Nepal has a great deal of variation in climate... and a tropical and subtropical climate exists in the Tarai Region. Outside the Tarai, 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-south distance. The presence of the east-west-trending Himalayan massifs to the north and the monsoonal alteration of wet and dry seasons also greatly contribute to local variations in climate.... five climatic zones in Nepal based on altitude: the tropical and subtropical zone of below 1,200 meters in altitude; the cool, temperate zone of 1,200 to 2,400 meters in altitude; the cold zone of 2,400 to 3,600 meters in altitude; the subarctic climatic zone of 3,600 to 4,400 meters in altitude; and the arctic zone above 4,400 meters in altitude. 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.
Altitude also affects annual rainfall or precipitation patterns. Up to about 3,000 meters, annual rainfall totals increase as the altitude increases; thereafter, annual totals diminish 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.... Second, the horizontal extension of hill and mountain ranges creates a moist condition on southand eastfacing slopes whereas it produces a major rain shadow 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.
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.... The premonsoon season generally occurs during April and May; it is characterized by the highest temperatures, reaching 40° C during the day in the Tarai 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 premonsoon season.... Even though the arrival of the summer monsoon can vary by as much as a month, in Nepal it generally arrives in early June, is preceded by violent lightning and thunderstorms, and lasts through September, 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 such problems as landslides; subsequent losses of human lives, farmlands, and other properties (not to mention great difficulty in the movement of goods and people); and heavy flooding in the plains. Conversely, when prolonged breaks in the summer monsoon occur, severe drought and famine often result.
The postmonsoon 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.... The postmonsoon season lasts until about December.
After the postmonsoon, comes the winter monsoon, a strong northeasterly flow, 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 meltwaters, 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.
LOC data as of September 1991