You can trek Nepal throughout the year depending on the region. Nepal has tremendous variation in climates due to the vast differences in altitude. The low land plains are subtropical (600-1200 m), the midland hills are warm temperate (1200-2100 m) and cool temperate (2100-3300 m), and the high mountains are alpine (3300-5000 m) and tundra (5000-8848 m). Mid-September to mid-December is considered the best trekking season in Nepal.
Autumn Season (September-November).
Autumn is considered the best trekking season in Nepal. You can experience better visibility of tantalizing mountains, green hills, and excellent weather. Weather is generally clear with mild to warm days and cold nights. The possibilities of natural disasters diminish, making it a safe season to trek. The Nepalese festivals Dashain and Tihar are celebrated during this time.
Winter Season (December-February).
In winter, the Himalayan region freezes over, and the snow falls. Trekking can be challenging in the higher altitudes because the trail can be covered with snow. In the winter you should trek at lower altitudes below 3000 m. The temperature of the Himalayan region will be up to -18 degrees Celsius, hilly regions 0 to 5 degrees Celsius, and plain regions 10 to 15 degrees Celsius.
Spring Season (March-May).
The blooming flowers, lush greenery, and sparkling mountains make for an excellent background to spring trekking. Spring feels like a delight with singing birds, beautiful views, and abundant wildflowers. The weather will be mild at lower elevations but cold at higher elevations. In spring, mornings are usually crystal clear. However, in the afternoon clouds bring rain showers.
Summer Season (June-August).
In summer, mornings are cloudy with rain in the afternoons and evenings. Trekking is very uncomfortable because the trail becomes muddy, wet, and leech-infested. You risk injury in the wet terrain. Furthermore, there is a higher chance of natural disasters, mainly landslides, in the hilly regions. If you trek, you must not forget to carry insect repellent. The temperature of Himalayan regions will be 16 to 20 degrees Celsius, hilly regions 20 to 30 degrees Celsius, and the plain regions 34 to 39 degrees Celsius. Although this is not a good season to trek, the summer is best for botanists and researchers in forests and higher altitudes. However, places like Manang, Mustang, and Dolpo are located in a rain shadow. Summer is the best time for these regions. Temperatures are mild, and the weather is not so fierce.