Desert climates are defined by extreme dryness, intense heat during the day, and surprisingly cold temperatures at night. The defining feature is precipitation: deserts receive so little rain that the amount of water that could evaporate from their surface far exceeds what actually falls from the sky. In Phoenix, Arizona, for example, the land could theoretically lose six times more water to evaporation than it receives in rainfall each year.
How Deserts Are Classified
A desert isn’t simply “a place that doesn’t rain much.” Climate scientists use a formula that weighs annual rainfall against temperature and seasonal patterns. If a region’s precipitation falls below a calculated threshold, it qualifies as arid (true desert) or semi-arid (steppe). The key ratio compares actual precipitation to potential evapotranspiration, which is the amount of water the atmosphere could pull from the ground if water were unlimited. When that ratio drops below 0.4, the climate is classified as arid.
This means a cold region with very low rainfall can still be a desert, because cold air holds less moisture and drives less evaporation. Antarctica qualifies as a desert under this system. So does the Gobi in central Asia, where winter temperatures plunge well below freezing. The popular image of endless sand dunes represents only one type of desert climate.
Why Deserts Form Where They Do
Most of the world’s large hot deserts sit near 30 degrees latitude, north and south of the equator. That’s no coincidence. The atmosphere operates like a giant conveyor belt: warm, moist air rises near the equator, drops its moisture as tropical rain, then flows outward at high altitude. By the time it reaches roughly 30 degrees latitude, the air has dried out and begins sinking. Sinking air compresses and warms, which prevents clouds from forming. The result is a persistent high-pressure zone that suppresses rainfall across wide bands of the globe. The Sahara, Arabian, Sonoran, and Kalahari deserts all sit beneath this belt.
Mountains create deserts too. When moist air hits a mountain range, it rises, cools, and dumps rain or snow on the windward side. By the time that air crosses the peaks and descends on the other side, it carries very little moisture. Joshua Tree National Park in California is a textbook example: coastal storms collide with mountains exceeding 10,000 feet, lose most of their water, and leave the land beyond in a permanent dry zone. This is called the rain shadow effect, and it explains why deserts can form at almost any latitude if the right mountain barrier exists.
Extreme Temperature Swings
The most striking feature of desert climate is the gap between daytime and nighttime temperatures. During the day, desert air averages around 38°C (just over 100°F), and extreme highs can reach 43 to 49°C (roughly 110 to 120°F). At night, temperatures can plummet to around -4°C (about 25°F). That’s a swing of 40°C or more in a single day.
The reason is moisture, or rather the lack of it. Water vapor in the atmosphere acts like a blanket, trapping heat near the surface after the sun goes down. Humid places cool slowly at night because that blanket holds warmth in. Desert air is too dry to retain heat, so the ground radiates energy back into space almost as fast as it absorbed it during the day. Desert surfaces receive roughly twice the solar radiation of humid regions and lose nearly twice as much heat overnight. If you’ve ever shivered in a desert after a scorching afternoon, that’s the mechanism at work.
Humidity and Dryness
Desert relative humidity typically hovers around 20 to 25% during the hottest part of the afternoon. At a place where daytime temperatures reach 30°C, that means only about a fifth of the air’s moisture-holding capacity is actually filled with water vapor. This is what makes desert heat feel different from tropical heat: your sweat evaporates almost instantly, which can mask how much water your body is losing.
Overnight, the story changes. As air cools, its capacity to hold moisture shrinks, so relative humidity climbs even though no new water has entered the atmosphere. In some deserts, early morning humidity can briefly reach 100%, producing dew on rocks and plants. This overnight moisture is a critical water source for many desert organisms, but it evaporates quickly once the sun rises.
Solar Radiation and UV Exposure
Clear skies and dry air mean deserts receive intense solar radiation. The UV index on a typical clear day in the Arabian Peninsula reaches 10 to 11, which is considered “extreme” on the international scale. Only major dust storms reduce this significantly, temporarily dropping UV levels to around 6 or 7, comparable to a heavily overcast day in a humid climate. Lower humidity amplifies UV exposure because water vapor in the atmosphere absorbs some ultraviolet radiation before it reaches the ground. In summer, when humidity drops further and the sun is more directly overhead, UV and heat exposure peak together.
Hot Deserts vs. Cold Deserts
Hot deserts like the Sahara and Sonoran have mean annual temperatures between 20 and 25°C, with summer averages of 21 to 27°C and occasional spikes well above 40°C. Their defining quality is heat combined with aridity. Rain, when it comes, often arrives in brief, violent bursts that the baked ground can barely absorb.
Cold deserts, like the Gobi, Patagonian, and Antarctic deserts, share the same fundamental dryness but experience long, frigid winters. The Gobi regularly drops below -30°C in January. These deserts form either because of rain shadow effects, distance from ocean moisture sources, or (in Antarctica’s case) extreme cold that locks moisture into ice rather than cycling it through rainfall. Cold deserts typically receive their limited precipitation as snow rather than rain, and their vegetation, where it exists, looks nothing like the cacti and scrub of hot deserts.
Wind and Dust Storms
Wind is a constant force in desert climates. Without vegetation or moisture to hold soil in place, dry ground is easily lifted into the air. The most dramatic result is the haboob: a massive wall of dust and sand driven by the downdraft of a thunderstorm. Haboobs are common across the Sahara, the Arabian Peninsula, and parts of the American Southwest. Wind speeds can reach 60 mph, and the dust wall can tower as high as 10,000 feet.
These storms reshape the landscape, stripping soil from one area and depositing it hundreds of miles away. Saharan dust regularly crosses the Atlantic Ocean. Beyond the dramatic storms, steady desert winds drive ongoing erosion that carves rock formations, builds and moves sand dunes, and keeps fine particles suspended in the air for days. This persistent dustiness further affects how sunlight and heat interact with the desert surface.
Deserts Are Getting Drier
Global temperature increases are pushing desert boundaries outward. Observations over the past 60 years show that drylands have expanded, and atmospheric aridity has been increasing in recent decades. Climate projections under all warming scenarios show this trend continuing: the atmosphere over existing drylands will become even more arid. While large-scale desertification of currently green land remains limited in scope (one recent study in Communications Earth & Environment estimated less than 4% of dryland areas would fully desertify), the edges of deserts are creeping into regions that were previously semi-arid grasslands. For communities living on those margins, the practical effects of shifting rainfall patterns and rising evaporation rates are already measurable.