Moist, tropical climates are hot and humid. Steppes and deserts are dry, with large temperature variations. Plentiful lakes, rivers, or nearby oceans give humid, midlatitude climates cool, damp winters, but they have hot, dry summers. Some of these climates are also called Mediterranean. Continental climates occur in the centers of large continents. Mountain ranges (or sheer distance) block off sources of moisture, creating dry regions with large seasonal variations in temperature. Much of southern Canada, Russia, and parts of central Asia would fall into this category. Cold, or polar, climates round out Köppen's list. A sixth region, high elevations, was later added to the classification system.