindh)	~200,000 km²	India's only major desert; avg rainfall 100–250 mm; 23rd largest desert
Kalahari	Southern Africa	900,000 km²	Semi-desert (receives 150–500 mm rain); Bushmen/San people
Namib	SW Africa (Namibia coast)	~80,000 km²	World's oldest desert; extreme coastal fog desert; Benguela Current
Atacama	Chile-Peru coast	~105,000 km²	World's driest non-polar desert; some areas 400 years without rain
Sonoran	USA-Mexico	310,000 km²	Iconic saguaro cacti; straddles US-Mexico border

2. Cold Deserts (Continental/Rain Shadow Deserts) —

Desert	Location	Area	Key Feature
Gobi Desert	Mongolia-China	1.3 million km²	Asia's largest; cold continental interior; receives 50–200 mm snow/rain; dinosaur fossils
Taklamakan	Xinjiang, China	270,000 km²	Central Asian cold desert; shifting sand dunes 100–300 m tall; Silk Road oases
Patagonian Desert	Argentina	670,000 km²	World's 8th largest; rain shadow of Andes; cold, windy; guanacos, rheas
Great Basin	Western USA	492,000 km²	Between Sierra Nevada and Rockies; cold winters; sagebrush; Great Salt Lake
Ladakh (Cold Desert)	India (J&K, Himachal)	~45,000 km²	Rain shadow of Himalayas and Karakoram; high altitude (3,500 m+); Buddhist monasteries

3. Coastal Deserts

Occur along western continental coasts in subtropical latitudes where cold ocean currents chill and stabilise the coastal air, preventing precipitation despite proximity to ocean.

• Namib Desert (Namibia-Angola-South Africa): Cooled by the Benguela Current (cold); world's oldest desert (~55 million years); home to Welwitschia mirabilis (plant that lives 2,000+ years)
• Atacama Desert (Chile-Peru): Cooled by the Humboldt Current (Peru Current); some weather stations have never recorded rainfall; driest place on Earth outside Antarctica

5.3 Australian Deserts — PYQ 2023 (2 marks — now 5-mark format in 2026)

Australia is the world's driest inhabited continent — ~44% of its land area is desert. It has six major recognised deserts, all located in the interior and western regions:

Desert	Area (km²)	Location	Key Feature
1. Great Victoria Desert	424,400	SW Australia (WA + SA)	Largest Australian desert; sand dunes, salt lakes, scrubland
2. Great Sandy Desert	267,250	NW Western Australia	Red sandy plains; very hot; Pintupi Aboriginal homeland
3. Tanami Desert	184,500	NT + WA border	Spinifex grassland; gold mining (Tanami Gold Mine)
4. Simpson Desert	176,500	NT + QLD + SA border	World's largest parallel sand dune system (1,100+ dunes); French Line track
5. Gibson Desert	156,000	Central Western Australia	Between Great Sandy and Great Victoria; named by Ernest Giles (1874)
6. Little Sandy Desert	111,500	WA (SW of Great Sandy)	Smaller, separated from Great Sandy by vegetation corridor

Memory aid for Australian deserts (area order): G-G-T-S-G-L (Great Victoria, Great Sandy, Tanami, Simpson, Gibson, Little Sandy)

Why Does Australia Have So Many Deserts?

• Australia straddles the subtropical high-pressure belt (20°–35°S)
• Low elevation (avg 330 m) — no mountain barrier to capture moisture from the eastern seaboard for the interior
• Surrounded by relatively cold ocean currents on west and south; moisture from east blocked by Great Dividing Range
• Very old, flat, dry continent — minimal volcanic or tectonic activity to disturb dry circulation patterns

5.4 Thar Desert — India's Desert (India Context for RPSC)

The Thar Desert occupies most of western Rajasthan (12 of 33 districts) and extends into Sindh, Punjab, and Haryana (Pakistan-India border region).

Physical Characteristics

• Area: ~200,000–320,000 km² (depending on definition; some include semi-arid regions)
• Rainfall: 100–250 mm annually; decreases westward toward Pakistan border (Jaisalmer avg 150 mm)
• Temperature: Extreme range — summer max 48–50°C; winter min below 0°C (frost common)
• Landforms: Sand dunes (aeolian — barchan, longitudinal, transverse); rocky plains (reg/hamada); salt flats (playas — e.g., Sambhar Lake)
• Wind: Strong SW monsoon winds create the great sand dunes; north winds in winter create dust storms

Unique Features

• Not geologically ancient — formed in the last 10,000–12,000 years (after Holocene climate shift)
• Saraswati River system once flowed through this region; climate aridification caused its drying
• Indira Gandhi Canal (world's longest irrigation canal, 650 km) has partially reclaimed desert lands in Bikaner and Jaisalmer
• Wildlife: Critically endangered Great Indian Bustard (Godawan); Blackbuck; Desert Fox; Desert Cat; Spiny-tailed Lizard (Saara hardwickii)

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