Despite its small size, Bhutan has a wide range of climatic conditions due to the sharp variations in elevation from region to region. The country can be roughly divided into three distinct climatic zones from south to north. The southern foothills are characterized by a tropical climate with hot and humid summers, and warm dry winters. The temperate central highlands, where most of the population centers are located has four distinct seasons. Spring in the higher valleys (mid-March to May) has warm days (20 °C) and cool nights. June marks the beginning of summer when day temperatures warm up to 27 - 29 °C. By July, the rainy season starts and continues till mid-September. The autumn months of September to November are ideal for trekking with clear skies and mild weather. In December temperatures fall, but the days are warm and the clear, azure winter skies serve as a striking background to the snow-capped peaks. On sunny days, temperatures reach about 16-18° C. The nights, however, are cold with temperatures falling below freezing.