Trewartha climate classification

The Trewartha climate classification is a climate classification system published by American geographer Glenn Thomas Trewartha in 1966, and updated in 1980. It is a modified version of the 1899 Köppen system, created to answer some of the deficiencies of the Köppen system. 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 system. For example, under the standard Köppen system, Washington and Oregon are classed into the same climate zone as Southern California, even though the two regions have strikingly different weather and vegetation. Another example was classifying cities like London in the same climate group as Brisbane or New Orleans, despite great differences in seasonal temperatures and native plant life.

Trewartha's modifications to the 1899 Koppen 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 Koppen climate classification.

This the tropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C, 64 °F, or above). The "A" climates are the realm of the winterless frost-free zone.

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