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Desert

Desert

In geography, a desert is a landscape form or region that receives little precipitation - less than 250 mm per year. Deserts have a reputation for supporting very little life. Compared to wetter regions this may be true, although upon closer examination, deserts often harbor a wealth of life that usually remains hidden (especially during the daylight) to preserve moisture. Approximately one-third of Earth's land surface is desert. (See a map of the world's non-polar deserts, http://pubs.usgs.gov/gip/deserts/what/world.html.) Desert landscapes have certain common features. Desert soil is often composed mostly of rocky surfaces called regs. Sand dunes called ergs and stony or hamada surfaces compose the minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation. Bottom lands may be salt-covered flats. Eolian (wind-driven) processes are major factors in shaping desert landscapes. Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are dry, they are ideal places for human artifacts and fossils to be preserved. In the Köppen climate classification system, they are classed as (BW).

Types of desert

Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes. However, lack of rainfall alone can't provide an accurate description of what a desert is. For example, Phoenix, Arizona receives less than 250 millimeters, (10 inches), of precipitation per year, and is immediately recognized as being located in a desert. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters of precipitation per year, but is not generally recognized as a desert region. The difference lies in something termed "potential evapotranspiration." The water budget of an area can be calculated using the formula P-PE+/-S, whereby P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. Tucson, Arizona receives about 300 millimeters, (12 inches), of rain per year, however about 2500 millimeters, (100 inches), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower, so while these regions receive minimal precipitation, they should be designated as specifically different from the simple definition of a desert: a place where evaporation exceeds precipitation. That said, there are different forms of deserts. Cold deserts can be covered in snow; such locations don't receive much precipitation, and what does fall remains frozen as snow pack; these are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless. Most non-polar deserts are hot because they have little water. Water tends to have a cooling, or at least a moderating, effect in environments where it is plentiful. In some parts of the world deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture (this is true in some middle-latitude landmass interior locations, particularly in Asia). Deserts are also classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.

Montane deserts

Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalaya, in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (9,843 feet) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40mm) to being very far from the nearest available sources of moisture.

Desert features

hemiboreal] Sand covers only about 20 percent of Earth's deserts. Most of the sand is in sand sheets and sand seas—vast regions of undulating dunes resembling ocean waves "frozen" in an instant of time. In general, there are 6 forms of deserts: i.Mountain and basin deserts, ii. Hamada deserts, which comprise of a plateaux landforms, iii. Regs which consist of rock pavements, iv. Ergs which are formed by sand seas, v. Intermontane Basins, and vi. Badlands which are located at the margins of arid lands comprising of clay-rich soil. Nearly 100 percent of desert surfaces are plains where eolian deflation—removal of fine-grained material by the wind—has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles. The remaining surfaces of arid lands are composed of exposed bedrock outcrops, desert soils, and fluvial deposits including alluvial fans, playas, desert lakes, and oases/oasis. Bedrock outcrops commonly occur as small mountains surrounded by extensive erosional plains. There are several different types of dunes. Barchan dunes are produced by strong winds blowing across a level surface and are crescent shaped. Longitudinal or seif dunes are dunes that are parallel to a strong wind that blows in one general direction. Transverse dunes run are a right angle to the constant wind direction. Star dunes are star-shaped and have several ridges that spread out around a point. Oases are vegetated areas moistened by springs, wells, or by irrigation. Many are artificial. Oases are often the only places in deserts that support crops and permanent habitation.

Soils

irrigation Soils that form in arid climates are predominantly mineral soils (classified as Aridisols) with low organic content such as salt. The repeated accumulation of water in some soils causes distinct salt layers to form. Calcium carbonate precipitated from solution may cement sand and gravel into hard layers called "calcrete" that form layers up to 50 meters thick. Caliche is a reddish-brown to white layer found in many desert soils. Caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction between water and carbon dioxide released by plant roots or by decaying organic material.

Vegetation

Most desert plants are drought- or salt-tolerant, such as xerophytes. Some store water in their leaves, roots, and stems. Other desert plants have long tap roots that penetrate the water table, anchor the soil, and control erosion. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Deserts typically have a plant cover that is sparse but enormously diverse. The Sonoran Desert of the American Southwest has the most complex desert vegetation on Earth. The giant saguaro cacti provide nests for desert birds and serve as "trees" of the desert. Saguaro grow slowly but may live 200 years. When 9 years old, they are about 15 centimeters high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro are 15 meters tall and weigh as much as 10 tons. They dot the Sonoran and reinforce the general impression of deserts as cacti-rich land. Although cacti are often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. They include the pea family and sunflower family. Cold deserts have grasses and shrubs as dominant vegetation.

Water

sunflower Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous. Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams fed by rain and snow from adjacent highlands. These streams fill the channel with a slurry of mud and commonly transport considerable quantities of sediment for a day or two. Although most deserts are in basins with closed, or interior drainage, a few deserts are crossed by 'exotic' rivers that derive their water from outside the desert. Such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts, but their volumes are such that they maintain their continuity. The Nile River, the Colorado River, and the Yellow River are exotic rivers that flow through deserts to deliver their sediments to the sea. Lakes form where rainfall or meltwater in interior drainage basins is sufficient. Desert lakes are generally shallow, temporary, and salty. Because these lakes are shallow and have a low bottom gradient, wind stress may cause the lake waters to move over many square kilometers. When small lakes dry up, they leave a salt crust or hardpan. The flat area of clay, silt, or sand encrusted with salt that forms is known as a playa. There are more than a hundred playas in North American deserts. Most are relics of large lakes that existed during the last ice age about 12,000 years ago. Lake Bonneville was a 52,000-square-kilometer lake almost 300 meters deep in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of Lake Bonneville include Utah's Great Salt Lake, Utah Lake, and Sevier Lake. Because playas are arid land forms from a wetter past, they contain useful clues to climatic change. When the occassional preticipation does occur, it erodes the desert rocks quickly and powerfully. Wind is the other factor that erodes deserts- they are constant yet slow. The flat terrains of hardpans and playas make them excellent race tracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records are commonly established on Bonneville Speedway, a race track on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base, California.

