It’s hard to generalize about the climate of a country ranging in elevation from near sea level to Mount Everest (8850mt). About the only thing that can be said is that all but a few parts of Nepal are governed by the same monsoon pattern with temperatures varying according to elevation.

Autumn: Mostly half of all tourists visit Nepal in this season (Mid September – November) and for good reasons. The weather is clear and dry, and temperatures aren’t too cold in the high country nor too hot in the Tarai. With the air washed clean by the monsoon rains, the mountains are at their most visible, making this the most popular time for trekking. Two major festivals also fall during this season.

Winter (December to January), the snow line descends to 2000 and 3000m, and though it never snows in Kathmandu, the "mists of Indra" make the capital feel cold and clammy.

Spring (February to mid-April) brings warmer temperatures, longer days, weddings and more festivals. The rhododendrons are in bloom in the hills towards the end of this period, and in the Tarai that has been cut, making this the best time for viewing wildlife. The one factor that keeps people away is a disappointing haze that obscures the mountains from lower elevations, though it’s usually possible to trek above it.

The pre-monsoon (Mid April - early June) is stifling at lower elevations, and dusty wind squalls are common. Trek high, where the temperatures are more tolerable.

Monsoon: (June to September), which breaks the enervating monotony of the previous months, and makes the fields come alive with rushing water and green shoots. The rains rinse and renew the land. This can be a fascinating time to visit, when Nepal is at its most Nepali, but there are many drawbacks, mountain views are rare, leeches come out in force along the mid-elevation trekking routes, roads wash out, flights get cancelled.

The Monsoon
Nepal’s climate is governed by the monsoon, one of the world’s great weather phenomena. A seasonal wind (the word derives from the Arabic for "season"; the Nepali word for monsoon is Varkhaa), the monsoon is driven by extreme temperature fluctuations in Central Asia. As air over the Asian land mass warms in late spring and early summer, it rises, sucking air in from the ocean periphery to take its place. The air drawn from the south, passing over the Indian Ocean, is laden with moisture; as soon as it’s forced aloft and cooled (whether by updrafts over hot land, or by a barrier, such as the hills and mountains of Nepal), it reaches its saturation point and drops its moisture. With the arrival of autumn, the flow reverses cooling over the continent blows dry air outwards, bringing clear, stable conditions. That’s the theory, though in practice this huge, complex system is affected by countless variables such as land temperatures, jet-stream patterns, topography and late-season typhoons. The further inland you are, the harder it is to predict the outcome. Nepal is at the end of the line of the eastern arm of the South Asian monsoon sweeping up from the Bay of Bengal, which means it gets a month or so of pre-monsoon - a period of false storms and dry lightning - before the moist air arrives.

In Nepal, the rains generally advance from east to west in early to mid-June, and drop more precipitation overall in the east than in the west. They build slowly, reaching a peak in July and early August, then taper off again until clear weather returns by early October. Even at the monsoon’s height, however, it doesn’t bring continuous torrential rain - more usually it’s intermittent showers and longer overnight soaks. Local terrain and other factors can affect rainfall considerably. Areas lying in the "rain shadow" north of the Himalaya see very little monsoon moisture, while south-facing slopes may receive precipitation long before the plains to the south do. The latter effect is most dramatic where monsoon winds slam into high ranges with few intervening foothills, as they do around Pokhara