Nepal has a great deal of variation in climate. Its latitude is about the same as that of Florida, so 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. Scholar Sharad Singh Negi identifies five climatic zones in Nepal based on altitude: a tropical and subtropical zone below 1,200 meters (the frost limit in winter), a cool temperate zone between 1,200 and 2,400 meters (where there is at least occasional snowfall in winter), a cold zone between 2,400 and 3,600 meters (tree line), a subarctic zone from 3,600 to 4,400 meters, and an arctic zone above 4,400 meters.
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 forests that have evolved in response to the monsoonal 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. Bishop divides the monsoon cycle into four seasons: premonsoon, summer monsoon, postmonsoon, and winter monsoon. The premonsoon 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 premonsoon season. 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. 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% 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. 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 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.
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