Trewartha climate classification

From Wikipedia, the free encyclopedia
Jump to: navigation, search

The Trewartha climate classification is a climate classification system published by American geographer Glenn Thomas Trewartha in 1966, and updated in 1980. It is a modified version of the 1899 Köppen system, created to answer some of the deficiencies of the Köppen system.[1] The Trewartha system attempts to redefine the middle latitudes to be closer to vegetation zoning and genetic climate systems. It was considered[by whom?] a more true or "real world" reflection of the global climate.[2] The changes were seen as most effective in Asia and North America, where many areas fall into a single group (C) in the Köppen system.[3] For example, under the standard Köppen system, western Washington and Oregon are classed into the same climate zone as southern California, even though the two regions have strikingly different weather and vegetation. The Köppen system also classes parts of the Northeastern US like New York City into the same zone as Louisiana or South Carolina.[4]

Scheme[edit]

Trewartha's modifications sought to reclass the middle latitudes into three groups: C (subtropical)—8 or more months have a mean temperature of 10 °C (50 °F) or higher; D (temperate)—4 to 7 months have a mean temperature of 10 °C or higher; and E (boreal or subarctic)—1 to 3 months have a mean temperature of 10 °C or higher.

Group A: Tropical climates[edit]

This the tropical climate realm, 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 60 mm 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 60 mm average precipitation).

Group B: Dry (arid and semi-arid) climates[edit]

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(T − 10) + 3P, with T equaling 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).[5]

If the precipitation 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: Subtropical climates[edit]

In the Trewartha scheme this category encompasses Subtropical climates (C) 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 890 mm (35 in) in addition to the driest summer month having less than 30 mm precipitation and being less than one-third as wet as the wettest winter month).

Group D: Temperate and continental climates[edit]

This group represents Temperate climates (D) with (4 to 7 months above 10 °C). Temperate oceanic maritime 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 like Philadelphia 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 Temperate 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 precipitation 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 −3 °C in the coldest month, 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: Boreal climates[edit]

This represents subarctic climate realms, 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 subarctic 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: Polar climates[edit]

This is the polar climate group, split into FT (Köppen ET) and FI (Köppen EF). All months must have a monthly mean air temperature of below 10°C. The subtypes are Ft (tundra) with the warmest month’s air temperature above 0°C and Fi (ice cap) where the air temperature in all months remains below 0°C.

Group H: Highland climates[edit]

Highland climates, in which altitude plays a role in determining climate classification.[6] 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[edit]

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 (95 °F) or higher
h — very hot: 28 to 34.9 °C (82.4 to 94.8 °F)
a — hot: 23 to 27.9 °C (73.4 to 82.2 °F)
b — warm: 18 to 22.9 °C (64.4 to 73.2 °F)
l — mild: 10 to 17.9 °C (50.0 to 64.2 °F)
k — cool: 0.1 to 9.9 °C (32.2 to 49.8 °F)
o — cold: −9.9 to 0 °C (14.2 to 32.0 °F)
c — very cold: −24.9 to −10 °C (−12.8 to 14.0 °F)
d — severely cold: −39.9 to −25 °C (−39.8 to −13.0 °F)
e — excessively cold: −40 °C (−40 °F) or below.

Examples of the resulting designations include Afaa for Surabaya, Indonesia, BWhl for Aswan, Egypt, Crhk for Dallas, Texas, U.S. DObk for London, EClc for Arkhangelsk, Russia, and FTkd for Barrow, Alaska, U.S..

See also[edit]

  • Holdridge life zones climate classification by three dimensions: precipitation, humidity, and potential evapotranspiration ratio

References[edit]

  1. ^ Peel MC, Finlayson BL, McMahon TA (2007) Updated world map of the Köppen-Geiger climate classification. Hydrol Earth Syst Sci 11: 1633−1644
  2. ^ Akin, Wallace E. (1991). Global Patterns: Climate, Vegetation, and Soils. University of Oklahoma Press. p. 52. ISBN 0-8061-2309-5. 
  3. ^ Köppen, 1936, Trewartha & Horn 1980, Bailey 2009, Baker et al. 2010
  4. ^ Bailey RG (2009) Ecosystem geography:from ecoregions to sites, 2nd edn. Springer, New York, NY
  5. ^ Patton CP (1962) A note on the classification of dry climate in the Köppen system. California Geographer 3: 105−112
  6. ^ McKnight, 237–40

External links[edit]