Trewartha climate classification

The Trewartha climate classification 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. It was considered a more true or 'real world' reflection of the global climate. The Trewartha climate classification 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. It was considered a more true or 'real world' reflection of the global climate. The changes were seen as most effective on the large landmasses in Asia and North America, where many areas fall into a single group (C) in the Köppen-Geiger system. For example, under the standard Köppen system, Washington and Oregon are classed into the same climate zone (Csb) as parts of Southern California, even though the two regions have strikingly different weather and vegetation. Another example was classifying cities like Bordeaux in the same climate group (C) as Brisbane or New Orleans, despite great differences in seasonal temperatures and native plant life. Trewartha's modifications to the 1899 Köppen climate system 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 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. This 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, 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). 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. In the Trewartha scheme the 'C' climate group encompasses Subtropical climates that 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, 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).