Climate

The climate is the weather averaged over a long period of time. A descriptive saying is that "climate is what you expect, weather is what you get." The Intergovernmental Panel on Climate Change (IPCC) glossary definition is:

Climate vs. Weather

The exact boundaries of what is climate and what is weather are not well defined and depend on the application. For example, in some senses an individual El Niño event could be considered climate; in others, as weather. When the original conception of climate as a long-term average came to be considered, perhaps towards the end of the 19th century, the idea of climate change was not current, and a 30 year average seemed reasonable (but see note 1). Given the current availability of long-term trends in the temperature record, it is harder to give a precise contradiction-free definition of climate: over a 30 year period, averages may shift; over a shorter period, the statistics are less stable. In a given geographical region, the climate generally does not vary over time on the scale of a human life span. However, over geological time, climate can vary considerably for a given place on the Earth. For example, Scandinavia has been through a number of ice ages over hundreds of thousands of years (the last one ending about 10,000 years ago). Paleoclimatology is the study of these past climates.

Classifications

In the original sense, climate is a concept used to divide the world into regions sharing similar climatic parameters. Climate regions can be classified on the basis of temperature and precipitation alone. Examples of such climate schemes are the Köppen climate classification or the Thornthwaite climate classification schemes.

Tropical Climate

The tropics are the geographic region of the Earth centered on the equator and limited in latitude by the two tropics: the Tropic of Cancer in the north and the Tropic of Capricorn in the southern hemisphere. This area lies approximately between 23.5° N latitude and 23.5° S latitude, and includes all the parts of the Earth where the sun reaches a point directly overhead at least once during the solar year (north of the Tropic of Cancer and south of the Tropic of Capricorn the sun never reaches an altitude of 90° or directly overhead). The word "tropics" comes from Greek tropos meaning "turn", because the apparent position of the Sun oscillates between the two tropics with a period which we call a year. Tropical plants and animals are those species native to the tropics. Tropical is also sometimes used in a general sense of a place that is warm and moist year-round, often with the sense of lush vegetation. However, there are places in the tropics that are anything but "tropical" in this sense, even with alpine tundra and snow-capped peaks! (See, for example, Mauna Kea). In Köppen's scheme of climate classification, a tropical climate is defined as a non-arid climate in which all twelve months have mean temperatures above 18 °C (64.4 °F).

Subtropical Climate

Subtropical climate is the form of climate found immediately poleward of the zone of tropical climate. There are various definitions of what characteristics make a climate "subtropical." Climatologist John Griffiths, for example, includes all places with a mean temperature in their coldest month of 6 °C (42.8°F) or warmer but colder than 18° C (64.4°F), while in Glenn Trewartha's scheme this climate is defined as having eight or more months with mean temperatures of 10 °C (50 °F) or warmer but with at least one month averaging colder than 18 °C. Precise conditions vary tremedously within the subtropical climate zone, as it is one of the largest thermal climate zones on Earth. Precipitation traits may be humid, semiarid or arid (some of the world's great deserts fit into the category) and summers may be hot, or merely warm. Most places that have Mediterranean climates (with a pronounced dry season in the summer) are subtropical from a thermal standpoint.

Desert Climate

In geography, a desert is a landscape form or region that receives little precipitation. As a consequence, 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. Desert landscapes have certain common features. Desert soil is often composed mostly of sand and sand dunes may be present. 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.

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, aridity 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 the 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.” 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.

Semiarid climate (Steppe)

In physical geography, steppe (from Slavic step') is a plain without trees (apart from those near rivers and lakes); it is similar to a prairie, although a prairie is generally reckoned 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. 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.

Mediterranean Climate

A Mediterranean climate is one that resembles those of the lands bordering the Mediterranean Sea. It is generally encountered between 30 to 50 degrees north and 30 to 40 degrees south of the Equator.

