Major chunk of Landscape in Nepal is ruled by Himalayas range except Terai or the plains in Nepal. The defining factors that determines the weather of area governed by Himalayas range is altitude. One conspicuous feature of altitude based weather is its propensity to dramatic change of weather during the span of day and night. For instance, if the temperature during October during November is 17 C max at day then it can plummet to 4 or 5 C at evening in Kathmandu.
Nepal weather can best be categorized into four seasons. The dry season runs from October to May and there’s the wet (monsoon) season from June to September. Autumn (September to November) and spring (March to May) bring almost perfect weather and are definitely the best times to come to Nepal. October to November, the start of the dry season, is in many ways the absolute best time. With the monsoon only recently finished, the countryside is green and lush, the air is sparkling clean and the Himalayan views are near perfect. .In December and January the climate and visibility are still good, though it can get very cold at high altitudes. Heading for the Everest Base Camp at this time of year can be a real feat of endurance and the Annapurna Circuit is often closed by snow on the Thorung La. October to February are considered the best times to visit the Terai and Royal Chitwan National Park. February to April, the tail end of the dry season, is the second-best time to visit. The weather gets warmer so high-altitude treks are not as arduous. Visibility is not as good as earlier in the dry season, but Nepal’s wonderful rhododendrons and other flowers are in Technicolor bloom.
May and early June are hot and dusty, with temperatures often above 30°C. By Mid-June to September, the monsoon finally arrives, because of its lower altitude; Pokhara is warmer and more pleasant than Kathmandu during winter