Group B: dry climates and coastal deserts

Detail

1. What makes a climate "dry" (Group B)

• Dry climate (B): a place where potential evaporation is greater than precipitation.
• Potential evaporation is the amount of water that the sun and wind could remove if there were unlimited water.
• Precipitation is all the water that falls from the sky (rain, snow, hail).

• If more water can leave than arrives, the land stays short of water all year. That is Group B.

• Koeppen sets Group B apart by moisture, not temperature. Groups A, C, D and E are set apart by temperature.

• Extent: Group B covers the largest area of any group, about a quarter to a third of the world's land. The exact share depends on the dataset and the map used.
• Latitude belt: it lies between 15° and 60° in both hemispheres.

2. Three settings where dry climates form

Setting	Latitude	Main cause	Examples
Under the subtropical highs	15°–30°	Sinking air and temperature inversion stop clouds from forming	Sahara, Arabia, Thar, Kalahari, Great Australian
West coasts beside cold currents	Can reach close to the equator	Cold water chills the air from below, so the air stays stable	Atacama (Peru–Chile), Namib
Mid-latitude interiors	35°–60°	Far from the sea and often ringed by mountains, so moist sea winds cannot reach	Gobi, Taklamakan, Great Basin, Patagonia

a) Under the subtropical highs (15°–30°)

• Subtropical high: a belt of high air pressure near 30° N and 30° S.
• Air that rose at the equator flows towards the poles high up. It then sinks over these belts.

• Sinking air is squeezed and warms up, so it can hold more water vapour. Clouds do not form.

• Temperature inversion: air near the ground is colder than the air above it, which is the reverse of the usual pattern.

• The warm layer above acts like a lid. Air near the ground cannot rise through it.
• Without rising air there is no cooling, no condensation and no rain.

b) On west coasts beside cold currents

• Cold ocean currents flow towards the equator along the western sides of continents.
• Because of them, the dry belt reaches closer to the equator and right down to the coast. The clearest case is Peru–Chile, where desert reaches about 5° S.

c) In mid-latitude interiors (35°–60°)

• These places are deep inside continents and cut off from moist sea winds, often by surrounding mountains.
• The winds that do arrive have already lost most of their moisture on the way.

3. Subdivisions: BS vs BW, h vs k

Letter	Meaning	Key point
BS	Steppe (semi-arid)	Enough rain for short grass
BW	Desert (arid)	Too dry for continuous grass. "W" comes from German Wüste (desert)
h	Hot (heiss)	Subtropical, 15°–35°
k	Cold (kalt)	Mid-latitude, 35°–60°

• Combining them gives four types: BWh, BSh, BWk and BSk.
• Steppe is a transition belt. It lies between the desert and the humid climates (A, C or D) around it.

4. Subtropical desert (BWh) and subtropical steppe (BSh)

Rainfall

• Rain is scanty and highly variable from year to year.
• In deserts it comes as short, intense thundershowers.
• The ground is hard and dry, so much of the water runs off at once in flash floods.
• The rest evaporates quickly in the heat.
• As a result, the rain does not build up soil moisture.

BSh on the desert margins

• BSh lies on the edges of the hot deserts. It gets enough rain for sparse grassland.
• Rain variability hurts the steppe more than the desert.
• People in the steppe grow crops and graze animals that depend on the rain.
• A run of dry years causes crops to fail and animals to die. This leads to famine.
• Example: the Sahel (the semi-arid strip south of the Sahara, from Senegal to Sudan) suffered the 1967–77 drought (see climate-change).

Examples of BWh (BSh lies on their margins)

Desert	Location
Sahara	North Africa, the largest hot desert
Arabian	Arabian Peninsula
Thar (Great Indian Desert)	Western Rajasthan (India) and Punjab and Sindh (Pakistan), about 200,000 sq km [6]
Kalahari	Southern Africa (Botswana, Namibia, South Africa)
Great Australian	Interior and west of Australia

Temperature

• Annual range is large: summers are very hot and winters are mild to cool.
• Daily (diurnal) range is large.
• Clear skies and dry air let the sun heat the ground strongly by day.

• At night the heat escapes quickly into space, because there are no clouds to hold it in.

• Very high summer maxima.

• The world record is 56.7°C at Furnace Creek, Death Valley, California, USA, on 10 July 1913 [2].
• The WMO (World Meteorological Organization) certified Death Valley as the world record on 12 September 2012 [3].
• The 58°C at Al Aziziyah (El Azizia), Libya, on 13 September 1922 was invalidated by the WMO in 2012 (NCERT: 58°C at Al Aziziyah, Libya, 13 September 1922, the world's highest).
• An international expert committee found a recording error. The observer was new and inexperienced and probably misread the instrument. The reading was taken over a paved surface that reflected heat [4][3].

India and land degradation

• About 25% of India's geographical area is affected by desertification, and about 32% by land degradation (UNCCD 4th National Report, 2010) [7].
• Desertification means land in dry areas losing its fertility because of human misuse and climate.

5. Coastal deserts (cold-current deserts)

• Coastal desert: a desert that runs right along a sea coast, beside a cold ocean current. Fog is common in these deserts.

Cause, process and result

• Cause: a cold current flows towards the equator along a west coast.
• Process:
• The cold water chills the air from below.
• Cold, heavy air near the ground cannot rise. This creates an inversion, and the air stays stable.

