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: C (subtropical)—8 or more months have a mean temperature of or higher; D temperate—4 to 7 months have a mean temperature of 10 °C or higher; and E boreal climate—1 to 3 months have a mean temperature of 10 °C or higher. Otherwise, the tropical climates and polar climates remained the same as the original Köppen climate classification.
The "highland" climate is ambiguously defined. Newer users of KTC generally omit this option.
Group A: Tropical climates
This is the tropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average , 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 , average precipitation, same as per Köppen) are classified Ar, 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
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 and October through March in the Southern).
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
In the Trewartha scheme the "C" climate group encompasses Subtropical climates that have 8 or more months with a mean temperature of or higher.
There are only two types within the "C" or subtropical climate group, Cs which is a dry -summer or Mediterranean climate, and a Cf or humid Subtropical climate. Cw types occur within the Cf group and mean 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 or higher.
"D" climate groups have two types – an Oceanic type (Do), where the coldest month has a mean temperature or higher, and a Continental type (Dc), where the coldest monthly mean temperature reaches below 0 °C in some interior landmasses like North America and Asia.
For the continental climates (Dc), sometimes the third letter (a or b) is used to denote a hot or cold summer. "Dca" is where the warmest month has a mean temperature of 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 Do or Dc climate types.