Mineral resources

Some mineral deposits are formed, improved, or preserved by geologic processes that occur in arid lands as a consequence of climate. Ground water leaches ore minerals and redeposits them in zones near the water table. This leaching process concentrates these minerals as ore that can be mined. Evaporation in arid lands enriches mineral accumulation in their lakes. Playas may be sources of mineral deposits formed by evaporation. Water evaporating in closed basins precipitates minerals such as gypsum, salts (including sodium nitrate and sodium chloride), and borates. The minerals formed in these evaporite deposits depend on the composition and temperature of the saline waters at the time of deposition. Significant evaporite resources occur in the Great Basin Desert of the United States, mineral deposits made forever famous by the "20-mule teams" that once hauled borax-laden wagons from Death Valley to the railroad. Boron, from borax and borate evaporites, is an essential ingredient in the manufacture of glass, ceramics, enamel, agricultural chemicals, water softeners, and pharmaceuticals. Borates are mined from evaporite deposits at Searles Lake, California, and other desert locations. The total value of chemicals that have been produced from Searles Lake substantially exceeds US$1 billion. The Atacama Desert of South America is unique among the deserts of the world in its great abundance of saline minerals. Sodium nitrate has been mined for explosives and fertilizer in the Atacama since the middle of the 19th century. Nearly 3 million tonnes were mined during World War I. Valuable minerals located in arid lands include copper in the United States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia; chromite in Turkey; and gold, silver, and uranium deposits in Australia and the United States. Nonmetallic mineral resources and rocks such as beryllium, mica, lithium, clays, pumice, and scoria also occur in arid regions. Sodium carbonate, sulfate, borate, nitrate, lithium, bromine, iodine, calcium, and strontium compounds come from sediments and near-surface brines formed by evaporation of inland bodies of water, often during geologically recent times. The Green River Formation of Colorado, Wyoming, and Utah contains alluvial fan deposits and playa evaporites created in a huge lake whose level fluctuated for millions of years. Economically significant deposits of trona, a major source of sodium compounds, and thick layers of oil shale were created in the arid environment. Some of the more productive petroleum areas on Earth are found in arid and semiarid regions of Africa and the Mideast, although the oil fields were originally formed in shallow marine environments. Recent climate change has placed these reservoirs in an arid environment. Other oil reservoirs, however, are presumed to be eolian in origin and are presently found in humid environments. The Rotliegendes, a hydrocarbon reservoir in the North Sea, is associated with extensive evaporite deposits. Many of the major U.S. hydrocarbon resources may come from eolian sands. Ancient alluvial fan sequences may also be hydrocarbon reservoirs.

List of deserts

Americas

- Atacama desert in Chile
- Mojave, Great Basin, Sonoran, and Chihuahuan See also: List of North American deserts

Africa

- Libyan
- Kalahari
- Sahara
- Namib

Asia-Pacific

- Dasht-e Kavir, central Iran.
- Gobi desert of Mongolia; Taklamakan desert in China.
- Kara Kum deserts in Central Asia.
- Thar-Cholistan desert in India and Pakistan.
- Kavir-e Lut, souteast Iran.
- Kyzyl Kum - Kazakhstan and Uzbekistan
- Negev - southern Israel
- Judean Desert - eastern Israel and Palestine
- Simpson Desert, Great Sandy Desert, Sturt's Stony Desert, Tanami Desert, Great Victoria Desert, Big Desert, Little Desert (all in Australia)
- Taklamakan - Xinjiang Uighur Autonomous Region of the People's Republic of China
- Rangipo Desert - New Zealand

Precipitation (meteorology)

In meteorology, precipitation is any kind of water that falls from the sky as part of the weather. This includes snow, rain, sleet, freezing rain, hail, and virga. Precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet. Precipitation is generated in clouds, which reach a point of saturation; at this point larger and larger droplets (or pieces of ice) form, which then fall to the earth under gravity. It is possible to 'seed' clouds to induce precipitation by releasing a fine dust or appropriate chemical (commonly silver nitrate) into a cloud, encouraging droplets to form, and increasing the probability of precipitation.

Orographic precipitation

Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This upwards movement cools the air, resulting in a cloud formation and rainfall. In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation, leaving drier air (see katabatic wind) on the descending (generally warming), leeward side where a rain shadow is formed. Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20 to 30 inches (500 to 750 mm) per year, and interior uplands receiving over 100 inches (2.5 m) per year. Leeward coastal areas are especially dry (<20 in (500 mm) per year at Waik&#299;k&#299;), and the tops of moderately high uplands are especially wet (~475 in (12 m) per year at Wai'ale'ale on Kaua'i).

Convectional rainfall

Convectional rainfall occurs when the air is heated up, usually by the land below it (land tends to heat up faster than air or water bodies). As the air heats up it rises. Inevitably cooling will result, and water vapor will condense out of the air to form droplets, and eventually clouds, if there is enough vapour. This kind of precipitation is most commonly found in tropical areas.

Frontal Rainfall

Frontal rainfall occurs at both warm fronts and cold fronts. At a warm front the warm air, being lighter, rides up over the cold air. As it rises it also cools down (adiabatic process). Moisture in the air condenses to form clouds, and precipitation occurs. At a cold front the warm air is forced up by the cold and the same process occurs.

Rainfall Patterns

Western

Major elements are prevailing westerlies and ocean currents moving equatorward. At high lattitudes the current is warmer than land, westerlies pick up moisture and cool when moving over land. When the land is warmer than the ocean clouds don't drop precipitation, but pick up additional moisture; it rains in the mountains. When the land is cooler than the ocean, then westerlies cool as they move inland and rain occurs in the lowlands.

Eastern and Central

Polar air masses (above 50° latitudes) are distinct from lower latitude air masses. The westerlies are warm air masses that move poleward from 30°N. In Eastern US the westernlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westernlie air masses meet, precipitation occurs.

Inland Continental Areas

Areas not proximate to large bodies of water, warm faster than surrounding areas. Hot air masses rise from the center of the continent forming a low pressure area. This low pressure draws water laden clouds from the coasts. This area is heated & rises, this causes cooling adiabatically and precipitation. Resulting in summer rain, and less winter precipitation.

Reg

Reg is slang terminology for a regular poster in a BBS, newsgroup, blog, or any kind of online community. They have generally been community members for a long period of time. Because of this they often are the first candidates for moderators. Regs constitute a small minority of the community members but contribute most of the content in the community. They are conscious of their situation in the community and often react strongly against newcomers because they consider them to be outsiders. Others in the other hand are very welcoming towards them and even form a sort of "welcome troupe".

Additionally

- it may be a short form of the forename Reginald - as in Reg Dwight;
- it may be an abbreviation (typically on coins) for 'Regina' the Latin for 'Queen';
- it may be an abbreviation for 'Registration' - thus 'Reg No' corresponds to 'Registration number' Category:Customary categories of people Category:Abbreviations Category:Given names category:Internet slang

Erg

:For other uses see Erg (disambiguation) An erg is the unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units, symbol "erg". Its name is derived from the Greek word meaning "work". The erg is a quite small unit, equal to a force of one dyne exerted for a distance of one centimetre. In the CGS base units, it is equal to one gram-square centimetre per second squared (g·cm²/s²). It is thus equal to 1 × 10^-7 joules or 0.1 microjoule (µJ) in SI units. It is approximately the amount of energy that a mosquito uses to take flight. 1 erg = 10^-7 joule 1 joule = 10⁷ ergs Category:Units of energy Category:CGS units From Greek ergon work OED] ja:エルグ

Hamada

Hamada (浜田市; -shi) is a city located in Shimane, Japan. As of 2003, the city has an estimated population of 46,532 and the density of 286.19 persons per km². The total area is 162.59 km². The city was founded on November 3, 1940. It is scheduled to merge with the surrounding towns of Asahi, Kanagi and Misumi, and the village of Yasaka in October 2005. See also: Hamada, Shoji, Master Japanese Potter, 20th Century.

