Trewartha climate types for the contiguous United States
TheTrewartha climate classification (TCC), or theKöppen–Trewartha climate classification (KTC), is aclimate classification system first published by American geographerGlenn Thomas Trewartha in 1966. It is a modified version of theKöppen–Geiger system, created to answer some of its deficiencies.[1] The Trewartha system attempts to redefine themiddle latitudes to be closer tovegetation zoning and geneticclimate systems.[2]
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:
This is thetropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C (64.4 °F) or above). TheA 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 classifiedAr.
Others are classifiedAw if thedry season is at the time of low-sun/short days orAs if the dry season is at the time of high-sun/long days.
There was no specificmonsoon climate identifier in the original scheme, butAm 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).
BW andBS mean the same as in the Köppen scheme. However, a different formula is used to quantify the aridity threshold: 10(T − 10) + 3P, withT equaling the mean annual temperature in degrees Celsius, andP denoting the percentage of total precipitation received in the six high-sun months (April through September in theNorthern Hemisphere, October through March in theSouthern).[4]
If the precipitation for a given location is less than the above formula — that is, ifP < 10(T − 10) + 3P — the climate is said to be that of adesert (BW).
If it is equal to or greater than the formula but less than twice that amount, the climate is classified assteppe (BS).
If the precipitation is more than double the value of the formula, the climate is not in GroupB.
Unlike in Köppen's scheme, no thermal subsets exist within this group in Trewartha's, unless theUniversal Thermal Scale (see below) is used.
In the Trewartha scheme theC climate group encompassessubtropical climates, which have 8 or more months with a mean temperature of 10 °C (50 °F) or higher. There are only two types within theC or subtropical climate group:
In the Trewartha scheme theD climate group encompassestemperate 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 orb) 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, andDcb 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 44thparallel exhibits aDo orDc climate type.
This representssubarctic andsubpolar 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 designationsEo andEc 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 GroupD, a third letter can be added to indicate seasonality of precipitation. There are no separate counterparts to the KöppenDfd,Dwd, andDsd 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).
This is thepolar climate group, where all months have a monthly mean air temperature below 10 °C (50 °F). Polar climates have two subtypes,Ft (tundra) andFi (ice cap):
In theFt 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 ascrub or tundra vegetation cover is possible.
In theFi 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 theNorth Pole, and the permanent iceplateaus ofAntarctica andGreenland.
Highland climates are those in whichaltitude plays a role in determining climate classification.[5] Specifically, this would apply if correcting the average temperature of each month to asea-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.
SometimesG is used instead ofH if the above is true and the altitude is between 500 and 2,500 meters (1,640 and 8,202 ft), but theG orH is placed in front of the applicable thermal letter rather than replacing it. The second letter used reflects thecorrected monthly temperatures, not the actual monthly temperatures.
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]
