Chapter 18 of G.C. Leong’s
Certificate Physical and Human Geographyprovides an in-depth analysis of The Hot Desert and Mid-Latitude Desert Climates. Below is a detailed, point-wise breakdown of the chapter:
1. Distribution of Desert Climates
Deserts cover about 1/5th of the Earth’s land surface.
- Hot Deserts: Located on the western margins of continents between 15° and 30° North and South latitudes. Major examples include the Sahara (the largest), Arabian, Thar, Atacama (driest), and Great Australian deserts.
- Mid-Latitude (Cold) Deserts: Found in the interiors of continents in temperate latitudes, often in rain-shadow areas or deep inland basins. Key examples are the Gobi and Turkestan deserts.
2. Causes of Aridity
Aridity is the primary characteristic of deserts, defined by rainfall of less than 10 inches (25 cm) annually.
- Offshore Trade Winds: In hot deserts, trade winds blow from land to sea, carrying little moisture.
- High-Pressure Belts: Located in the Horse Latitudes (Subtropical High Pressure Belt), where descending air inhibits cloud formation and precipitation.
- Cold Currents: Deserts on western coasts (e.g., Atacama, Namib) are influenced by cold ocean currents (like the Humboldt or Benguela). These currents cool the air, causing fog but preventing rain.
- Rain-Shadow Effect: Mid-latitude deserts are often cut off from rain-bearing winds by high mountain ranges (e.g., the Patagonian Desert behind the Andes).
3. Climatic Characteristics
- Extreme Temperature: Hot deserts have no cold season, with average summer temperatures around 30°C to 35°C. The highest recorded temperature was 58°C (136°F) at Al-Azizia, Libya.
- Diurnal Range: Deserts have an extremely high diurnal (daily) temperature range—often 14°C to 25°C—due to rapid heating under clear skies by day and rapid cooling by radiation at night.
- Humidity & Evaporation: Low relative humidity (often below 30%) and high evaporation rates further deplete moisture.
4. Desert Vegetation (Xerophytes)
Plants are adapted to survive intense drought and salinity:
- Water Conservation: Waxy, leathery, or needle-shaped leaves to reduce transpiration.
- Storage: Succulent stems (like cacti) store water for long periods.
- Root Systems: Long taproots to reach deep groundwater or wide-spreading surface roots to catch rare rainfall.
- Dormancy: Seeds have tough skins and can remain dormant for years until rain occurs.
5. Human Adaptations and Economy
- Nomadic Herders: Groups like the Bedouin of Arabia and Tuaregs of the Sahara migrate with sheep, goats, and camels for pasture. The Bushmen of the Kalahari are nomadic hunters.
- Settled Agriculture: Concentrated around Oases where groundwater is available, or along permanent rivers like the Nile or Indus. Date palms are the most common commercial crop.
- Mining: Deserts are often rich in minerals. Examples include copper in the Atacama, gold in Australia, and oil/gas in the Middle East and Sahara.
6. Types of Desert Landscapes
- Erg (Sandy Desert): Vast “seas of sand” with undulating dunes.
- Reg (Stony Desert): Extensive sheets of angular pebbles and gravels left behind after the wind blows away finer sand.
- Hamada (Rocky Desert): Large stretches of bare, wind-swept rock.
revise again
Deserts are regions of extreme aridity. They are generally defined as areas receiving less than 25 cm (10 inches) of annual rainfall.
1. Types of Deserts
- Hot Deserts (Trade Wind Deserts): Located on the western coasts of continents between
15∘ and
30∘ North and South (e.g., Sahara, Arabian, Thar, Atacama, Namib, Kalahari, Great Australian).
- Mid-Latitude (Cold) Deserts: Located in the deep interiors of continents, far from the sea (e.g., Gobi, Turkestan, Patagonian).
2. Why are Deserts Dry? (The 3 Main Reasons)
- Off-shore Trade Winds: In the
15∘–
30∘ belt, winds blow from land to sea, carrying no moisture.
- Rain Shadow Effect: Mountains block moisture-laden winds (e.g., the Andes block moisture for the Atacama).
- Cold Ocean Currents: Cold currents (like the Peru or Benguela) chill the air, causing it to hold its moisture as fog over the sea rather than rain over the land.
3. Climate: Extremes of Everything
- Temperature: Hot deserts have the highest temperatures on Earth (
50∘C+ in summer). However, they have a high Diurnal Range (meaning it’s scorching by day but can drop to freezing at night because there are no clouds to trap the heat).
- Rainfall: Rare and unreliable. When it does rain, it often comes as a violent thunderstorm causing flash floods.
4. Vegetation: Xerophytic (Drought-loving)
Desert plants must survive long periods without water. They are called Xerophytes.
- Water Storage: Thick, succulent stems (e.g., Cactus) store water.
- Small Leaves: Leaves are replaced by spines or have waxy coatings to prevent water loss (transpiration).
- Long Roots: Roots go very deep (to reach groundwater) or spread very wide (to catch surface dew).
5. Life and Economy
- Nomadic Herdsmen: Bedouins (Arabia) and Tuaregs (Sahara) move with camels/goats.
- Oasis Cultivation: Where water reaches the surface, Date Palms and cereals are grown.
- Mineral Wealth: Deserts are rich in minerals:
- Atacama: Nitrates and Copper.
- Sahara/Middle East: Oil and Gold.
- Australia: Gold.
10 Revision Questions with Mnemonics
Q1: What is the defining feature of a desert?
- A: Less than 25 cm of annual rainfall.
- Mnemonic: “Desert = Dry” (Under 25).
Q2: Why is the Atacama Desert so dry?
- A: It is in the Rain Shadow of the Andes and affected by the Cold Peru Current.
- Mnemonic: “Atacama is Arid Always.”
Q3: What are “Xerophytes”?
- A: Plants adapted to survive in dry conditions.
- Mnemonic: “Xerophyte = Xtra dry plant.”
Q4: Why are desert nights so cold?
- A: Lack of cloud cover allows heat to escape rapidly into space.
- Mnemonic: “No Clouds = No Coat” (for the Earth).
Q5: What is the most important tree of the desert oasis?
- A: The Date Palm.
- Mnemonic: “Date in the Desert.”
Q6: Name a Mid-Latitude (Cold) Desert.
- A: The Gobi Desert (Mongolia/China).
- Mnemonic: “Gobi is Generally Glacially cold” (in winter).
Q7: Which cold current affects the Namib Desert?
- A: The Benguela Current.
- Mnemonic: “Benguela Brings Brrr” (to the Namib).
Q8: What is “Diurnal Range”?
- A: The difference between daily maximum and minimum temperatures.
- Mnemonic: “Diurnal = Daily change.”
Q9: Who are the Bedouins?
- A: Nomadic tribes of the Arabian Deserts.
- Mnemonic: “Bedouins = Beasts of the Burning sand.”
Q10: What is a “Wadi”?
- A: A dry riverbed that fills only during rare flash floods.
- Mnemonic: “Wadi = Waiting for Water.”
Quick Summary Table: Desert Survival
| Feature | Adaptation | Mnemonic |
|---|---|---|
| Cactus | Succulent Stem | Stem = Storage |
| Camels | Humps (fat) & Padded Feet | Camel = Carrier |
| Date Palm | Deep Roots | Palm = Pump |