External links

- [http://www.city.hamada.shimane.jp/ Hamada official website] in Japanese
- [http://fish.miracle.ne.jp/hamakoku/ Hamada International Association homepage] in English (slightly out of date) Category:Cities in Shimane Prefecture ja:浜田市

Rock (geology)

, plutonic, metamorphic rock types of North America. ]] Rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks are classified by mineral and chemical composition; the texture of the constituent particles; and also by the processes that formed them. These indicators separate rocks into igneous, sedimentary, and metamorphic. Igneous rocks are formed from molten magma, and are divided into two main categories: Plutonic rock and Volcanic rock. Plutonic rocks result when the magma cools and crystallises slowly within the Earth's crust, while Volcanic rocks result from the magma reaching the surface either as lava or fragmental ejecta. Sedimentary rocks are formed by deposition of either detrital or organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation. The latter can occur at or near the earth's surface, especially in the case of carbonate-rich sediments. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the earth's surface, and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). The transformation of one rock type to another is described by the geological model called the rock cycle. The Earth's crust (including the lithosphere) and mantle are formed of rock.

External links

- [http://www.geol.lsu.edu/henry/Geology3041/2IgneousClassify/IgneousClassFlow.htm Classification of Igneous Rocks] Category:Geology Category:Rocks ja:岩石 ms:Batu th:หิน

Eolian

Eolian (or aeolian) processes pertain to the activity of the winds. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments. Although water is much more powerful than wind, eolian processes are important in arid environments such as deserts.

Eolian erosion

Wind erodes Earth's surface by deflation, the removal of loose, fine-grained particles by the turbulent eddy action of the wind, and by abrasion, the wearing down of surfaces by the grinding action and sand blasting of windborne particles. Most eolian deflation zones are composed of desert pavement, a sheetlike surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of Earth's desert surfaces are stony deflation zones. The rock mantle in desert pavements protects the underlying material from deflation. A dark, shiny stain, called desert varnish or rock varnish, is often found on surfaces of some desert rocks that have been exposed at the surface for a long period of time. Manganese, iron oxides, hydroxides, and clay minerals form most varnishes and provide the shine. Deflation basins, called blowouts, are hollows formed by the removal of particles by wind. Blowouts are generally small, but may be up to several kilometers in diameter. Wind-driven grains abrade landforms. Grinding by particles carried in the wind creates grooves or small depressions. Ventifacts are rocks which have been cut, and sometimes polished, by the abrasive action of wind. Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long and are forms that have been streamlined by desert winds. The famous sphinx at Giza in Egypt may be a modified yardang.

Eolian transportation

Particles are transported by winds through suspension, saltation, and creep. Small particles may be held in the atmosphere in suspension. Upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air. Typical winds near Earth's surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft as dust or haze. Saltation is downwind movement of particles in a series of jumps or skips. Saltation normally lifts sand-size particles no more than one centimeter above the ground, and proceeds at one-half to one-third the speed of the wind. A saltating grain may hit other grains that jump up to continue the saltation. The grain may also hit larger grains that are too heavy to hop, but that slowly creep forward as they are pushed by saltating grains. Surface creep accounts for as much as 25 percent of grain movement in a desert. Eolian turbidity currents are better known as dust storms. Air over deserts is cooled significantly when rain passes through it. This cooler and denser air sinks toward the desert surface. When it reaches the ground, the air is deflected forward and sweeps up surface debris in its turbulence as a dust storm. Crops, people, villages, and possibly even climates are affected by dust storms. Some dust storms are intercontinental, a few may circle the globe, and occasionally they may engulf entire planets. When the Mariner 9 spacecraft arrived at Mars in 1971, the entire planet was enshrouded in global dust. Most of the dust carried by dust storms is in the form of silt-size particles. Deposits of this windblown silt are known as loess. The thickest known deposit of loess, 335 meters, is on the Loess Plateau in China. In Europe and in the Americas, accumulations of loess are generally from 20 to 30 meters thick. Small whirlwinds, called dust devils, are common in arid lands and are thought to be related to very intense local heating of the air that results in instabilities of the air mass. Dust devils may be as much as one kilometer high.

Eolian deposition

Wind-deposited materials hold clues to past as well as to present wind directions and intensities. These features help us understand the present climate and the forces that molded it. Wind-deposited sand bodies occur as sand sheets, ripples, and dunes. Sand sheets are flat, gently undulating sandy plots of sand surfaced by grains that may be too large for saltation. They form approximately 40 percent of eolian depositional surfaces. The Selima Sand Sheet, which occupies 60,000 square kilometers in southern Egypt and northern Sudan, is one of the Earth's largest sand sheets. The Selima is absolutely flat in some places; in others, active dunes move over its surface. Wind blowing on a sand surface ripples the surface into crests and troughs whose long axes are perpendicular to the wind direction. The average length of jumps during saltation corresponds to the wavelength, or distance between adjacent crests, of the ripples. In ripples, the coarsest materials collect at the crests. This distinguishes small ripples from dunes, where the coarsest materials are generally in the troughs. Wind-blown sand moves up the gentle upwind side of the dune by saltation or creep. Sand accumulates at the brink, the top of the slipface. When the buildup of sand at the brink exceeds the angle of repose, a small avalanche of grains slides down the slipface. Grain by grain, the dune moves downwind. Accumulations of sediment blown by the wind into a mound or ridge, dunes have gentle upwind slopes on the wind-facing side. The downwind portion of the dune, the lee slope, is commonly a steep avalanche slope referred to as a slipface. Dunes may have more than one slipface. The minimum height of a slipface is about 30 centimeters. Some of the most significant experimental measurements on eolian sand movement were performed by Ralph Bagnold, a British engineer who worked in Egypt prior to World War II. Bagnold investigated the physics of particles moving through the atmosphere and deposited by wind. He recognized two basic dune types, the crescentic dune, which he called "barchan," and the linear dune, which he called longitudinal or "sief" (Arabic for "sword"). Category:Deserts Category:Geological processes

Köppen climate classification

The Köppen climate classification is one of the most widely used climate classification systems. It was developed by Wladimir Köppen, a German climatologist, around 1900 (with several further modifications by himself, notably in 1918 and 1936). It is based on the concept that native vegetation is the best expression of climate, thus climate zone boundaries have been selected with vegetation distribution in mind. It combines average annual and monthly temperatures and precipitation, and the seasonality of precipitation.

