<p>Because of its elevation, Nepal can be divided into three zones – the high mountains, the mid-hills and the flat plains called the Terai. The Everest, Langtang and Annpurna trekking destinations in the mountains are cool throughout the year. The mid-hills, including Kathmandu and Pokhara, are cool except in the summer months from May to July. Temperatures in these cities during the summer remain much cooler than in many cities of the region. Lumbini, Chitwan and Janakpur in the Terai plains are hot in summer (March –July) but cold in winter (December-February).</p> <p>So if you are going trekking, the best months for walking are between September and November and between February and May when the weather is fair during the day and the temperatures do not drop rapidly during the night.</p> <p> </p> <p>The year is divided into a wet season from June to September—as summer warmth over Inner Asia creates a low pressure zone that draws in air from the Indian Ocean—and a dry season from October to June as cold temperatures in the vast interior creates a high pressure zone causing dry air to flow outward. April and May are months of intense water stress when cumulative effects of the long dry season are exacerbated by temperatures rising over 40 °C (104 °F) in the tropical climate belt. Seasonal drought further intensifies in the Siwaliks hills consisting of poorly-consolidated, coarse, permeable sediments that do not retain water, so hillsides are often covered with drought-tolerant scrub forest. In fact much of Nepal’s native vegetation adapted to withstand drought, but less so at higher elevations where cooler temperatures mean less water stress.</p> <p>The summer monsoon may be preceeded by a buildup of thunderstorm activity that provides water for rice seedbeds. Sustained rain on average arrives in mid-June as rising temperatures over Inner Asia creates a low pressure zone that draws in air from the Indian Ocean, but this can vary up to a month. Significant failure of monsoon rains historically meant drought and famine while above-normal rains still cause flooding and landslides with losses in human lives, farmland and buildings. The monsoon also complicates transportation with roads and trails washing out while unpaved roads and airstrips may become unusable and cloud cover reduces safety margins for aviation. Rains diminish in September and generally end by mid-October, ushering in generally cool, clear, and dry weather, as well as the most relaxed and jovial period in Nepal. By this time, the harvest is completed and people are in a festive mood. The two biggest and most important Hindu festivals—Dashain and Tihar (Dipawali)—arrive during this period, about one month apart. The postmonsoon season lasts until about December.</p> <p>After the postmonsoon comes the winter monsoon, a strong northeasterly flow marked by occasional, short rainfalls in the lowlands and plains and snowfalls in the high-altitude areas.In this season the Himalaya function as a barrier to cold air masses from Inner Asia, so southern Nepal and northern India have warmer winters than would otherwise be the case. April and May are dry and hot, especially below 1,200 meters (4,000 ft) where afternoon temperatures may exceed 40 °C (100 °F).</p>