Trewartha climate types for the contiguous United States
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.[1] The Trewartha system attempts to redefine the middle latitudes to be closer to vegetation zoning and genetic climate systems.[2]
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:
The tropical climates and polar climates remained the same as in the original Köppen climate classification.
The "highland" climate is ambiguously defined. Newer users of KTC generally omit this option.[3]
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).[4]
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 two types within the C or subtropical climate group:
Cf, or humid subtropical climate. Cw types occur within the Cf group and refer to subtropical monsoon climates (like much of east Asia).
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.
Group H: Highland climates
Highland climates are those in which altitude plays a role in determining climate classification.[5] 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 (10.1 °F)[citation needed] for each 1,000 meters (3,281 ft) of elevation—would cause the climate to fit into a different thermal group from 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 between 500 and 2,500 meters (1,640 and 8,202 ft), but the G or H is placed in front of the applicable thermal letter rather than replacing it. 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, denoting mean monthly temperature, conform to the following scale:[6]
Code
Description
Temperature range
i
Severely hot
35 °C (95 °F) or higher
h
Very hot
28 to 34.9 °C (82.4 to 94.8 °F)
a
Hot
22.2 to 27.9 °C (72.0 to 82.2 °F)
b
Warm
18 to 22.1 °C (64.4 to 71.8 °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
It might be helpful to convert these into a table.