The scheme

Köppen climate classification scheme divides the climates into five main groups and several types and subtypes. Each particular climate type is represented by a 2 to 4 letter symbol:

GROUP A: Tropical/Megathermal climates

Tropical climates (see tropics) are characterized by constant high temperature (at sea level and low elevations) — all twelve months of the year have average temperatures of 18 °C (64.4 °F) or higher. They are subdivided as follows:
- Tropical rain forest climate (Af): All twelve months have average precipitation of at least 60 mm (2.36 inches). These climates, usually occurring within 5-10° latitude of the equator. In some eastern-coast areas, they may extend to as much as 25° away from the equator. This climate is dominated by the Doldrums Low Pressure System all year round, and thus has no real seasons. :Examples: Singapore,Belém, Brazil. :Some of the places that have this climate are indeed uniformly and monotonously wet throughout the year (e.g., the northwest Pacific coast of South and Central America, from Ecuador to Costa Rica, see for instance, Andagoya, Colombia), but in many cases the period of higher sun and longer days is distinctly wettest (as at Palembang, Indonesia) or the time of lower sun and shorter days may have more rain (as at Sitiawan, Malaysia). : A few places with this climate are found at the outer edge of the tropics, almost exclusively in the Southern Hemisphere; one example is Santos, Brazil.
Note. The term aseasonal refers to the lack in the tropical zone of large differences in day light hours and mean monthly (or daily) temperature throughout the year. There are annual cyclic changes in the tropics, not as predictable as thosse in the temperate zone, albeit unrelated to temperature but to water availability whether as rain, mist, soil, or ground water. Plant response (e. g., phenology), animal (feeding, migration, reproduction, et cetera), and human activities (plant sowing, harvesting, hunting, fishing, et cetera) are tuned to this seasonality. Indeed, in tropical South America and Central America, the rainy season (and the high water season) is called Invierno, even though it could occur in the northern hemisphere summer; likewise, the dry season (and low water season) is called Verano and can occur in the northern hemisphere winter.
- Tropical monsoon climate (Am): This type of climate, most common in southern Asia and West Africa, results from the monsoon winds which change direction according to the seasons. This climate has a driest month (which nearly always occurs at or soon after the "winter" solstice for that side of the equator) with rainfall less than 60 mm, but more than (100 − [total annual precipitation /25]): : Examples: Conakry, Guinea
Chittagong, Bangladesh. : There is also another scenario under which some places fit into this category; this is referred to as the trade-wind littoral climate because easterly winds bring enough precipitation during the "winter" months to prevent the climate from becoming a tropical wet-and-dry climate. Jakarta, Indonesia and Miami, Florida are included among these location.
- Tropical wet and dry or savanna climate (Aw): These climates have a pronounced dry season, with the driest month having precipitation less than 60 mm and also less than (100 − [total annual precipitation /25]): : Examples: Bangalore, India
Veracruz, Mexico
Townsville, Australia. : Most places that have this climate are found at the outer margins of the tropical zone, but occasionally an inner-tropical location (e.g., San Marcos, Antioquia, Colombia) also qualifies. Actually, the Caribbean coast, eastward from Urabá gulf on the Colombia–Panamá border to the Orinoco river delta, on the Atlantic ocean (ca. 4,000 km), have long dry periods (the extreme is the BSh climate (see below), characterised by very low, unreliable precipitation, present, for instance, in extensive areas in the Guajira, and Coro, western Venezuela, the northernmost peninsulas in South America, which receive <300 mm total annual precipitation, practically all in two or three months). This condition extends to the Lesser Antilles and Greater Antilles forming the Circumcaribbean dry belt. The length and severity of the dry season diminishes inland (southward); at the latitude of the Amazon river — which flows eastward, just south of the equatorial line — the climate is Af. East from the Andes, between the dry, arid Caribbean and the ever-wet Amazon are the Orinoco river' Llanos or Savannas, from where this climate takes its name. : Sometimes As is used in place of Aw if the dry season occurs during the time of higher sun and longer days. This is the case in parts of Hawaii (Honolulu), East Africa (Mombasa, Kenya) and Sri Lanka (Trincomalee), for instance. In most places that have tropical wet and dry climates, however, the dry season occurs during the time of lower sun and shorter days.

GROUP B: Dry (Arid and semiarid) climates

These climates are characterized by the fact that precipitation is less than potential evapotranspiration. The threshold is determined as follows:
- 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.
- If the annual precipitation is less than half the threshold for Group B, it is classified as BW (desert climate); if it is less than the threshold but more than half the threshold, it is classified as BS (steppe climate).
- A third letter can be included to indicate temperature. Originally, h signified low latitude climate (average annual temperature above 18 °C) while k signified middle latitude climate (average annual temperature below 18 °C), but the more common practice today (especially in the United States) is to use h to mean that the coldest month has an average temperature that is above 0 °C (32 °F), with k denoting that at least one month averages below 0 °C.
- Examples: Yuma, Arizona (BWh)
Turpan, China (BWk)
Cobar, Australia (BSh)
Medicine Hat, Alberta (BSk).
Some desert areas, situated along the west coasts of continents at tropical or near-tropical locations, are characterized by cooler temperatures than encountered elsewhere at comparable latitudes (due to the nearby presence of cold ocean currents) and frequent fog and low clouds, despite the fact that these places rank among the driest on earth in terms of actual precipitation received. This climate is sometimes labelled BWn and examples can be found at Lima, Peru and Walvis Bay, Namibia.
- On occasion, a fourth letter is added to indicate if either the winter or summer is "wetter" than the other half of the year. To qualify, the wettest month must have at least 60 mm of average precipitation if all twelve months are above 18 °C, or 30 mm (1.18 inches) if not; plus at least 70% of the total precipitation must be in the same half of the year as the wettest month — but the letter used indicates when the dry season occurs, not the "wet" one. This would result in Khartoum, Sudan being reckoned as BWhw, Niamey, Niger as BShw, El Arish, Egypt as BWhs, Asbi'ah, Libya as BShs, Umnugobi, Mongolia as BWkw, and Xining, China as BSkw (BWks and BSks do not exist if 0°C in the coldest month is recognized as the h/k boundary). If the standards for neither w nor s are met, no fourth letter is added.

