Climate

The climate is the weather averaged over a long period of time. 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.

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 — one of the most popular methods of climate classification is that developed by Vladimir Köppen:

Köppen's climate classification scheme

• GROUP A - Megathermal or Tropical climates: Tropical climates (see tropics) are characterized by constant high temperature - all twelve months of the year have average temperatures of 18 °C (64.4 °F) or higher. They are subdivided as follows:
  • Af (Tropical rain forest) climate: - all twelve months have average precipitation above 60 mm. These climates, usually occurring within 5° latitude of the equator, are dominated by the Doldrums Low Pressure all year round, and thus have no seasons.
    Examples: - Singapore; Andagoya, Colombia. 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.
  • Am (Tropical monsoon) climate: This type of climate, most common in southern Asia and West Africa, results from the monsoon winds which switch direction according to the seasons. This climate has a driest month 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 known 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. Tegal, Indonesia and Miami, Florida are included among these locations.
  • Aw (Tropical wet and dry) climate - these climates have a pronounced dry season, with the driest month having precipitation less than (100 - (total annual precipitation (mm)/25)) mm:
    Examples: Dongfang, China; Tampico, Mexico. Most places that have this climate are found at the outer margins of the tropical zone, but occasionally an inner-tropical location (i.e., San Marcos, Colombia) also qualifies.
    Sometimes As is used instead of Aw if the dry season occurs during the time of higher sun and longer days. This is the case in parts of Hawaii, East Africa and Sri Lanka; 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 climates: These climates are characterized by the fact that precipitation is less than evaporation. The threshold is determined as follows:
  • To find the precipitation threshold (in millimetres), multiply the average annual temperature in °C by 20, then add 280 (if 70%+ precipitation is in warmest 6 months) or 140 (if 30%-70% precipitation is in warmest 6 months).
  • 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 added to indicate temperature: h signifies hot climate (average annual temperature above 18 °C), while k signifies temperate climate (average annual temperature below 18 °C). In some schemes h means the coldest month has an average temperature that is above freezing and k means that at least one month averages below freezing.
  • Examples: In Salah, Algeria (BWh)
    Turpan, China (BWk)
    Cobar, Australia (BSh)
    Williston, North Dakota (BSk)
  • Rarely, 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 60mm of average precipitation if the temperature regime is tropical, or 30mm if it is not (but the letter used indicates when the dry season occurs, not the "wet" one). Examples would include Khartoum, Sudan as BWhw, Niamey, Niger as BShw, Al Arish, Egypt as BWhs, Asbiah, Libya as BShs, Omnogovi, Mongolia as BWkw, and Xining, China as BSkw (BWks and BSks do not exist).

• GROUP C - Mesothermal or temperate 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.
  • 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), 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 rain in all seasons (neither above mentioned condition fulfilled).
  • The third letter indicates the degree of summer heat - a indicates warmest month average temperature above 22 °C, 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.
  • Group C climates may be divided as follows:
    • Mediterranean climates (Csa, Csb) - these climates usually occur on the western sides of continents between the latitudes of 30° and 45°. These climates are in the polar front region in winter, and thus have moderate temperatures and changeable weather. Summers are hot and dry, due to the domination of the subtropical high pressure systems.
      Examples: Lisbon, Portugal (Csa)
      Athens, Greece (Csa)
      Santiago, Chile (Csb)
      San Francisco, California (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 mainland Asia, winters are sometimes dry, due to monsoonal influence.
      Examples: Memphis, Tennessee (Cfa)
      Buenos Aires, Argentina (Cfa)
      New Delhi, India (Cwa)
      Tokyo, Japan (Cfa).
    • Maritime Temperate climates (Cfb) - these climates usually occur on the western sides of continents between the latitudes of 45° and 55°. 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: London, England
      Vancouver, British Columbia
      Valdivia, Chile
      Christchurch, New Zealand.
      Cwb is found only at higher altitudes, if not for which the climate would have been 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.
      Examples: Punta Arenas, Chile
      Torshavn, Faroe Islands.

• GROUP D - Microthermal or Continental climates: These climates have an average temperature above 10 °C in their warmest months, and a coldest month average below -3 °C. 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, except along a few intermediate slopes of the Andes mountains.
  • 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.
  • Group D climates may be divided as follows:
    • Hot Summer Continental climates (Dfa, Dwa) - these climates usually occur in the forties latitudes. In mainland Asia, winters are sometimes dry due to the influence of the Siberian anticyclone, and summers are very wet because of monsoon circulation.
      Examples: Chicago, Illinois (Dfa)
      Beijing, China (Dwa).
      Dsa also exists, but only at higher elevations adjacent to areas with Mediterranean climates, such as parts of Idaho and Iranian Kurdistan.
    • Warm Summer Continental climates (Dfb, Dwb) - these climates are immediately north of Hot Summer Continental climates, and also in central and eastern Europe, between the Maritime Temperate and Continental Subarctic climates.
      Examples: Montreal, Quebec (Dfb)
      Warsaw, Poland (Dfb)
      Vladivostok, Russia (Dwb).
      Dsb arises from the same scenario as Dsa, but at even higher altitudes.
    • Continental Subarctic climates (Dfc, Dwc) - these climates occur poleward of the other Group D climates, mostly north of 50° North latitude.
      Examples: Sept-Iles, Quebec, Irkutsk, Russia.
      Dsc, like Dsa and Dsb, is confined exclusively to high-altitude locations near lowlands that have Mediterranean climates, and is the rarest of the three as a still higher altitude is needed to produce this climate.
    • Continental Subarctic Climates With Severe Winters (Dfd, Dwd) - these climates occur only in eastern Siberia. The names of some of the places that have this climate - such as Verkhoyansk and Oimekon - are 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:
  • ET (Tundra) climate: - 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.
    Examples: - Point Barrow, Alaska
    Norilsk, Russia
    Svalbard, Norway. ET is also found at high elevations outside the polar regions, above the so-called timber line.
  • EF (Ice Cap) climate: - all twelve months have average temperatures below 0 °C. This climates is dominant in Antarctica and in inland Greenland.
  • Occasionally, a third, lower-case letter is added to ET climates if either the summer or winter is cleary drier than the other half of the year; thus Pic du Midi in the French Pyrenees becomes ETs with Qikiqtaruk, or Herschel Island, off the coast of Canada's Yukon Territory, acquiring an ETw designation (if the precipitation is more or less evenly spread throughout the year, ETf may be used, such as for Hebron, Labrador). 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.

See also climate change.