The Köppen climate classification system has been further modified, within the Trewartha climate classification system in the middle 1960s (revised in 1980). The Trewartha system sought to create a more refined middle latitude climate zone, which was one of the criticisms of the Köppen system (the C climate group was too broad).:200–1
The Köppen climate classification scheme divides climates into five main climate groups: A (tropical), B (dry), C (temperate), D (continental), and E (polar). The second letter indicates the seasonal precipitation type, while the third letter indicates the level of heat.
Group A: Tropical (megathermal) climates
This type of climate has every month of the year with an average temperature of 18 °C (64.4 °F) or higher, with significant precipitation.
Group B: Dry (arid and semiarid) climates