GROUP C: Temperate/mesothermal climates

These climates have an average temperature above 10 °C (50 °F) in their warmest months, and a coldest month average between −3 °C and 18 °C. (Some climatologists, particularly in the United States, however, prefer to observe 0 °C rather than −3 °C in the coldest month as the boundary between this group and Group D; this is done to prevent certain headland locations in New England — principally Cape Cod — and such nearby islands as Nantucket and Martha's Vineyard, from fitting into the Maritime Temperate category noted below; this category is alternately known as the Marine West Coast climate, and eliminating the aforementioned locations indeed confines it exclusively to places found along the western margins of the continents, at least in the Northern Hemisphere).
- The second letter indicates the precipitation pattern — w indicates dry winters (driest winter month average precipitation less than one-tenth wettest summer month average precipitation; one variation also requires that the driest winter month have less than 30 mm average precipitation), s inidicates dry summers (driest summer month less than 30 mm average precipitation and less than one-third wettest winter month precipitation) and f means significant precipitation in all seasons (neither above mentioned set of conditions fulfilled).
- The third letter indicates the degree of summer heat — a indicates warmest month average temperature above 22 °C (71.6 °F), b indicates warmest month average temperature below 22 °C, with at least 4 months averaging above 10 °C, while c means 3 or fewer months with mean temperatures above 10 °C.
- The order of these two letters is sometimes reversed, especially by climatologists in the United States.
- Group C climates are subdivided as follows:
- Mediterranean climates (Csa, Csb): These climates usually occur on the western sides of continents between the latitudes of 30° and 45° (though on the west coast of North America, they occur in small patches as far north as 48°). These climates are in the polar front region in winter, and thus have moderate temperatures and changeable, rainy weather. Summers are hot and dry, due to the domination of the subtropical high pressure systems, except in the immediate coastal areas, where summers are milder due to the nearby presence of cold ocean currents.
Examples: Palermo, Sicily (Csa)
Gaziantep, Turkey (Csa)
San Francisco, California (Csb)
Victoria, Canada (Csb).
- Humid Subtropical climates (Cfa, Cwa): These climates usually occur in the interiors of continents, or on their east coasts, between the latitudes of 25° and 40°. Unlike the Mediterranean climates, the summers are humid due to unstable tropical air masses, or onshore Trade Winds. In eastern Asia, winters can be dry (and colder than other places at a corresponding latitude) because of the Siberian high pressure system, and summers very wet due to monsoonal influence.
Examples: Houston, Texas (Cfa — uniform precipitation distribution)
Brisbane, Australia (Cfa — summer wetter than winter)
Yalta, Ukraine (Cfa — summer drier than winter)
Luodian, China (Cwa).
- Maritime Temperate climates (Cfb, Cwb): Cfb climates usually occur on the western sides of continents between the latitudes of 45° and 55°; they are typically situated immediately poleward of the Mediterranean climates, although in Australia this climate is found immediately poleward of the Humid Subtropical climate, and at a somewhat lower latitude. In western Europe, this climate occur in coastal areas up to 62° latitude. These climates are dominated all year round by the polar front, leading to changeable, often overcast weather. Summers are cool due to cloud cover, but winters are milder than other climates in similar latitudes.
Examples: Limoges, France (uniform precipitation distribution)
Langebaanweg, South Africa (summer wetter than winter)
Prince Rupert, British Columbia (summer drier than winter).
Bergen, Norway (Cfb - uniform precipitation distribution)
Cfb climates are also encountered at high elevations in certain tropical areas, where the climate would be that of a tropical rain forest if not for the altitude. Bogotá, Colombia is perhaps the best example.
Cwb is found only at higher altitudes, without which the climate would be tropical wet and dry; examples include Addis Ababa, Ethiopia and Mexico City.
- Maritime Subarctic climates (Cfc): These climates occur poleward of the Maritime Temperate climates, and are confined either to narrow coastal strips on the western poleward margins of the continents, or, especially in the Northern Hemisphere, to islands off such coasts.
Examples: Punta Arenas, Chile (uniform precipitation distribution)
Monte Dinero, Argentina (summer wetter than winter)
Tórshavn, Faroe Islands (summer drier than winter)
Bodø, Norway (uniform precipitation distribution).

GROUP D: Continental/microthermal climate

These climates have an average temperature above 10 °C in their warmest months, and a coldest month average below −3 °C (or 0 °C in some versions, as noted previously). These usually occur in the interiors of continents, or on their east coasts, north of 40° North latitude. Group D climates do not exist at all in the Southern hemisphere due to the smaller land masses here.
- The second and third letters are used as for Group C climates, while a third letter of d indicates 3 or fewer months with mean temperatures above 10 °C and a coldest month temperature below −38 °C (−36.4 °F).
- Group D climates are subdivided as follows:
- Hot Summer Continental climates (Dfa, Dwa, Dsa): Dfa climates usually occur in the low 40s in latitude, and in eastern Asia Dwa climates extend further south due to the influence of the Siberian high pressure system, which also causes winters here to be dry, and summers can be very wet because of monsoon circulation.
Examples: Lowell, Massachusetts (Dfa — uniform precipitation distribution)
Peoria, Illinois (Dfa — summer wetter than winter)
Santaquin, Utah (Dfa — summer drier than winter)
Beijing, China (Dwa).
Dsa exists only at higher elevations adjacent to areas with Mediterranean climates, such as Cambridge, Idaho and Saqqez in Iranian Kurdistan.
- Warm Summer Continental or Hemiboreal climates (Dfb, Dwb, Dsb): Dfb and Dwb climates are immediately north of Hot Summer Continental climates, generally in the high 40s in latitude, and also in central and eastern Europe, between the Maritime Temperate and Continental Subarctic climates, where it extends up to high 50s and even lowest 60 degrees latitude.
Examples: Moncton, New Brunswick (Dfb — uniform precipitation distribution)
Minsk, Belarus (Dfb — summer wetter than winter)
Revelstoke, British Columbia (Dfb — summer drier than winter)
Rudnaya Pristan, Russia (Dwb).
Stockholm, Sweden (Dfb — summer wetter than winter)
Dsb arises from the same scenario as Dsa, but at even higher altitudes, and chiefly in North America since here the Mediterranean climates extend further poleward than in Eurasia; Mazama, Washington is one such location.
- Continental Subarctic or Boreal (taiga) climates (Dfc, Dwc, Dsc): Dfc and Dwc climates occur poleward of the other Group D climates, mostly in the 50s North latitude, although it might occur as far north as 69° latitude.
Examples: Sept-Îles, Quebec (Dfc — uniform precipitation distribution)
Anchorage, Alaska (Dfc — summer wetter than winter)
Mount Robson, British Columbia (Dfc — summer drier than winter)
Irkutsk, Russia (Dwc).
Kirkenes, Finnmark (Dfc - summer wetter than winter)
Dsc, like Dsa and Dsb, is confined exclusively to highland locations near areas that have Mediterranean climates, and is the rarest of the three as a still higher altitude is needed to produce this climate. Example: Galena Summit, Idaho.
- Continental Subarctic climates with extremely severe winters (Dfd, Dwd): These climates occur only in eastern Siberia. The names of some of the places that have this climate — most notably Verkhoyansk and Oymyakon — have become veritable synonyms for extreme, severe winter cold.