• Stable air forms no tall rain clouds. Only a low layer of fog forms over the cold sea and drifts onto land [5].

• Result: the coast is rainless but foggy. The air along the Atacama coast is unusually humid but rarely becomes saturated (fully loaded with water vapour). At most it forms coastal fog, called garúa or camanchaca [8].

Cold current and coastal desert pairs

Coastal desert	Country or region	Cold current	Ocean
Atacama	Northern Chile and southern Peru	Humboldt (Peru)	South Pacific
Namib	Namibia and southern Angola	Benguela	South Atlantic
Coastal Sahara	Morocco, Western Sahara and Mauritania	Canary	North Atlantic
Baja California / Sonoran coast	Mexico	California	North Pacific
Western Australian coast	Western Australia	West Australian	Indian Ocean

• See ocean-movements.

Atacama

• The South Pacific subtropical high creates a subsidence inversion (the lid formed by sinking air), and the cold Humboldt Current makes it stronger. Together they block moist Pacific air and stop clouds from forming [8].
• Fog is therefore the main water source. Fog-catching nets (camanchaca nets) in Chile collect drinking water from it.

Namib

• The Namib coast is one of the foggiest places on Earth. Fog forms where cold Benguela water meets air sinking from the Hadley cell (the loop of air that rises at the equator and sinks near 30°) [9].
• The Namib and the Atacama are called "fog deserts", because fog is their main source of water [9].
• Fog-basking beetles: Namib darkling beetles climb to dune crests and tilt their bodies into the foggy wind. Droplets collect on their backs and run down to their mouths [9].

6. Mid-latitude steppe (BSk) and mid-latitude desert (BWk)

• Both lie in continental interiors at 35°–60°, far from the sea.
• Continentality (distance from the sea's moderating effect) gives them hot summers and severely cold winters.

Mid-latitude steppe (BSk)

• Semi-arid climate with short grass.
• Examples:
• Eurasian steppes, from Ukraine through Kazakhstan to Mongolia

• Great Plains of the USA and Canada

• Steppe and prairie farming is covered in world-physical-features.

Mid-latitude desert (BWk)

• Arid climate. It forms in interiors and in rain shadows.
• Rain shadow: the dry side of a mountain range, facing away from the moist wind.
• Moist wind is pushed up the windward side of the range. The rising air cools and drops its rain there.
• The air then comes down the other side. Sinking air warms and dries, so little rain falls on the leeward side.

BWk desert	Location	Main reason it is dry
Gobi	Mongolia and China	Deep interior, shut off by the Himalaya–Tibet block
Taklamakan	Tarim Basin, Xinjiang (China)	Basin surrounded by the Tien Shan, Pamirs and Kunlun
Patagonia	Southern Argentina	Rain shadow of the Andes (westerlies drop their rain in Chile)
Great Basin	Western USA (Nevada and Utah)	Rain shadow of the Sierra Nevada and Cascades
Ladakh	India (Union Territory)	Cold desert in the rain shadow of the Great Himalaya

Prelims Hooks

• Group B is defined by moisture (potential evaporation greater than precipitation), not temperature. It is the largest group by land area.
• BS = steppe (semi-arid); BW = desert (arid); h = 15°–35° (hot); k = 35°–60° (cold).
• World's highest temperature: 56.7°C at Furnace Creek, Death Valley, USA (10 July 1913). Al Aziziyah, Libya (58°C, 1922) was invalidated by the WMO in 2012.
• Cold current and desert pairs: Humboldt–Atacama; Benguela–Namib; Canary–coastal Sahara; California–Baja California; West Australian–Western Australian coast.
• Camanchaca = coastal fog of the Atacama, Chile, and the nets that harvest it. Fog-basking darkling beetles = Namib.
• Patagonia is a desert on an east coast. It owes its dryness to the Andes rain shadow, not to a cold current. This is a common trap.
• Ladakh = BWk (cold desert). Thar = BWh (hot desert).
• The Sahel is a semi-arid (BSh) belt. Its 1967–77 drought shows that rain variability hurts steppe more than desert.
• Cold-current deserts are foggy but rainless, because cold air near the surface makes the air stable (an inversion).

Mains Points

• Three causes of aridity:
• Planetary: sinking air under the subtropical highs
• Oceanic: cold currents on west coasts
• Continental and relief: interiors far from the sea and rain shadows

• This explains why deserts sit on the western sides of continents between 15° and 30°, and in mid-latitude interiors.

• Steppe vulnerability (GS-I/III):

• Rain variability causes harvest failure, overgrazing and famine (Sahel, 1967–77).
• This links to desertification. About 25% of India's area is affected (2010) [7].

• Responses include dryland farming, watershed management and shelterbelts (rows of trees planted to block wind).

• Fog harvesting as adaptation (GS-III):

• Camanchaca nets in Chile and beetle-inspired water-collecting surfaces show how coastal deserts can get water without rain.

• This is relevant to water security in dry regions.

• India's dry zones:

• The Thar (BWh) and Ladakh (BWk) need different strategies. The Thar needs canal irrigation (Indira Gandhi Canal) and drought management. Ladakh needs glacier-fed water and ice stupas.