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 Coulomb 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.