GROUP E: Polar climates

These climates are characterized by average temperatures below 10 °C in all twelve months of the year:
- Tundra climate (ET): Warmest month has an average temperature between 0 °C and 10 °C. These climates occur on the northern edges of the North American and Eurasian landmasses, and on nearby islands; they also exist along the outer fringes of Antarctica (especially the Palmer Peninsula) and on nearby islands.
Examples: Iqaluit, Nunavut
Provideniya, Russia
Deception Island, Antarctica.
Longyearbyen, Svalbard
ET is also found at high elevations outside the polar regions, above the timber line — as at Mount Washington, New Hampshire.
- Ice Cap climate (EF): All twelve months have average temperatures below 0 °C. This climate is dominant in Antarctica (e.g., Scott Base) and in inner Greenland (e.g., Eismitte or North Ice).
- Occasionally, a third, lower-case letter is added to ET climates if either the summer or winter is clearly drier than the other half of the year; thus Qikiqtaruk, or Herschel Island, off the coast of Canada's Yukon Territory, becomes ETw, with Pic du Midi de Bigorre in the French Pyrenees acquiring an ETs designation. If the precipitation is more or less evenly spread throughout the year, ETf may be used, such as for Hebron, Labrador. When the option to include this letter is exercised, the same standards that are used for Groups C and D apply, with the additional requirement that the wettest month must have an average of at least 30 mm precipitation (Group E climates can be as dry or even drier than Group B climates based on actual precipitation received, but their rate of evaporation is much lower). Seasonal precipitation letters are almost never attached to EF climates, mainly due to the difficulty in distinguishing between falling and blowing snow, as snow is the sole source of moisture in these climates.

Trewartha climate classification scheme

The Trewartha climate classification scheme is a modified version of the Köppen system. It attempts to redefine the broad climatic groups in such a way as to be closer to vegetational zoning.
- Group A: This the tropical climate group, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C or above). Climates with no more than two dry months (defined as having less than 60mm average precipitation, same as per Köppen) are classified Ar (instead of Köppen's Af), while others are classified Aw if the dry season is at the time of low sun/short days or As if the dry season is at the time of high sun/long days. There was no specific monsoon climate identifier in the original scheme, but Am was added later, with the same parameters as Köppen's (except that at least three months, rather than one, must have less than 60mm average precipitation).
- Group B: BW and BS mean the same as in the Köppen scheme, with the Köppen BWn climate sometimes being designated BM (the M standing for "marine"). However, a different formula is used to quantify the aridity threshold: 10 X (T − 10) + 3P, with T equalling the mean annual temperature in degrees Celsius and P denoting the percentage of total precipitation received in the six high-sun months (April through September in the Northern Hemisphere and October through March in the Southern). If the precipition for a given location is less than the above formula, its climate is said to be that of a desert (BW); if it is equal to or greater than the above formula but less than twice that amount, the climate is classified as steppe (BS); and if the precipitation is more than double the value of the formula the climate is not in Group B. Unlike in Köppen's scheme, no thermal subsets exist within this group in Trewartha's, unless the Universal Thermal Scale (see below) is used.
- Group C: In the Trewartha scheme this category encompasses subtropical climates only (8 or more months above 10 °C). Cs and Cw have the same meanings as they do in Köppen's scheme, but the subtropical climate with no distinct dry season is designated Cr instead of Köppen's Cf (and for Cs the average annual precipitation must be less than 890mm [35 inches] in addition to the driest summer month having less than 30mm precipitation and being less than one-third as wet as the wettest winter month).
- Group D: This group represents temperate climates (4 to 7 months above 10 °C). Maritime temperate climates (most of Köppen's Cfb and Cwb climates, though some of these would fit into Trewartha's Cr and Cw respectively) are denoted DO in the Trewartha classification (although some places near the east coasts of both North America and Asia actually qualify as DO climates in Trewartha's scheme when they fit into Cfa/Cwa rather than Cfb/Cwb in Köppen's), while continental climates are represented as DCa (Köppen Dfa, Dwa, Dsa) and DCb (Köppen Dfb, Dwb, Dsb). For the continental climates, sometimes the third letter (a or b) is omitted and DC is simply used instead, and occasionally a precipitational seasonality letter is added to both the maritime and continental climates (r, w, or s, as applicable). The dividing point between the maritime and continental climates is 0 °C in the coldest month, rather than the Köppen value of −3 °C (as noted in the section on the Köppen scheme, however, some climatologists — particularly in the United States — now observe 0 °C in the coldest month as the equatorward limit of the continental climates in that scheme as well).
- Group E: This represents subarctic climates, defined the same as in Köppen's scheme (1 to 3 months with average temperatures of 10 °C or above; Köppen Cfc, Dfc, Dwc, Dsc, Dfd, Dwd). In the original scheme, this group was not further divided; later, the designations EO and EC were created, with EO (maritime subarctic) signifying that the coldest month averages above −10 °C, while EC (continental subartctic or "boreal") means that at least one month has an average temperature of −10 °C or below. As in Group D, a third letter can be added to indicate seasonality of precipitation. There is no separate counterpart to the Köppen Dfd/Dwd climate in Trewartha's scheme.
- Group F: This is the polar climate group, split into FT (Köppen ET) and FI (Köppen EF).
- Group H: Highland climates, in which altitude plays a role in determining climate classification. Specifically, this would apply if correcting the average temperature of each month to a sea-level value using the formula of adding 5.6°C for each 1,000 meters of elevation would result in the climate fitting into a different thermal group than that into which the actual monthly temperatures place it. Sometimes G is used instead of H if the above is true and the altitude is 500 meters or higher but lower than 2,500 meters; but the G or H is placed in front of the applicable thermal letter rather than replacing it — and the second letter used reflects the corrected monthly temperatures, not the actual monthly temperatures.
- Universal Thermal Scale: An option exists to include information on both the warmest and coldest months for every climate by adding a third and fourth letter, respectively. The letters used conform to the following scale:
i — severely hot: Mean monthly temperature 35 °C or higher
h — very hot: 28 to 34.9°C
a — hot: 23 to 27.9°C
b — warm: 18 to 22.9°C
l — mild: 10 to 17.9°C
k — cool: 0.1 to 9.9°C
o — cold: −9.9 to 0 °C
c — very cold: −24.9 to −10 °C
d — severely cold: −39.9 to −25 °C
e — excessively cold: −40 °C or below.
Examples of the resulting designations include Afaa for Kuala Lumpur, Malaysia, BWhl for Aswan, Egypt, Crhk for Dallas, Texas, DOlk for London, EClc for Arkhangelsk, Russia, and FTkd for Barrow, Alaska.

