(a, b) A system for describing climates devised in 1931 and revised in 1948 by the American climatologist Charles Warren Thornthwaite which divides climates into groups according to the vegetation characteristic of them. Vegetation is the true index of climate is associated with Thornthwaite and Koppen. Like Thornthwaite, Koppen also considered natural vegetation of a region as the indicator of climate of that region.