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A region is said to be arid when it is characterized by a severe lack of available water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
, to the extent of hindering or even preventing the growth and development
Morphogenesis

Morphogenesis , is the physical process that gives rise to the shape of an organism. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation....
 of plant
Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
 and animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
 life. As a result, environments subject to arid climates tend to lack vegetation and are called xeric
Deserts and xeric shrublands

Desert and xeric shrublands is a biome characterized by, relating to, or requiring only a small amount of moisture. Deserts and xeric shrublands receive an annual average rainfall of ten inches or less, and have an arid or hyperarid climate, characterized by a strong moisture deficit, where annual potential loss of moisture from evapotransp...
 or desertic
Désert

?D?sert? is ?milie Simon's debut single, released in October 2002. The song was a huge success both critically and commercially in her homeland....
.

Concepts
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 . Water vapor is one Phase of the water cycle within the hydrosphere....
 but that water may not be generally accessible to plants, except for very specialized organisms (such as some species of lichen
Lichen

Lichens are composite organisms consisting of a symbiosis association of a fungus with a Photosynthesis partner , usually either a green algae or Cyanobacteria ....
).






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A region is said to be arid when it is characterized by a severe lack of available water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
, to the extent of hindering or even preventing the growth and development
Morphogenesis

Morphogenesis , is the physical process that gives rise to the shape of an organism. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation....
 of plant
Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
 and animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
 life. As a result, environments subject to arid climates tend to lack vegetation and are called xeric
Deserts and xeric shrublands

Desert and xeric shrublands is a biome characterized by, relating to, or requiring only a small amount of moisture. Deserts and xeric shrublands receive an annual average rainfall of ten inches or less, and have an arid or hyperarid climate, characterized by a strong moisture deficit, where annual potential loss of moisture from evapotransp...
 or desertic
Désert

?D?sert? is ?milie Simon's debut single, released in October 2002. The song was a huge success both critically and commercially in her homeland....
.

Concepts


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 . Water vapor is one Phase of the water cycle within the hydrosphere....
 but that water may not be generally accessible to plants, except for very specialized organisms (such as some species of lichen
Lichen

Lichens are composite organisms consisting of a symbiosis association of a fungus with a Photosynthesis partner , usually either a green algae or Cyanobacteria ....
). '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 arid climates, may thus depend on the use of the land
Land use

Land use is the human modification of natural environment or wilderness into built environment such as fields, pastures, and settlements. The major effect of land use on land cover since 1750 has been deforestation of temperate regions....
. Regards to the presence of life, what is more important than the degree of rainfall is the fraction of precipitation
Precipitation (meteorology)

File:MeanMonthlyP.gifIn meteorology, precipitation is any product of the condensation of Atmosphere water vapor that is deposited on the earth's surface....
 that is not quickly lost through evaporation
Evaporation

Evaporation is the slow vaporization of a liquid and the reverse of condensation. A type of phase transition, it is the process by which molecules in a liquid State of matter spontaneously become gaseous ....
 or runoff
Surface runoff

Surface runoff is the water flow which occurs when soil is infiltrated to full capacity and excess water, from rain, snowmelt, or other sources flows over the land....
. Attempts to quantitatively describe the degree of aridity of a place has often led to the development of aridity indexes
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 the effective use of the land for such activi...
. There is no universal agreement on the precise boundaries between classes such as 'hyper-arid', 'arid', 'semi-arid
Semi-arid

A Semi-arid climate or steppe climate generally describes climate regions that receive low annual rainfall . A more precise definition is given by the K?ppen climate classification that treats steppe climates as intermediates between the desert climates and humid climates in ecological characteristics and agricultural potential....
', etc.

Geographical distribution


If different classification schemes and maps differ in their details, there is a general agreement about the fact that large areas of the Earth are considered 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

For the Australian television series see Rain Shadow .A rain shadow or rainshadow, or more accurately, precipitation shadow, is a dry region of land that is leeward of a mountain range or other geographic feature, with respect to prevailing wind direction....
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 virtually rainless plateau in South America, covering a 966 km strip of land on the Pacific Ocean coast of South America, west of the Andes mountains....
).

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
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if 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 any long-term significant change in the expected patterns of average weather of a specific region over an appropriately significant period of time....
. In addition, changes in land use can result in greater demands on soil water and induce a higher degree of aridity. See aridification
Aridification

Aridification is the process of a region becoming increasingly Arid. It refers to long term change rather than seasonal variation.It is often measured as the reduction of average soil moisture content....
.

See also


  • Drought
    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 ....
  • Saturation vapor pressure
  • Relative humidity
    Relative humidity

    Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water....
  • 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 behavioral adaptations to withstand these period...