Key Terms

📖 Key Terms

Arid — A climate or environment receiving less than 250mm of rainfall per year, where evaporation significantly exceeds precipitation. Hot deserts (such as the Sahara and Thar) are classified as arid; semi-arid zones (like the Sahel) receive 250–500mm and are at high desertification risk.

Diurnal temperature range — The difference between the maximum and minimum temperature recorded in a single 24-hour period. Hot deserts have extreme diurnal ranges (often 40–50°C) because clear, cloudless skies allow rapid solar heating during the day and rapid heat loss by radiation at night.

Hadley Cell — A pattern of atmospheric circulation in which hot air at the equator rises, loses moisture (causing tropical rainfall), travels poleward at high altitude, and then descends at roughly 20°–30° latitude as dry, warming air — creating the world's hot desert belts.

Xerophyte — A plant adapted to survive in conditions of extreme water scarcity. Xerophytic adaptations include reduced leaf surface area, thick waxy cuticles, CAM photosynthesis (stomata open at night only), water storage in stems or leaves, and deep root systems. Examples: cacti, acacia, aloe vera.

Xerocole — An animal adapted to life in a desert environment. Xerocole adaptations include nocturnal behaviour, concentrated urine production, fat storage (as an energy and metabolic water reserve), and physical features for heat dissipation (e.g., large ears in the fennec fox). Examples: dromedary camel, fennec fox, banded gecko.

CAM Photosynthesis — Crassulacean Acid Metabolism: a specialised form of photosynthesis used by cacti and succulents in which stomata open only at night to absorb CO₂ (storing it as malic acid), and remain closed during the day when temperatures and evaporation rates are highest. This dramatically reduces water loss compared to conventional C3 photosynthesis.

Ephemeral plant — A plant that completes its entire life cycle (germination → growth → flowering → seed dispersal) very rapidly after rainfall, then dies, leaving its seeds dormant in the soil for months or years. Ephemerals "avoid" drought rather than tolerate it. The "super-blooms" of the Atacama and Mojave are caused by mass simultaneous germination of ephemerals after rare rain.

Desertification — The process by which fertile or semi-fertile land on the margins of deserts becomes permanently degraded through a combination of human pressures (overgrazing, over-cultivation, deforestation) and climate change (reduced and less reliable rainfall). Critically, desertification is NOT simply the natural spread of desert — it is a human-influenced degradation process that can theoretically be stopped and reversed.

Salinisation — The build-up of salt in irrigated soils. Occurs when irrigation water evaporates, leaving its dissolved salt content behind. Over time, salt concentrations rise to levels toxic to most crops. A major problem in irrigated zones of the Thar Desert (around the Indira Gandhi Canal) and in other heavily irrigated arid regions including the Murray-Darling basin in Australia.

Pastoralism — A livelihood based on herding livestock (cattle, goats, camels, sheep) across seasonal grazing routes. Traditional pastoralists in the Thar Desert move herds between summer and winter pastures following water availability. Pastoralism is sustainable at traditional densities but becomes a driver of desertification when herd sizes exceed land carrying capacity.

Subsistence farming — Growing food primarily to feed the farmer's own family, with little or no surplus for sale. Most farming in the Thar Desert is subsistence-based, making communities highly vulnerable to crop failure from drought, flood, or soil degradation.