Semi-arid climate
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 climates (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 grasses or shrubs.
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). [1]
Furthermore, to delineate "hot semi-arid climates" from "cold semi-arid climates", there are three widely used isotherms: 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).
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[edit] Hot Semi-arid climates
Hot semi-arid climates (type "BSh") tend to be located in the tropics 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 climates, 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 monsoons, 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.[2]
[edit] Cold Semi-arid climates
Cold semi-arid climates (type "Bsk") tend to be located in temperate zones. The precipitation inclusion is a somewhat complicated format, to find the precipitation threshold (in millimeters), multiply the average annual temperature in °C by 20, then add 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. They are typically found in continental interiors some distance from large bodies of water, temperatures of over 38°C (100 °F) are experienced with varying regularity or length during the peak of summer heat. Cold semi-arid climates usually feature hot and dry summers, but a much shorter duration than hot semi-arid climates. Unlike hot semi-arid climates, areas with cold semi-arid climates tend to have cold, rather than mild winters. These areas usually see some snowfall during the winter, though snowfall is much lower than locations at similar latitudes with more humid climates and melting often occurs quicker with evaporation. Greater temperature swings over short time frames are less likely in hot semi-arid climates. Cold semi-arid climates at higher latitudes tend to have drier winters and wetter summers from instability created by warm air rising over mountain slopes, while cold semi-arid climates at lower latitudes tend to have precipitation patterns more akin to Mediterranean climates, with greater winter precipitation (often valley fog and drizzle) sometimes this also occurs in the spring or autumn seasons. In North America they exist in much of the inter-mountain US West and Northwestern Mexico, for example Salt Lake City, Utah typifies this type of climate. Further north up 50° north latitude they exist in the southern interior of British Columbia and in the border region between the provinces of Alberta and Saskatchewan in Canada. With colder winters, they occur in parts of Central Russia, Kazakhstan, Mongolia and China. Less extreme versions with more modified winters are located in mid-high altitude regions of Spain, North Africa, Middle East, Iran, Pakistan, South America (mostly in Argentina), South Africa and South Australia.
[edit] 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.
[edit] See also
- Continental climate
- Dust Bowl (an era of devastating dust storms, mostly in the 1930s, in semi-arid areas on the Great Plains of the United States and Canada)
- Goyder's Line (a boundary marking the limit of semi-arid climates in the Australian state of South Australia)
- Palliser's Triangle (semi-arid area of Canada)
- Köppen climate classification
[edit] References
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