Nepal's climate varies with its topography and altitude ranging from arctic climate in the mountains to the tropical in the southern lowlands. The northern mountain region, around an altitude above 3,300m has an alpine climate with considerably lower temperature in winter as can be expected.
The low-land Terai region with its maximum altitude to approximately 305 m, which lies in the tropical southern part of the country, for instance, has a hot and humid climate that can rise above 45 Degree Celsius (113 Degree Fahrenheit) during summer. The mid-land regions are pleasant almost all the year round, although winter nights are cool.
Nepal has four climatic seasons.
Autumn (September – November): Autumn or fall is definitely the best season to travel to Nepal with monsoon gone and winter to set in. Skies are clear and smog is down and we have clear views of the mountain and valleys. The weather is highly pleasant. This is also the season of festivities as Nepal celebrates the biggest Hindu festivals Dashain followed by Tihar.
Spring (March – May): The temperature is mildly warm in low lands while moderate in higher altitudes with plenty of opportunities to have tryst with the mountain views. It is also the time for flowers to blossom and the national flower of Nepal – rhododendron sweeps the ascending altitudes with its magnanimous color and beauty. Spring is the second preferable season for traveling to Nepal and if you are plant lovers, definitely you will choose Spring over Fall trekking in Nepal as the mountainous forests and orchids are in bloom during spring.
Summer or Monsoon (June – August): This is also the monsoon season in Nepal. The weather is hot and wet at times. It rains almost every day for a couple of hours and sometime days and nights with occasional thunderstorms in the evening. Rains usually occur during the night-time leaving the sky clean and clear by the morning making the Himalayan view even more dramatic. The rain spreads the pleasantness around with lush green vegetation. Flashfloods and landslides can disrupt surface travel. Monsoon cannot be an obstacle for a cultural trip to Kathmandu, Chitwan, Pokhara. Some parts of the Himalayas in Manang, Mustang and Dolpo are in rain-shadow areas; the mountains are high enough to block the clouds. Tibet's high travel season also corresponds to Nepal's monsoon.
Winter (December – February): The weather is cool and the sky is clear with occasional snowfalls at higher elevations. This season is good for trekking in lower elevations. The morning and night is cold and the days are warm when sunny.
Temperature and rainfall
Nepal is the country of extremes. The low-land plains of the Terai can have tropical temperatures and also the mosquitoes. The Himalayas can get to sub-zero temperatures, but the sun blaze can bring some warmth during the day, even in the mountains. The temperature of Kathmandu goes below 1 Degree Celsius (34 Degree Fahrenheit) in winter and rises to an average of 25 Degree Celsius (77 Degree Fahrenheit) in summer.
The average temperature in Kathmandu during the four seasons:
Spring season ranges between 16-23 Degree Celsius (61-73 Degree Fahrenheit)
Summer season ranges between 23-25 Degree Celsius (73-77 Degree Fahrenheit)
Autumn season ranges between 15-24 Degree Celsius (59-75 Degree Fahrenheit)
Winter season ranges between 9-12 Degree Celsius (48-54 Degree Fahrenheit)
During the rainy monsoon season between June to August, it rains to an average between 200-375 millimeters in Kathmandu. There is occasional rainfall during the other seasons too. In an average, 1300 millimeters of rain falls in Kathmandu every year.
Nepal can be traveled throughout the year but in terms of weather, the fall season (September, October and November) is the best as you can enjoy mild temperature, clear skies and good mountain views. Trekkers and hikers prefer spring season (March, April and May) as it's dry and also you are able to see rhododendrons and wild orchid in bloom during this time of the year.