Trewartha climate classification

The Trewartha climate classification (TCC), or the Köppen–Trewartha climate classification (KTC), is a climate classification system first published by American geographer Glenn Thomas Trewartha in 1966. It is a modified version of the Köppen–Geiger system, created to answer some of its deficiencies. The Trewartha system attempts to redefine the middle latitudes to be closer to vegetation zoning and genetic climate systems.

Scheme Trewartha's modifications to the 1884 Köppen climate system sought to reclass the middle latitudes into three groups, according to how many months have a mean temperature of 10 °C (50 °F) or higher:

C (subtropical)—8 or more months; D (temperate)—4 to 7 months; E (boreal)—1 to 3 months. The tropical climates and polar climates remained the same as in the original Köppen climate classification.

Group A: Tropical climates This is the tropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C (64.4 °F) or above). The A climates are the realm of the winterless frost-free zone.

Climates with no more than two dry months (defined as having less than 60 mm (2.4 inches) average precipitation, same as per Köppen) are classified Ar. 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 BW and BS mean the same as in the Köppen scheme. 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, October through March in the Southern).

If the precipitation for a given location is less than the above formula — that is, if P < 10(T − 10) + 3P — the climate is said to be that of a desert (BW). If it is equal to or greater than the formula but less than twice that amount, the climate is classified as steppe (BS). 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 In the Trewartha scheme the C climate group encompasses subtropical climates, which have 8 or more months with a mean temperature of 10 °C (50 °F) or higher. There are only three types within the C or subtropical climate group:

Cs, which is a dry-summer or mediterranean climate; Cf, or humid subtropical climate with no dry season; Cw, a monsoon-influenced humid subtropical climate with dry winters.

Group D: Temperate and continental climates In the Trewartha scheme the D climate group encompasses temperate climates that have 4 to 7 months with a mean temperature of 10 °C (50 °F) or higher. D climate groups have two types:

Oceanic (Do), where the coldest month has a mean temperature 0 °C (32 °F) or higher Continental (Dc), where the coldest monthly mean temperature reaches below 0 °C, as in some interior landmasses in North America and Asia. For the continental climates (Dc), sometimes a third letter (a or b) is added to denote a hot or cold summer. Dca is used where the warmest month has a mean temperature of 22.2 °C (72.0 °F) or higher, and Dcb is used for cool-summer temperate climates, where the warmest month has a mean temperature below 22.2 °C. Most of Europe north of the 44th parallel exhibits a Do or Dc climate type.

Group E: Boreal climates This represents subarctic and subpolar oceanic climate realms, defined the same as in Köppen's scheme, where 1 to 3 months have an average temperature of 10 °C (50 °F) or above. In this climate zone there is only a short period (normally 50 to 90 days) that is frost free. In the original scheme, this group was not further divided; later, the designations Eo and Ec were created:

Eo (maritime subarctic) signifies that the coldest month averages above −10 °C (14 °F). 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 are no separate counterparts to the Köppen Dfd, Dwd, and Dsd climate types in Trewartha's scheme, but a letter can optionally be added to the end of the symbol to indicate the temperature of the coldest month (see below).

Group F: Polar climates This is the polar climate group, where all months have a monthly mean air temperature below 10 °C (50 °F). Polar climates have two subtypes, Ft (tundra) and Fi (ice cap):

In the Ft climate type, at least one month has an average temperature above 0 °C (32 °F) (but not above 10 °C (50 °F)), so that there is a brief time when the surface might be free of snow or ice and a scrub or tundra vegetation cover is possible. In the Fi climate type, all months have an average temperature below 0 °C (32 °F). This is the region of the vast deserts of perpetually frozen ocean in the North Pole, and the permanent ice plateaus of Antarctica and Greenland.

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, denoting mean monthly temperature, conform to the following scale:

Examples

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