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.
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.