Glenn Thomas Trewartha (1896 – 1984) was an American geographer and climatologist of Cornish American descent. He is best known for his eponymous Trewartha climate classification, an expanded and modified version of the Köppen–Geiger system.
He graduated from the University of Wisconsin–Madison, with a Ph.D. in 1924. He taught at the University of Wisconsin.
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.