Desert Climate: Hot, Dry, Extreme, and Expanding

A **desert climate (Köppen BWh / BWk)** is defined by **aridity**—precipitation so low that evaporation exceeds it, creating persistent water deficit. It covers ~1/3 of Earth's land surface.

Core Characteristics

• Precipitation: **< 250 mm/yr (10 in)**; often < 50 mm. Highly unpredictable—years may pass between rains.
• Temperature Extremes:
  • **Hot Deserts (BWh):** Sahara, Arabian, Sonoran, Lut (Iran), Australian Outback. **Daily max 40–50°C (104–122°F)**; surface temps 70°C+. **Record:** 56.7°C (134°F), Death Valley, 1913.
  • **Cold Deserts (BWk):** Gobi, Great Basin, Patagonia, Central Asia. **Hot summers, freezing winters** (-20 to -40°C).
• Diurnal Swing: **20–30°C (36–54°F)** day-night difference (dry air, no cloud cover, low heat capacity of sand/rock).
• Humidity: **< 20%** (often < 5%). Rapid dehydration.
• Wind & Aeolian Processes: Sand/dust storms (haboobs), dune migration, ventifacts, yardangs.
• Soils: Low organic matter, high salinity/alkalinity, calcrete, gypsum crusts.
• Vegetation: Xerophytes—succulents (cacti, euphorbias), deep-rooted shrubs (creosote, sagebrush), ephemerals (bloom post-rain). Sparse cover.
• Fauna: Nocturnal, burrowing, water-metabolic (kangaroo rat, fennec fox, reptiles).

Types & Distribution

• Subtropical (Hadley Cell descent): Sahara, Arabian, Australian, Kalahari, Atacama, Sonoran, Mojave.
• Rain Shadow (Orographic): Patagonia (Andes), Great Basin (Sierra Nevada), Gobi (Himalayas).
• Coastal (Cold Current): Atacama (Humboldt), Namib (Benguela), Western Sahara (Canary).
• Polar Deserts: Antarctica, High Arctic (precip < 250 mm, but frozen).

Climate Change Impact

• Expansion: Deserts advancing poleward (Sahel, Mediterranean, SW US).
• Intensification: Hotter, drier, more extreme events.
• Dust Fertilization: Saharan dust feeds Amazon & Atlantic phytoplankton—changing.

Deserts are not "empty"—they are **precisely calibrated ecosystems** now under unprecedented stress.