Criticisms of the Köppen scheme

Some climatologists have argued that Köppen's system could be improved upon. One of the most frequently-raised objections concerns the temperate Group C category, regarded by many as overbroad (it includes both Tampa, Florida and Cape May, New Jersey, for example). In Applied Climatology (first edition published in 1966), John Griffiths proposed a new subtropical zone, encompassing those areas with a coldest month of between 6 °C (42.8 °F) and 18 °C, effectively subdividing Group C into two nearly equal parts (his scheme assigns the letter B to the new zone, and identifies dry climates with an additional letter immediately following the temperature-based letter). Another point of contention involves the dry B climates; the argument here is that their separation by Köppen into only two thermal subsets is inadequate. Those who hold this view (including Griffiths) have suggested that the dry climates be placed on the same temperature continuum as other climates, with the thermal letter being followed by an additional capital letter — S for steppe or W (or D) for desert — as applicable (Griffiths also advances an alternate formula for use as an aridity threshold: R = 160 + 9T, with R equalling the threshold, in millimeters of mean annual precipitation, and T denoting the mean annual temperature in degrees Celsius). A third idea is to create a maritime polar or EM zone within Group E to separate relatively mild marine locations (such as Ushuaia, Argentina and the outer Aleutian Islands) from the colder, continental tundra climates. Specific proposals vary; some advocate setting a coldest-month parameter, such as −7 °C (19.4 °F), while others support assigning the new designation to areas with an average annual temperature of above 0 °C. The accuracy of the 10 °C warmest-month line as the start of the polar climates has also been questioned; Otto Nordenskiöld, for example, devised an alternate formula: W = 9 − 0.1 C, with W representing the average temperature of the warmest month and C that of the coldest month, both in degrees Celsius (for instance, if the coldest month averaged −20 ° C, a warmest-month average of 11 °C or higher would be necessary to prevent the climate from being polar). This boundary does appear to more closely follow the tree line, or the latitude poleward of which trees cannot grow, than the 10 °C warmest-month isotherm; the former tends to run poleward of the latter near the western margins of the continents, but at a lower latitide in the landmass interiors, the two lines crossing at or near the east coasts of both Asia and North America.

References

# # # # # # # # # # # # # # # #

External links

- [http://www.atm.dal.ca/~glesins/climatechange/koeppen.jpg Köppen climate classification] - world map
- [http://snow.ag.uidaho.edu/Clim_map/koppen_usa_map.htm Köppen Climate Classification] - United States
- Climate records
- [http://www.worldclimate.com WorldClimate]
- [http://www.weatherbase.com Weatherbase] ja:ケッペンの気候区分 Category:Climate

Rain

: For other uses see Rain (disambiguation). Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air. When none of it reaches the ground, it is a precipitation called virga.

Rain in nature

Rain plays a major role in the hydrologic cycle in which [http://wiktionary.org/wiki/moisture moisture] from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds. The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm). Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [http://www.ems.psu.edu/~fraser/Bad/BadRain.html] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain. Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.

Culture

lightning Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally has a sad and negative connotation — reflected in children's rhymes like Rain Rain Go Away — in contrast to the bright and happy sun. In dry places such as India and the Middle East, the rain is greeted with euphoria. Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be collected for drinking water since rainwater is pure, or used as greywater. Excessive rain, particularly after a dry period has hardened the soil so that it cannot absorb water, can cause floods. Many people find the scent smelt during and immediately after rain especially pleasant or distinctive. The source of this smell is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall.

Humidity

Humidity is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity, or relative humidity. A device used to measure humidity is called a hygrometer. A humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a thermometer and thermostat for climate control. Due to the changing partial pressure of water vapor in air as temperature changes, the water content of air at sea level can get as high as 3% at 30 °C (86 °F), and no more than about 0.5% at 0 °C (32 °F).

Absolute humidity

Absolute humidity defines the mass of water vapor in a given volume of moist air or gas, and is usually reported in grams per cubic meter, although grains per cubic foot is more commonly used in the United States.

Specific humidity

Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of water vapor to dry air. Specific humidity is expressed as a ratio of kilograms of water vapor,

m_w

, per kilogram of air,

m_a

. That ratio can be given as: :

x =

Partial pressure of water vapor and air can also be used to express the ratio.

Relative humidity

Relative humidity is the ratio of the current vapor pressure of water in any gas (especially air) to the equilibrium vapor pressure, at which the gas is called saturated at the current temperature, expressed as a percentage. Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas. The numerators of these ratios are the two ways of expressing absolute humidity. The following graph compares dew point (maximum humidity in red) to 50% relative humidity (green line halfway between zero and the dew point across the range of temperatures). Image:relative_humidity.jpg A gas in this context is referred to as saturated when the vapor pressure of water is at the equilibrium vapor pressure for water vapor; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It also corresponds to the possibilility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of droplets of liquid. (Even though these droplets may be so small as to fall imperceptibly slowly through the mixed gas we call air, this behavior is too different from that of water vapor to reflect it in the same scale. This explains the restriction of relative-humidity discussions to 100% and below.) The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.

Effects on human body

Humidity can make the warmth of the surrounding air feel as if it is warmer than the actual temperature would suggest, because the cooling effect of evaporation from the skin is reduced. This is because as environmental temperatures approach normal skin temperature, cooling of the body becomes more difficult. If the atmosphere is as warm as or warmer than the skin, blood brought to the body surface cannot lose its heat. Under these conditions, the heart continues to pump blood to the body surface, the sweat glands pour liquids containing electrolytes onto the surface of the skin and the evaporation of the sweat becomes the principal effective means of maintaining a constant body temperature. Sweating does not cool the body unless the moisture is removed from the skin by evaporation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs; physical strength declines; and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. First spoken by Warren Hymer in the 1939 movie Mr. Moto on Danger Island, the expression It's not the heat, it's the humidity refers to unpleasantly muggy weather; that is, the fact that humid air can be significantly less comfortable than drier air at the same temperature.

External links

- [http://nsidc.org/arcticmet/glossary/specific_humidity.html Glossary definition of specific humidity]
- [http://web.archive.org/web/20041010063646/http://www.nsdl.arm.gov/Library/glossary.shtml Archived National Science Digital Library - Absolute Humidity]
- [http://nsidc.org/arcticmet/glossary/psychrometric_tables.html Glossary definition of psychrometric tables] Category:Weather Category:Psychrometrics Category:Meteorology Humidity ja:湿度

Steppes

In physical geography, a steppe (from Russian step') is a plain without trees (apart from those near rivers and lakes); it is similar to a prairie, although a prairie is generally considered as being dominated by tall grasses, while short grasses are said to be the norm in the steppe. It may be semi-desert, or covered with grass or shrubs, or both depending on the season. The term is also used to denote the climate encountered in such regions, too dry to support a forest but not so dry as to make it a desert. They are usually found in areas of the world less prone to moisture. The soil is considered too moist to be a desert, but also too dry to support normal forest life. The climate of steppes can be summarized by hot summers and cold winters, averaging 10-20 inches of rain every year. Plant life is usually greater than one foot tall, including the blue grama and buffalo grass, cacti, sagebrush, speargrass, and other small relatives of the sunflower. Animal life includes but isn't limited to the Corsac Fox, Mongolian Gerbil, Saiga Antelope, Northern Lynx, and the Saker Falcon. The world's largest zone of steppes are found in central Russia and neighbouring republics of Central Asia. The steppes begin east of the Volga river and extend through desert or semi-desert south of the Ural Mountains and to the north and east of the Caspian Sea. To the east of the Caspian Sea they extend through Turkmenistan, Uzbekistan and Kazakhstan to the mountain ranges of Mongolia, China, Kyrgyzstan, Tajikistan and Afghanistan. To the north on the eastern side of the Urals is the forested West Siberian Plain which extends nearly to the Arctic Ocean. Another large steppe area is located in the central United States and western Canada. The High Plains steppe is the westernmost part of the Great Plains region. Category:Grasslands Category:Climate

