Nepal is a small country but it has a great variation in climate. A tropical and subtropical climate exists in the Terai Region. Outside the Terai, however, the climate is completely different. The remarkable differences in climatic conditions are primarily related to the enormous range of altitude within such a short north-to-south distance.
The presence of the east-to-west trending Himalayan massifs towards the north and the monsoonal alteration of wet and dry seasons heavily contribute to variations in local climate. In terms of natural vegetational regimes or distribution patterns, altitude again plays a significant role. Below 1,200 meters, the dominant form of vegetation consists of tropical and subtropical rain forests.
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 warmer winters in these regions.
Altitude and precipitation
Vairation in altitude affects annual precipitation patterns. Up to about 3,000 meters, annual rainfall increase as the altitude increases; thereafter, it diminishes with increasing altitude and latitude. In addition to this latitudinal differentiation in rainfall, 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. However, there are certain pockets with heavy annual rainfall e.g. the Pokhara Valley in central Nepal.
Second, the horizontal extension of hill and mountain ranges creates a moist condition on southland east facing slopes, whereas it produces a major rainfall on the northern sides of the slopes. The aridity increases with altitude and latitude, especially on the northern slopes, and 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 about 1,420 millimeters, and western Nepal about 1,000 millimeters.
For the vast majority of southern Asians, including Nepalese, the term monsoon is synonymous with the summer rainy season, which makes or breaks the lives of hundreds of millions of farmers on the subcontinent. Monsoon cycle affects the seasonal variation in the amount of rainfall. Monsoon can be divided into four seasons: pre-monsoon, summer monsoon, post-monsoon, and winter monsoon.
The pre monsoon season generally occurs during April and May; it is characterized by the highest temperatures, reaching 40° C during the day in the Terai Region and other lowlands. The hills and mountains, however, remain cool.
The summer monsoon, a strong flow of moist air from the southwest, follows the pre monsoon season. The arrival of the summer monsoon can vary by a month, but it generally arrives in early June causing violent lightning and thunderstorms, and lasts through September that is when it begins to recede. The plains and lower Himalayas receive more than 70 percent of their annual precipitation during the summer monsoon. The amount of summer monsoon rain generally declines from southeast to northwest as the maritime wedge of air gradually becomes thinner and dryer. Although the success of farming is almost totally dependent on the timely arrival of the summer monsoon, it periodically causes landslides; subsequent losses of human lives, farmlands, and other properties; and heavy flooding in the plains. Conversely, when prolonged breaks in the summer monsoon occur results in severe drought and famine.
The post monsoon season begins with a slow withdrawal of the monsoon. This retreat leads to an almost complete disappearance of moist air by mid-October, thus ushering in generally cool, clear, and dry weather, as well as the most relaxed and jovial period in Nepal. By this time, the harvest is completed and people are in a festive mood. The two biggest and most important Hindu festivals-- Dashain and Tihar (Dipawali) arrive during this period, about one month apart. The post monsoon season lasts until about December.
After the post monsoon, comes the winter monsoon, a strong northeasterly flow of moist air, which is marked by occasional, short rainfalls in the lowlands and plains, and snowfalls in the high-altitude areas. The amount of precipitation resulting from the northeast land trade winds varies considerably but increases markedly with elevation. The secondary winter precipitation in the form of snowfalls in the Himalayas is important for generating a sufficient volume of spring and summer melt-waters, which are critical for irrigation in the lower hills and valleys where agriculture predominates. Winter precipitation is also are indispensable for the success of winter crops, such as wheat, barley, and numerous vegetables.
Original Source: Wikipedia (Edited by Explore Nepal)