In most places with Mediterranean climates, rainfall results almost entirely from the procession of the westerly zone of frontal storms during the low-sun season; during the summer, the subtropical high pressure zone dominates the seasonal weather, preventing rainfall by inhibiting rainstorms from forming. While examples of places with wet winters and dry summers exist in every thermal-based climate zone on Earth from the equatorial to the polar, the greatest concentration of Mediterranean climates is encountered immediately poleward of the great dry belt of subtropical deserts but equatorward of the zone of maritime temperate or oceanic climates. They are never on the east coasts of continents. As an example, San Francisco in California, USA, has an average of 448 mm (17.6 in.) of rain from November through April each year, but averages only 52 mm (2 in.) of rain for the rest of the year. Because of the proximity to large bodies of water, temperatures are generally moderate with a comparatively small range of temperatures between the winter low and summer high, although the daily range of temperatures during the summer is large, except along the immediate coasts. Temperatures during winter rarely reach freezing (except in areas with a high elevation), and snow is almost unheard of. In the summer, the temperatures, while warm, don't reach the high levels of inland desert areas. The average temperature for most lowland locations having this climate is about 10°C in winter and roughly 27°C in summer. The subtropical western coasts of continental landmasses typically possess such climates. Examples include Coastal California, Western Australia, Cape Province in South Africa, and central Chile. Inland locations sheltered from or distant from sea breezes can experience severe heat during the summer. Locations inside the Sacramento Valley of northern California, for example, are subject to summer temperatures characteristic of hot deserts (often around 40°C), although winters are rainy enough to allow lush vegetation. Unlike the coastal climates that have Köppen's Csb classification characteristic of places with cooler summers, the hotter, typically inland areas have the Csa classification that indicates a hot summer. Areas of high altitude adjacent to locations with Mediterranean climates may have the cold winters that are characteristic of a continental climate; under Köppen's scheme such places might earn the designation Dsa, Dsb or even Dsc.

Temperate Climate

In geography, temperate latitudes of the globe lie between the tropics and the polar circles. The north temperate zone extends from the Tropic of Cancer at about 23.5 degrees north latitude to the Arctic Circle at about 66.5 degrees north latitude. The south temperate zone extends from the Tropic of Capricorn at about 23.5 degrees south latitude to the Antarctic Circle at about 66.5 degrees south Latitude. Regions with a temperate climate are usually characterized by having roughly equally long winters and summers. Temperature extremes are uncommon in temperate climates but do occur in regions also affected by outer limit of their temperature tolerances. Leaf bearing trees dominate this climate close to the coastal regions and ferns dominate the northern region as well as some inland parts.

Oceanic Climate

An oceanic climate is the climate typically found along the west coasts at the middle latitudes of all the world's continents, and in southeastern Australia; similar climates are also found at high elevations within the tropics. Oceanic climates are characterized by a narrower annual range of temperatures than are encountered in other places at a comparable latitude, and differ from Mediterranean climates in that significant amounts of precipitation are received in summer. This means that precipitation is both adequate and reliable at all times of the year in oceanic climates, except in certain tropical highland areas, which would have tropical savanna climates (with a dry season in winter) if not for the high altitude making them cooler. Overall temperature characteristics vary among oceanic climates; those at the lowest latitudes are subtropical from a thermal standpoint, but more commonly a mesothermal regime prevails, with cool, but not cold, winters. Poleward of the latter is a zone of subpolar oceanic climate, with a summer growing season of less than four months; examples of this climate include much of coastal Iceland in the Northern Hemisphere and extreme southern Chile in the Southern Hemisphere (the fact that this form of climate exists in both hemispheres ruling out the use of such terms as subarctic or boreal to denote it; even though these terms are used to refer to climates characterized by short summers, they are also synonyms for "northern" and therefore inappropriate). With one significant exception, oceanic climates are classified as humid with respect to precipitation, the only major example of a dry oceanic climate being that of Patagonia.

