What Is a Desert Climate?
A desert is defined not by heat but by very low rainfall — less than 250 mm per year. Deserts are found on all continents and make up about 33% of Earth's land surface. They are the most extreme climates in terms of water shortage.
There are two main types of desert:
- Hot deserts: Very high daytime temperatures, cold nights, minimal rainfall.
- Cold deserts: Cold winters, warm/hot summers, also very dry.
Hot Deserts
Location: Around 30°N and 30°S latitude (the subtropics). This is where descending dry air creates high pressure and very little rainfall. Examples: Sahara (Africa), Arabian Desert, Sonoran Desert (Mexico/USA), Australian Outback.
Climate characteristics:
- Very hot days: Often above 40°C in summer, can exceed 50°C.
- Cold nights: Temperatures can drop below 0°C. Without moisture or cloud cover, the ground quickly loses its heat at night.
- Extreme temperature range: A daily range of 20–30°C is common.
- Low humidity: Very dry air — less than 250 mm rain per year, often much less.
The Sahara is the world's largest hot desert, covering about 9.2 million km² — roughly the size of the United States. It is not all sand dunes (erg) — only about 25% is sand. The rest is rocky plateau (hamada), gravel plains, and mountains. Contrary to popular belief, much of the Sahara is rocky, not sandy.
Why Are Hot Deserts Dry?
Hot deserts exist at about 30°N and 30°S because of global atmospheric circulation:
- Near the equator, warm air rises and loses moisture as heavy rain.
- This now-dry air spreads north and south at high altitude.
- Around 30° latitude, it descends, compressing and warming.
- This descending, dry air creates high pressure zones with very little rainfall.
Additional causes of deserts: rain shadows behind mountains; cold ocean currents cooling coastal air.
Cold Deserts
Examples: Gobi Desert (Central Asia), Atacama Desert (Chile — one of the driest places on Earth), Great Basin (USA).
- The Gobi is a cold desert with temperatures from −40°C in winter to +45°C in summer.
- The Atacama receives almost no rainfall (some areas have not recorded rainfall for decades) because it is in the rain shadow of the Andes AND adjacent to a cold ocean current.
The Atacama Desert in Chile is so dry that some areas receive 1–3 mm of rain per year — or even less. Paradoxically, fog often rolls in from the Pacific Ocean. Local communities use fog nets (mesh panels) to collect water droplets from the fog, which is enough to supply drinking water and irrigation.
Desert Ecosystems
Life in deserts has extraordinary adaptations:
- Camels: Store fat (not water) in their hump; can lose 25% of body weight in water without harm.
- Cacti: Shallow, wide roots to collect rain quickly; thick stems to store water; spines to reduce water loss and deter animals.
- Reptiles: Active at dawn and dusk (crepuscular) to avoid extreme midday heat.
Real world: Fog nets in the Atacama Desert
The Atacama Desert in Chile is one of the driest places on Earth — some areas receive almost no rainfall for decades. Yet fog regularly rolls in from the cold Pacific Ocean. Communities have installed large mesh fog nets (up to 4 metres tall) on hillsides that catch water droplets from the fog. A single net can collect 200–300 litres of water per day. The project has brought fresh water to otherwise inaccessible villages and is now being replicated in Morocco, Peru, and Eritrea.
Real world: Dubai — building a city in the desert
Dubai (UAE) sits in the Arabian Desert, receiving less than 100 mm of rain per year with summer temperatures exceeding 45°C. Despite this, it has become a global city of 3.5 million people through massive engineering — desalination plants convert seawater into drinking water (meeting 90% of needs), air conditioning is nearly universal, and irrigation supports golf courses and parks. Dubai's water and energy footprint per person is among the highest in the world, raising questions about desert urbanisation's sustainability.
Real world: Desertification in the Sahel region
The Sahel — the semi-arid zone just south of the Sahara spanning Senegal to Ethiopia — is experiencing desertification. Overgrazing, deforestation, and changing rainfall patterns are pushing the Sahara southward. Millions of people rely on subsistence farming in the Sahel, but declining rainfall and degraded soils are forcing migration to cities. The African Union's Great Green Wall project aims to plant an 8,000 km band of trees across the Sahel to halt desertification and restore 100 million hectares of land by 2030.
Where do you see this in real life? Look at the labels on food products in a supermarket — dates, oranges, and tomatoes often come from North Africa or the Middle East. These hot-desert countries use irrigation, greenhouses, and desalination to grow food in regions that would otherwise support almost no agriculture. Desert farming is an engineering achievement that depends on water management.
Practice Problems
Why can desert temperatures drop dramatically from day to night — often by 20–30°C?
Show answer
Explain why hot deserts tend to form at around 30°N and 30°S latitude, not at the equator.