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A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development
Morphogenesis , is the biological process that causes an organism to develop its shape...

 of plant and animal life. As a result, environments subject to arid climates tend to lack vegetation and are called xeric
Deserts and xeric shrublands
Deserts and xeric shrublands is a biome characterized by, relating to, or requiring only a small amount of moisture.-Definition and occurrence:...

 or desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...



The expression 'available water' refers to water in the soil in excess to the wilting point
Permanent wilting point
Permanent wilting point or wilting point is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours...

. The air over a hot desert may actually contain substantial amounts of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 but that water may not be generally accessible to plants, except for very specialized organisms (such as some species of lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

). 'Lack of water' refers to use by plants. The water that is actually present in the environment may be sufficient for some species or usages (such as climax vegetation
Climax vegetation
Climax vegetation is the vegetation which establishes itself on a given site for given climatic conditions in the absence of anthropic action after a long time ....

), and grossly insufficient for others. Aridity, the characteristic nature of an arid climate, may thus depend on the use of the land
Land use
Land use is the human use of land. Land use involves the management and modification of natural environment or wilderness into built environment such as fields, pastures, and settlements. It has also been defined as "the arrangements, activities and inputs people undertake in a certain land cover...

. Regards to the presence of life, what is more important than the degree of rainfall is the fraction of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 that is not quickly lost through evaporation or runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

. Attempts to quantitatively describe the degree of aridity of a place has often led to the development of aridity index
Aridity index
An aridity index is a numerical indicator of the degree of dryness of the climate at a given location. A number of aridity indices have been proposed ; these indicators serve to identify, locate or delimit regions that suffer from a deficit of available water, a condition that can severely affect...

es. There is no universal agreement on the precise boundaries between classes such as 'hyper-arid', 'arid', 'semi-arid
A semi-arid climate or steppe climate describes climatic regions that receive precipitation below potential evapotranspiration, but not extremely...

', etc.

Geographical distribution

Although different classification schemes and maps differ in their details, there is a general agreement about the fact that large areas of the Earth are arid. These include the hot deserts located broadly in sub-tropical regions, where the accumulation of water is largely prevented by either low precipitations, or high evaporation, or both, as well as cold deserts near the poles, where water may be permanently locked in solid forms (snow and ice). Other arid regions include areas located in the rain shadow
Rain shadow
A rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them. As shown by the diagram to the right, the warm moist air is "pulled" by the prevailing winds over a mountain...

s of major mountain ranges or along coastal regions affected by significant upwelling (such as the Atacama Desert
Atacama Desert
The Atacama Desert is a plateau in South America, covering a strip of land on the Pacific coast, west of the Andes mountains. It is, according to NASA, National Geographic and many other publications, the driest desert in the world...


Change over time

The distribution of aridity observed at any one point in time is largely the result of the general circulation of the atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. The latter does change significantly over time through climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

. In addition, changes in land use can result in greater demands on soil water and induce a higher degree of aridity. See aridification
Aridification is the process of a region becoming increasingly dry. It refers to long term change rather than seasonal variation.It is often measured as the reduction of average soil moisture content....


See also

  • Desert climate
    Desert climate
    A desert climate , also known as an arid climate, is a climate that does not meet the criteria to be classified as a polar climate, and in which precipitation is too low to sustain any vegetation at all, or at most a very scanty scrub.An area that features this climate usually experiences less than...

  • Desiccation tolerance
    Desiccation tolerance
    Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or drought-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or...

  • Drought
    A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

  • Relative humidity
    Relative humidity
    Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

  • Saturation vapor pressure