Continental Climate

A continental climate is the climate typical of the interiors of the large continents of the Northern Hemisphere; similar climates exist along the east coasts (but not the west coasts) of the same continents, and also at higher elevations in certain other parts of the world. This climate is characterized by winter temperatures cold enough to support a fixed period of stable snow cover each year, and relatively low precipitation occurring mostly in summer - although east coast areas (chiefly in North America) may show an even distribution of precipitation. Only a few areas in Iran, adjacent Turkey and Central Asia show a winter maximum in precipitation, which typically melts in early spring to give short-lived floods. The Midwestern United States and most of Russia are examples of areas of the world with continental climates, which do not exist at all in the Southern Hemisphere due to the lack of broad land masses at high enough latitudes there.

Alpine Climate

Alpine climate is the average weather (climate) for a region above the tree-line. This climate is colder at high elevations, due to the lapse rate of air: air will tend to get colder as it rises, since it expands. The dry adiabatic lapse rate is 10°C per km of elevation or altitude. Therefore, moving up 100 meters on a mountain is roughly equivalent to moving 80 kilometers (45' (0.75°) of latitude) towards the pole. This relationship is only approximate, however, since local factors such as proximity to oceans can drastically modify the climate. There have been several attempts at quantifying what constitutes an alpine climate. Climatologist Wladimir Köppen demonstrated a relationship between the Arctic and Antarctic tree lines and the 10°C summer isotherm; i.e., places where the average temperature in the warmest calendar month of the year is below 10°C cannot support forests. See Köppen climate classification for more information. However, Otto Nordenskiöld theorized that winter conditions also play a role: His formula is W = 9 - 0.1 C, with W denoting the average temperature in the warmest month and C the average of the coldest month, both in degrees Celsius (this would mean, for example, that if a particular location had an average temperature of -20°C in its coldest month, the warmest month would need to average 11°C or higher for trees to be able to survive there). Nordenskiöld's line tends to run to the north of Köppen's near the west coasts of the Northern Hemisphere continents, south of it in the interior sections, and at about the same latitude along the east coasts of both Asia and North America. In the Southern Hemisphere, all of Tierra del Fuego lies outside the polar region in Nordenskiöld's system, but part of the island (including Ushuaia, Argentina) is reckoned as being within the Antarctic under Köppen's. In 1947, Holdridge improved on these schemes, by defining biotemperature: the mean annual temperature, where all temperatures below 0°C are treated as 0°C (because it makes no difference to plant life, being dormant). If the mean biotemperature is between 1.5°C and 3°C, Holdridge quantifies the climate as alpine (or subpolar, of the low temperature is caused by latitude).

Subarctic Climate

Regions having a subarctic climate (also called boreal climate) are characterized by very cold winters, and brief, warm summers. This type of climate offers some of the most extreme seasonal temperature variations found on the planet: In winter, temperature can drop to -40°C (also -40°F) and in summer, the temperature may reach 30°C (86°F). The subarctic climate is a subset of the continental climate. Vegetation in the subarctic climate is generally sparse, as only hardy species can survive the long winters and make use of the short summers. Agriculture is almost nonexistent, and confined to vegetables that can grow very quickly. Trees are mostly limited to ferns and evergreen conifers, as few broadleafed trees are able to survive the very low temperatures in winter; this type of forest is also known as taiga, a term which is sometimes applied to the climate found therein as well.

Polar Climate

Regions with a polar climate are characterized by a lack of warm summers (specifically, no month having an average temperature of 10°C or higher), resulting in the absence of trees in such places, which may also be covered with glaciers or a permanent or semi-permanent layer of ice. There are two distinct subtypes of polar climate. The less severe of the two is the tundra climate, where at least one month has an average temperature of above freezing, while the colder one — known by various names including the ice cap climate and the perpetual frost climate — features sub-freezing average temperatures year-round. The only continent where the polar climate is predominant is Antarctica. All but a few isolated coastal areas on the island of Greenland also have a polar climate.