Trekking in Nepal can be undertaken throughout the year, depending on the region. Nepal has tremendous variations in climate as there is a vast variation in altitudes. The low land plains are sub- tropical (600-1200m), the midland hills are warm temperate (1200-2100m) and cool temperate (2100-3300m) and the high mountains are Alpine(3300-5000m) and Tundra(5000-8848m).
Autumn Season (Mid-September to mid- December) is considered the best time for trekking in Nepal.
You can experience better visibility during this time. The weather is generally clear with mild, warm days and cold nights. There are less possibilities of natural disasters, so safe and excellent trekking can be experienced. The main Nepalese festivals Dashain (Festival of Lights) and Tihar are also marked in this season.
Winter Season (December- February)
In winter, the Himalayan area can freeze with a mass of snow fall. Trekking can be challenging in the higher altitudes because the trail can be covered with snow. Winter season is suitable for trekking in lower altitudes below 3000m. The temperature in the Himalayan region will be up to -18 degree Celsius, the hilly region 0 to 5 degrees Celsius whereas in the low plains region it will be 10 to 15 degree Celsius.
Spring Season (March-May)
Trekking in spring is really a lovely time as the hilly slopes will be seen as a garden of blooming wild flowers with lush green fields and clear views of the mountains. With birds singing around you, it feels like being in the right place at the right time. Weather will be mild and warm at lower elevations but cold at higher elevation. In spring, mornings are usually very clear, whereas the afternoon clouds bring rain showers.
Summer Season (June-August)
In summer, the mornings are cloudy, and the afternoon and evenings are mostly rainy. Trekking is very uncomfortable because the trail becomes risky, muddy, wet, and leech-infested. There is high chance of natural disasters – mainly landslides in hilly regions. If you trek, you must not forget to carry insect repellants. The temperature of the Himalayan region will be 16 to 20 degree Celsius, the hilly region 20 to 30 degree Celsius, where as in plain region 34 to 39 degree Celsius. Although trekking is not good at this time, summer is best for botanists and researchers in forests and higher altitudes. Different places of Nepal like Manang, Mustang and Dolpo are somewhat sheltered from the rain and will be neither hot nor cold.