permanent dry season and only receives minimum summer rainfall. Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi Desert in Mongolia west-southwest through Pakistan and Iran into the Arabian Desert in Saudi Arabia. In Asia, rainfall is favored across its southern portion from India east and northeast across the Philippines and southern China into Japan due to the monsoon advecting moisture primarily from the Indian Ocean into the region. Similar, but weaker, monsoon circulations are present over North America and Australia. In Europe, the wettest regions are in the Alps and downwind of bodies of water, particularly the Atlantic west coasts. Within North America, the drier areas of the United States are the Desert Southwest, Great Basin, valleys of northeast Arizona, eastern Utah, central Wyoming, and the Columbia Basin. Other dry regions within the continent are far northern Canada and the Sonoran Desert of northwest Mexico. The Pacific Northwest United States, the Rockies of British Columbia, and the coastal ranges of Alaska are the wettest locations in North America. The equatorial region near the Intertropical Convergence Zone (ITCZ), or monsoon trough, is the wettest part of the world's continents. Annually, the rain belt within the tropics marches northward by August, then moves back southwards into the Southern Hemisphere by February and March. Role in Köppen climate classification The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. Specifically, the primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as rain forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert. Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between and . A tropical savanna is a grassland biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes, with rainfall between and a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia, and Australia. The humid subtropical climate zone where winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional