Nepal is known for its high-altitude mountain treks and snow-covered peaks. It's actually a very climatically diverse country, and many would-be visitors are surprised by just how hot and tropical much of it is.
Broadly speaking, Nepal's climate can be divided into four distinct regions and four seasons. When planning a trip to Nepal, it's important to factor in the season, the regions you'll be traveling to, and the altitude. It's also important to consider the activities you want to experience and when the best times to do them are. If you're an inexperienced hiker, then heading into the mountains in the middle of winter is not a good idea, although if you've done winter treks before and are well prepared with the right gear, then even winter treks can be fun. Likewise, if your trip coincides with the monsoon season, you need to know how the conditions will affect your plans and what might not be possible.
Weather by Region
The Terai is the collective name for the lowland areas of Nepal bordering North India. Parts are covered in jungles and national parks, such as Chitwan and Bardia, and marshy bird habitats in the confluence of major South Asian rivers flowing from Tibet. Although there are some hill ranges on the Terai, the altitude is generally low. For example, the town of Lumbini is less than 500 feet.
Being so close to India, the climate of the Terai is more similar to a North Indian climate than a hill or mountain Nepali climate. That is scorching temperatures between March and October (often above 95 degrees F) and a short, cool, and often foggy winter between November and February.
The Hill Areas
Popular and busy cities like Kathmandu and Pokhara are located in the Nepali hills, the area between the lowland Terai and the high Himalaya mountains. Altitudes vary, but settlements in the hills are generally not high enough to cause health issues but are just high enough to be cooler than the Terai. For example, Kathmandu is located at 4,600 feet, and Pokhara at 2,700 feet.
The Nepali hill areas' climate is the most moderate of anywhere in the country, with hot but not-too-uncomfortable temperatures between March and October and cool-to-cold but short winters. Temperatures in Kathmandu can get as low as 32 degrees F, but mostly at night and not for very long. The coldest time of year in the hills is between mid-December and mid-January.
Few Nepalis actually live in the high Himalaya mountains, but you're likely to want to get deep into the mountains if you're coming to Nepal for trekking. Despite mountain giants like Everest, Annapurna, and Dhaulagiri always being covered in snow, unless you're trekking in winter (November-February) or actually climbing a mountain, you're unlikely to have to walk through deep snow on most mainstream trekking routes.
Altitudes, as well as seasons, affect how cold temperatures are in the mountains. Even gateway towns to trekking regions, like Lukla (9,400 feet), are significantly higher than the hill areas you're likely to be traveling from. For example, you may start the day in 77-degree Kathmandu and land in 50-degree Lukla a couple of hours later when embarking on an October trek. Most treks will greatly ascend in altitude, so it will progressively get colder as you walk, and any rainfall is more likely to be snowfall the higher you go.
In the Rainshadow of the Himalaya
While most of Nepal's mountain areas are on the southern side of the Tibetan Plateau, a few places sit on the "other" side of the mountains. Mustang, Dolpo, the Nar-Phu Valley, Manang, and some other smaller and lesser-known areas are in the Himalaya's rainshadow, meaning that the mountains stop the monsoon rains that sweep up from India between June and September. These areas in the rainshadow are much drier than the rest of Nepal, so the landscape is very different.
Accessibility is also different from the rest of Nepal. While most trekking areas are too wet for trekking between June and September, these are the best months to visit places in the rainshadow as they're dry. However, getting there can still be problematic. Reaching Mustang, for example, requires a short flight from Pokhara through the mountains (or a long and painful bus ride), which is often canceled during the monsoon because of the rain.
Except for the areas in the Himalayas' rainshadow, all of Nepal's regions experience variations of the following four seasons. Areas in the rainshadow experience freezing winters (due to their higher altitude) and warmer and drier conditions at other times.
Monsoon Season in Nepal
As temperatures rise to an uncomfortable level in late May and early June, Nepalis eagerly await the monsoon's arrival, which sweeps up the continent from India. Rains usually begin in Kathmandu in mid-June and continue well into September. It does not rain all day throughout the monsoon, but skies are usually cloudy (and streets muddy). Temperatures are actually less than in the stifling pre-monsoon weeks, but the humidity is high.
Hill and mountain areas of Nepal don't harbor malaria-carrying mosquitoes. Still, monsoon-season dengue outbreaks in Kathmandu in recent years mean that good insect repellent is important if you must travel to Nepal during the monsoon.
Spring in Nepal
The Hindu festivals of Shivaratri and Holi herald the coming of spring in Nepal, and these usually fall in early March. Temperatures vary throughout the country, but in the capital, early March is generally a comfortable 68 degrees in the daytime, rising to a less comfortable 86 degrees by late May.
Hotter temperatures arrive earlier on the Terai and later in the Himalaya, but the general pattern of increasingly warm temperatures throughout March, April, and May remains consistent.
Autumn in Nepal
Between the soggy monsoon and the cold winter, the autumn is generally warm, clear, and pleasant throughout Nepal. This is also the peak season for travelers. Late September to late November offers ideal conditions for sightseeing, trekking, and other outdoor activities. Nights can get cold by late November, and you may experience snow showers in the mountains.
Winter in Nepal
Nepal's winter is relatively short, with the coldest period falling in December and January (although the higher altitude you go, the longer and colder the winters are). Lack of indoor heating, even in good hotels, can make the winter seem colder than it is, but daytime temperatures in Kathmandu and Pokhara are usually at least 50 degrees. It rarely rains in winter, so skies are clear, and conditions good for lower-altitude trekking or general sightseeing. Daylight hours don't vary significantly throughout the year in Nepal, but the days are the shortest in winter, with the sun rising around 7 a.m. and setting around 5:30 p.m.
When to Visit Nepal
With warm temperatures and clear skies, the autumn season (late September to late November) is Nepal's peak season, with spring (March to May) slightly less busy but still popular. Building dust and humidity make the spring somewhat less pleasant than autumn.
Few tourists visit Nepal in winter, but if you want to do general sightseeing activities in the hill areas and main cities, it's not a bad time to visit.
Unless you want to trek in the mountain areas in the Himalayas' rainshadow, avoid visiting Nepal during the monsoon. Not only will you experience a lot of rain, but flooding in Kathmandu and washed-out highways can make getting around difficult.