North Slope

North Slope can refer to:
- Alaska North Slope
- North Slope Borough, Alaska
- North Slope neighborhood of Tacoma, Washington

Brooks Range

The Brooks Range is a mountain range that stretches from west to east across northern Alaska and into Canada's Yukon Territory, a total distance of about 1100 km (700 mi). The mountains are not especially high, topping out at over 2,700 m (9,000 ft), and well north of the tree-line, so they are mostly covered by tundra. Mount Isto is the highest peak in the range, at 2762m (9060 ft.) The range is mostly uninhabited, but the Dalton Highway and the Alaska Pipeline run through the Atigun Pass (1,415 m, 4,643 ft) on their way to the North Slope and the oil fields at Prudhoe Bay. The Alaska Native villages of Anaktuvuk and Arctic Village, as well as the very small communities of Coldfoot, Wiseman, and Bettles are the only settlements in the 700-mile Brooks Range. As one of the remotest and least-disturbed wildernesses of North America, the mountains are teeming with wildlife, including Dall sheep, grizzly bears, and caribou. The range was named by the USGS in 1925 after Alfred Hulse Brooks, who was the chief USGS geologist for Alaska from 1903 to 1924. Various historical records also referred to the range as the Arctic Mountains, Hooper Mountains, Meade Mountains and Meade River Mountains — and the Canadian portion is still often referred to as the British Mountains. The British Mountains are part of Ivvavik National Park. Category:Mountain ranges of Alaska Category:Yukon mountains

Evaporation

Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapour increases. When the process of escape and return reaches an equilibrium, the vapour is said to be "saturated," and no further change in either vapour pressure and density or liquid temperature will occur. Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the entropy change for evaporation (ΔH_evaporation) is always positive. Forced evaporation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component (e.g. water), leaving behind the dry, less volatile, component. It is a misconception that at 1 atm, water vapour only exists at 100°C. Water molecules are in a constant state of evaporation and condensation flux near the surface of liquid water. If a surface molecule receives enough energy, it will leave the liquid and turn into vapour pending an allowable vapor pressure. Under a pressure of 1 atm, water will boil at 100°C.

Factors influencing rate of evaporation

- Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
- Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
- Temperature of the substance. If the substance is hotter, then evaporation will be faster.
- Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.
- Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid or solid state the more energy that must be input in order to evaporate them.

Combustion vaporisation

The fuel droplets vapourise as they receive heat by mixing with the hot gases in the combustion chamber. Heat(energy) can also be received by radiation from any hot refractory wall of the combustion chamber.

Snow

) high forests.]] Snow is precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes. Since it is composed of small rough particles it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snow is commonly formed when water vapor undergoes deposition high in the atmosphere at a temperature of less than 0°C (32°F), and then falls to the ground.

Types

Flurries are similar to rainshowers and only last for short periods of time. Snow which has partially thawed while falling is called sleet; if this re-freezes on further descent, the resulting small icy pellets or granules of snow are called soft hail. A related phenomenon is freezing rain, where rain falls on ground sufficiently cold for it to freeze on contact, forming black ice on the ground. A snow squall is a brief, very intense snowstorm while a blizzard is a long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteout conditions where visibility is reduced to less than 1 m, while blizzards can also create large snowdrifts. A ground blizzard occurs when a strong wind drives already fallen snow to create drifts and whiteouts. Snow can be also manufactured using snow cannons, which actually create tiny granules more like soft hail (this is sometimes called "grits" by those in the southern U.S. for its likeness to the texture of the food). In recent years, snow cannons have been produced that create more natural looking snow, but these machines are very expensive and are found only on the most prestigious places.

Occurrence

Snowfall varies by time and location, including geographic latitude, elevation and other factors which affect weather in general. In latitudes closer to the equator, there is less chance of snow fall, 35° N and 40°S are often quoted as a rough delimiter. The western coasts of the major continents remain snowless to much higher latitudes. As temperature decreases with altitude, high mountains, even at or near the Equator, have permanent snow cover on their top. Examples include Mount Kilimanjaro in Tanzania and the Tropical Andes in South America; the only snow actually on the Equator is at 4,690 m altitude on the south slope of Volcán Cayambe in Ecuador (Google Earth images). Conversely, many regions of the Arctic and Antarctic receive very little precipitation and therefore little snow despite the bitter cold (below a certain temperature, air essentially loses its ability to carry water vapor). Although density of fresh snow varies widely, a guide is that the depth of snowfall is 10 times that of a rainfall containing the same mass of water. Substantial snowfall sometimes disrupts infrastructure and services even in regions that are accustomed to them. Traffic may be snarled or even completely stop. Basic infrastructure such as electricity, phones and gas supply can be shut down. This can lead to a snow day, a day on which school or other services are cancelled owing to unusually heavy snowfall. In areas that normally have very little snow, this may occur even with light accumulation — something often made fun of by those people used to colder climates, where streets would remain passable given the same amount of snow. The highest seasonally cumulative precipitation of snow ever measured in the world was on Mount Baker, Washington, U.S.A during 1998–1999 season when they received 28.96 meters (1,140 in); this surpassed the previous record holder, Mount Rainier, Washington, U.S.A which during 1971–1972 season received 28.5 meters (1,122 in) of snow; and the world record daily precipitation was record

Desert

Types of desert

Montane deserts

Desert features

Soils

Vegetation

Water

Mineral resources

List of deserts

Americas

Africa

Asia-Pacific

See also

Precipitation (meteorology)

Orographic precipitation

Convectional rainfall

Frontal Rainfall

Rainfall Patterns

Western

Eastern and Central

Inland Continental Areas

See also

Reg

See also

Additionally

Erg

Hamada

External links

Rock (geology)

See also

External links

Eolian

Eolian erosion

Eolian transportation

Eolian deposition

Köppen climate classification

The scheme

GROUP A: Tropical/Megathermal climates

GROUP B: Dry (Arid and semiarid) climates

GROUP C: Temperate/mesothermal climates

GROUP D: Continental/microthermal climate

GROUP E: Polar climates

Trewartha climate classification scheme

Criticisms of the Köppen scheme

References

External links

Rain

Rain in nature

Culture

See also

Humidity

Absolute humidity

Specific humidity

Relative humidity

Effects on human body

See Also

External links

Steppes

North Slope

Brooks Range

Evaporation

Factors influencing rate of evaporation

Combustion vaporisation

See also

Snow

Types

Occurrence