The German-born, Russian climatologist Wladimir Köppen (1846–1940) developed a climate classification system that is still used today, albeit with some modifications. He classified climates into six categories: tropical humid, dry, mid-latitude, severe mid-latitude, polar, and highland. He also created sub-categories for five of these classifications. His climate map is often found in geography texts and atlases. In 1931, American geographer and climatologist Charles Warren Thornthwaite (1899–1963) published The Climates of North America: According to a New Classification, which takes into more thorough consideration how differences in geography affect local climates.