The tropical zone below experiences frost less than once per decade. It can be subdivided into lower tropical (below 300 meters or 1,000 ft.) with 18% of the nation's land area) and upper (18% of land area) tropical zones. The best mangoes and well as papaya and banana are largely confined to the lower zone. Other fruit such as litchee, jackfruit, citrus and mangoes of lower quality grow in the upper tropical zone as well. Winter crops include grains and vegetables typically grown in temperate climates. The Outer Terai is virtually all in the lower tropical zone. Inner Terai valleys span both tropical zones. The Sivalik Hills are mostly upper tropical. Tropical climate zones extend far up river valleys across the Middle Hills and even into the Mountain regions.