Nepal is a landlocked country, surrounded by India on three sides and by China to the north. Nepal has a great deal of variation in climate. The low elevation Terai has a tropical and subtropical climate. Above the Terai, the climate is completely different due to higher elevations. The east-west-trending Himalayan ranges to the north and the monsoonal alteration of wet and dry seasons also contribute to local variations in climate.
Altitude also affects annual rainfall or precipitation patterns. Up to about 3,000 meters, annual rainfall totals increase with elevation, but above this they decrease with elevation and latitude.
Two other patterns can be discerned. First, given the northwestward movement of the moisture-laden summer monsoon (June to September), the amount of annual rainfall generally decreases from east to west, although there are exceptional areas such as the Pokhara Valley in Central Nepal with higher rainfall due to generally lower "hills" to the south and the main Himalayan Range immediately to the north that stops the northward passage of moist air. Second, adabiatic effects increase rainfall on south- and east-facing mountain slopes, with a rain shadow on northern sides. This reaches its climax in the inner Himalayan region and on the Tibetan Plateau. Eastern Nepal receives approximately 2,500 millimeters of rain annually, the Kathmandu area gets about 1,400 millimeters, and western Nepal about 1,000 millimeters.
The towering Himalayas play a critical role, blocking the northwesterly advances of moist, tropical air from the Bay of Bengal, and ultimately leading to its conversion to rain in the summer. In the winter, this range prevents the outbursts of cold air from Inner Asia from reaching southern Nepal and northern India, thus ensuring warmer winters in these regions than otherwise would be the case.
In addition, there are seasonal variations in the amount of rainfall, depending on the monsoon cycle.: premonsoon, summer monsoon, postmonsoon, and winter monsoon.
