Consequently, the Terai is the hottest place in summer, whereas the high Himalayas remain cold all around the year. It is customary in Nepal to divide the time around the year in six seasons, with two months in each: spring, summer, monsoon, autumn, fall, and winter.
In the terai, summer temperatures can reach 37 degrees centigrade or even higher, whereas in winter they vary from 7 to 23. The temperature of hills remains temperate mostly throughout the year, whereas in high mountain region, winters are more severe. Except in rainy season that lasts three to four months from June to early September, the days in Nepal are bright and sunny. This allows visitors to enjoy the sceneries around the year. Even in rainy season, there are places in Nepal like Upper Mustang and Dolpa, the hubs of Tibetan culture, which remain in rain shadow. Trekking to these areas makes excellent choice in rainy (monsoon) season.
Customarily the best trekking season in Nepal is autumn, when the sky is at its clearest after long monsoon rains, and when greenery greets you from all around after the nourishing rains. This season is followed by spring and summer, when the slopes of hills and mountains burst into colorful spectacle with rhododendron blooming all around. But Nepal can be visited all around the year where different activities are available for your entertainment and refreshment.
The Himalayas of Nepal are not just the Mecca for adventure tourism like mountain trekking and mountaineering expeditions; they serve as the natural barrier to the cold wind blowing from central Asia in winter, thus keeping the hilly region temperate. In monsoon, the wet winds blowing from the Bay of Bengal are again stopped by them, thus causing rainfall in the south. In short, Nepal is just 150 kilometers wide in average, but due to its staggering difference in altitude, almost all kinds of climate are found here, ranging from the tropical to the polar.