Pros:

After the short but chilly winter, temperatures really increase in Nepal by March. There are fun and colourful festivals such as Shivaratri and Holi. There is very little rain in the spring, and trekking trails are generally in good condition. In March and April you can see beautiful red, pink, white, and purple rhododendron blooms in the forests. Lower Mustang is significantly colder than Pokhara or Kathmandu at this time, but still comfortable to visit.

Cons:

The spring is generally very dry, and the dust levels in Kathmandu can be intense. Kathmandu residents eagerly await the monsoon, to freshen the air. This also means that the mountain views can be obscured by haze and humidity. If you’re wanting to go white-water rafting, the water levels in some rivers can be too low. The temperatures on the Terai can also be uncomfortably hot, so this is not an ideal time to visit the Chitwan or Bardia National Parks.

Pros:

The monsoon normally starts in mid-June, and lasts into mid-September. However, it doesn’t tend to rain all day every day, so if this is the only time you have free to visit Nepal, don’t be put off. You can still go sightseeing in Kathmandu around the rainshowers. Although the weather can be humid, it’s not exceptionally hot (in Kathmandu) and the rains have the benefit of keeping the dust levels at bay. Also, this is the ideal time to visit Mustang, which is in the rainshadow of the Himalaya, and doesn’t experience the monsoon. (The only glitch is that flights from Pokhara to Jomsom are more likely to be cancelled or delayed due to the weather on the south side of the Himalaya).

Cons:

In Kathmandu, the monsoon can mean muddy and flooded streets, so leave your best shoes at home! Elsewhere, the rains can lead to landslides and even flooding on the Terai. Most popular trekking trails (with the exception of Mustang) are inaccessible, or uncomfortable, during the monsoon, and the mountains will be hidden behind clouds.

Pros:

Autumn is Nepal’s number one peak season, and this is because it can be a glorious time to be here. The temperatures are warm, but start to cool down (especially by the end of November, when you’ll need a sweater). The whole landscape turns a dazzling green as the rice that was planted during the monsoon matures. Nepal’s two most important festivals (for Hindus) are held between late September and early November—Dashain and Tihar. You’re likely to get good mountain views on the trails, and even from Kathmandu from November. Water levels are optimum for white-water rafting.

Cons:

Because this is Nepal’s peak season, some places can get quite crowded. The most popular trekking trails, such as the Everest Base Camp trek, can get very busy, with the best lodges filling up fast, and flights to and from Lukla booking out in advance. Pre-planning and pre-booking is required at this time, especially if you’re coming during Dashain or Tihar, when many locals take holidays.

Pros:

Some parts of Nepal—especially in the high Himalaya—can get very cold in the winter. But much of the country remains quite mild. Popular cities like Kathmandu and Pokhara never get really cold in the winter (compared to North American or European winters, that is!), but they are a bit chilly.

In Kathmandu, the lack of reliable heating can be the biggest problem, but days are often sunny and above 10 degrees, especially once you’re over the ‘hump’ of late December and early January. Pokhara remains very mild, and the mountain views from the lakeside city are spectacular in winter.

Winter is one of the best times to go trekking as long as you’re prepared for the cold with a good sleeping bag, thermals, and a down jacket. Stick to treks at lower altitudes that don’t require crossing high passes.

Not too many travellers come to Nepal in the winter, so you can find good deals at some nice hotels.

Cons:

Kathmandu can be foggy and smoggy in the winter, and the air quality can be poor.

Trekking to high altitude in the winter can be difficult or impossible, and high passes snowbound. Accommodation on some routes are closed. If you want to trek at altitude—and most medium to long-distance treks in Nepal rise to at least 2500 metres—warmer conditions are preferable.