The climate of India comprises a wide range of weather conditions across a vast geographic scale and varied topography, making generalisations difficult. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid desert in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The nation has four seasons: winter (January and February), summer (March to May), a monsoon (rainy) season (June–September), and a post-monsoon period (October–December).