Semi-arid
Encyclopedia
A semi-arid climate or steppe climate describes climatic
regions that receive precipitation below potential evapotranspiration, but not extremely. A more precise definition is given by the Köppen climate classification
that treats steppe
climates (BSk and BSh) as intermediates between desert climate
s (BW) and humid climates in ecological characteristics and agricultural potential. Semi-arid climates tend to support short or scrubby vegetation, with semi-arid areas usually being dominated by either grass
es or shrub
s.
To determine if a location has a semi-arid climate, the precipitation threshold must first be determined. Finding the precipitation threshold (in millimeters) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30%–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received. If the area's annual precipitation is less than the threshold but more than half the threshold, it is classified as a BS (steppe climate).
Furthermore, to delineate "hot semi-arid climates" from "cold semi-arid climates", there are three widely used isotherm
s: Either a mean annual temperature of 18°C, or a mean temperature of 0°C or -3°C in the coldest month, so that a location with a "BS" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot semi-arid" (BSh), and a location with the appropriate temperature below the given isotherm is classified as "cold semi-arid" (BSk).
and subtropics
. These climates tend to have hot, sometimes extremely hot, summers and mild to warm winters. Snow rarely (if ever) falls in these regions. Hot semi-arid climates are most commonly found around the fringes of subtropical deserts. At higher latitudes, hot semi-arid climates typically feature a Mediterranean
precipitation pattern, with dry summers and wetter winters, while hot semi-arid climates at lower latitudes typically feature precipitation patterns more like those of tropical savanna climate
s, with warmer wet seasons and cooler dry seasons. Hot semi-arid climate zones in regions such as West Africa
, India
and Pakistan
experience the seasonal effects of monsoon
s, being extremely wet during the monsoon months and extremely dry during the rest of the year, with few or no months bringing moderate levels of precipitation. In Australia
, a large portion of the Outback
, surrounding the central desert regions, lies within the hot semi-arid climate regime.
s, with dry summers, relatively wet winters, and even wetter springs and autumns.
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
regions that receive precipitation below potential evapotranspiration, but not extremely. A more precise definition is given by the Köppen climate classification
Köppen climate classification
The Köppen climate classification is one of the most widely used climate classification systems. It was first published by Crimea German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself, notably in 1918 and 1936...
that treats steppe
Steppe
In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...
climates (BSk and BSh) as intermediates between 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...
s (BW) and humid climates in ecological characteristics and agricultural potential. Semi-arid climates tend to support short or scrubby vegetation, with semi-arid areas usually being dominated by either grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
es or shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s.
To determine if a location has a semi-arid climate, the precipitation threshold must first be determined. Finding the precipitation threshold (in millimeters) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30%–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received. If the area's annual precipitation is less than the threshold but more than half the threshold, it is classified as a BS (steppe climate).
Furthermore, to delineate "hot semi-arid climates" from "cold semi-arid climates", there are three widely used isotherm
Isotherm
Isotherm may refer to:* Isotherm a type of contour line that connects points of equal temperature at a given date or time on a geographic map* Isotherm in thermodynamics, a curve on a p-V diagram for an isothermal process...
s: Either a mean annual temperature of 18°C, or a mean temperature of 0°C or -3°C in the coldest month, so that a location with a "BS" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot semi-arid" (BSh), and a location with the appropriate temperature below the given isotherm is classified as "cold semi-arid" (BSk).
Hot Semi-arid climates
Hot semi-arid climates (type "BSh") tend to be located in the tropicsTropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
and subtropics
Subtropics
The subtropics are the geographical and climatical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
. These climates tend to have hot, sometimes extremely hot, summers and mild to warm winters. Snow rarely (if ever) falls in these regions. Hot semi-arid climates are most commonly found around the fringes of subtropical deserts. At higher latitudes, hot semi-arid climates typically feature a Mediterranean
Mediterranean climate
A Mediterranean climate is the climate typical of most of the lands in the Mediterranean Basin, and is a particular variety of subtropical climate...
precipitation pattern, with dry summers and wetter winters, while hot semi-arid climates at lower latitudes typically feature precipitation patterns more like those of tropical savanna climate
Tropical savanna climate
Tropical savanna climate or tropical wet and dry climate is a type of climate that corresponds to the Köppen climate classification categories "Aw" and '"As."...
s, with warmer wet seasons and cooler dry seasons. Hot semi-arid climate zones in regions such as West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
experience the seasonal effects of monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
s, being extremely wet during the monsoon months and extremely dry during the rest of the year, with few or no months bringing moderate levels of precipitation. In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, a large portion of the Outback
Outback
The Outback is the vast, remote, arid area of Australia, term colloquially can refer to any lands outside the main urban areas. The term "the outback" is generally used to refer to locations that are comparatively more remote than those areas named "the bush".-Overview:The outback is home to a...
, surrounding the central desert regions, lies within the hot semi-arid climate regime.
Cold Semi-arid climates
Cold semi-arid climates (type "Bsk") tend to be located in temperate zones. They are typically found in continental interiors some distance from large bodies of water. Cold semi-arid climates usually feature hot and dry (often exceptionally hot) summers, though their summers are typically not quite as hot as those of hot semi-arid climates. Unlike hot semi-arid climates, areas with cold semi-arid climates tend to have cold winters. These areas usually see some snowfall during the winter, though snowfall is much lower than locations at similar latitudes with more humid climates. Areas featuring cold semi-arid climates tend to have higher elevations than areas with hot semi-arid climates, and are sometimes subject to major temperature swings between day and night, sometimes by as much as 13°C/55°F in that time frame. These temperature swings are seldom seen in hot semi-arid climates. Cold semi-arid climates at higher latitudes tend to have dry winters and wetter summers, while cold semi-arid climates at lower latitudes tend to have precipitation patterns more akin to Mediterranean climateMediterranean climate
A Mediterranean climate is the climate typical of most of the lands in the Mediterranean Basin, and is a particular variety of subtropical climate...
s, with dry summers, relatively wet winters, and even wetter springs and autumns.
Regions of varying classification
As stated previously, there are three isotherms used to delineate between hot and cold semi-arid climates. As a result of this, some areas can have climates that are classified as hot or cold semi-arid depending on the isotherm used.See also
- Continental climateContinental climateContinental climate is a climate characterized by important annual variation in temperature due to the lack of significant bodies of water nearby...
- Goyder's LineGoyder's LineGoyder's Line is a boundary line across South Australia corresponding to a rainfall boundary believed to indicate the edge of the area suitable for agriculture. North of Goyder's Line, the rainfall is not reliable enough, and the land is only suitable for grazing and not cropping. The line traces a...
- Palliser's TrianglePalliser's TrianglePalliser's Triangle, or the Palliser Triangle, is a largely semi-arid steppe region in the Prairie Provinces of Western Canada that was determined to be unsuitable for agriculture because of its unfavourable climate. The soil in this area is dark brown or black in color and is very nutrient-rich....
- Köppen climate classificationKöppen climate classificationThe Köppen climate classification is one of the most widely used climate classification systems. It was first published by Crimea German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself, notably in 1918 and 1936...