The summer is the principal rainy season. The eastern lowlands and the Caribbean and Pacific coastal lowlands have tropical and equatorial climates with high temperatures and humidity throught the year and abundant precipation of over 1,000 millimeters (40 inches). There is no completely dry season, although the north and the northwest have a comparatively drier winter. In the mountain regions, climatic conditions are much more complex and varied. These variations depend of altitude, topographic position, and extent of exposure to prevailing winds and the sun. There are often extremes differences over short distances, especially with regard to precipitation. Slopes facing rain-bearing winds are amply supplied with rainfall, while the adjacent sheltered slopes and valleys have a dry climate. But climatic differences, which are reflected in the natural vegetation and which determine types of agricultural activity, are related mainly to the vertical zoning of the Andes, where natural conditions provide the full range of climates, from the torrid equatorial type to the permafrost of areas above 15,000 feet.