Deserts
Defined by lack of water, not by heat.
A desert is defined by aridity, not temperature: less than about 250 mm of precipitation a year, with evaporation exceeding it. Antarctica qualifies and is the largest desert on Earth, which surprises people who picture sand.
Four ways a desert forms
- Subtropical — sinking air at about 30° latitude warms and dries. The Sahara and Arabian deserts.
- Rain shadow — mountains force air up, it rains on the windward side, and descends dry. The Atacama and Great Basin.
- Coastal — cold ocean currents chill the air, reducing its capacity to hold moisture. The Namib.
- Polar — air too cold to hold much water vapour at all. Antarctica.
Surviving without water
Plants use three broad strategies: succulence, storing water in tissue; drought tolerance, surviving desiccation and reviving; and drought escape, completing a whole life cycle in the days after rain. Many use CAM photosynthesis, opening stomata only at night to cut water loss.
Animals mostly avoid the problem behaviourally — nocturnal, burrowing, active at dawn and dusk. The kangaroo rat goes further and never drinks, extracting metabolic water from dry seeds and producing extremely concentrated urine.