A desert can be defined as an arid biome (hot or cold) with low average rainfalls below 250mm. This definition is not enough to comprehend the entire meaning of a desert as several other characteristics are at play to determine a desert. Note that deserts are defined in many ways e.g. as sandy or low humidity or scarce vegetation, but low rainfall is included in most instances. Whatever you may define deserts, the low rainfall (below 250 mm) part is good to include.
Deserts can be arid or semi-arid (steppe). The latter is partially a desert, but found on margins of true deserts where humidity & precipitation is relatively high. Semi-arid regions are transitional between arid & more humid areas.
Hot Deserts (arid & semi-arid) cover about a third of the world's land area. Examples of deserts include Sahara (Africa),the largest, Great Basin (USA), Pentagonia (S. America), Atacama (S.America) and many others.
Commonly deserts exhibit the following characteristics:
Scarce water & Low RainfallsRainfall in arid regions can be as low as 100 mm on average. Water bodies are usually dry for most of the year. Evaporation exceeds water supply in deserts which makes survivability impossible. Outbursts of rainfall do exists in deserts for very short periods of time. These rainfalls can lead to 'flash floods'. The low rainfalls in deserts give rise to many other characteristics
Complete or partial dryness
Most desert are dry to support many lifeforms.
Few fauna & Flora
Deserts are broadly characterised as lifeless but life do exist in some few selected area. The toughest lifeforms survive by adapting to the arid region's characteristics. For example, the cactus plant stores water in its body, has thorny leaves to reduce water loss & long roots to reach a far away water table. Most animals in arid regions have also found a way to adapt to the scorching sun of deserts. Many animals have the ability to cool themselves. Chameleons change to reflective colours to reflect sunlight. Camels convert fat stored on their backs (bump) to water, they can go for months without water.
Deserts have very low humidities despite the high evaporation rates. This is mainly attributed to water scarcity for most of the time which reduces evaporation rate & lowers humidity.
Plenty of Sunlight (Hot)
Sunlight is abundant in most deserts except for cold ones. In hot deserts, temperatures can reach or exceeds 40oC. When people talk about deserts the major assumption is that they are always hot. This is not true as deserts can be arid with cold temperatures e.g. Antarctica and Gobi (East Aisa). Temperatures are below 0oC for most of the times in these deserts. Temperatures (hot or cold) in deserts can fall dramatically (below 0) at night due to rapid radiation attributed to clear skies with less cloud cover.
Sandy, Stony & Rocky
Most hot deserts are covered with sand except for cold deserts which are usually ice covered e.g. Antarctica. These sandy or stony deserts are called 'reg'. Depending on location these deserts can be called gibber in Australia or 'sai' in Central Asia. Rocky deserts with steep escarpments & bare protrusions of hard rock are called 'hamada' e.g Wadi Rum, Jordan & Monument Valley, Arizona (USA). Deserts covered largely by sand dunes are called 'ergs' e.g. Sossusvlei, Namibia & Gobi deserts. Many hot deserts contain significant portions of erg (dunes), reg (stony) or hamada (rocky).
Excess runoff during flash floods
The bare, baked & ice surfaces of arid regions impedes infiltration, therefore water excessively flows as overland flow. In addition, lack of vegetation cover greatly impedes infiltration and increases overland flow as few plants are available to capture the running water.