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.
The Mountain Region
The Mountain Region (Parbat in Nepali) is abruptly elevated thousands of meters into the zone of perpetual snow along the Main Central Thust fault zone. South of this fault system, “hills” do not greatly exceed treeline at about 3,500 meters. North of it the Himalayas rise as a virtual wall above the snowline at 5,000 to 5,500 meters to some 90 peaks over 7,000 meters (22,965′) and eight giants exceeding 8,000 meters (26,246′), including Mount Everest at 8,848 meters and Kanchenjunga at 8,598 meters.
Cutting between the various subranges of the Himalaya and north of them are alpine, often semi-arid valleys including Humla, Jumla, Mustang, Manang District and Khumbu that are lightly populated by people with Tibetan affinities called Bhotiya or Bhutia, the famous Sherpas in the Kumbu valley near Mount Everest. Bhote traditionally grazed yaks, grew cold-tolerant crops such as potatoes, barley and millet, and traded across the mountains, e.g. Tibetan salt for rice from lowlands in Nepal and India. Since the 1950s these mountain peoples have also found work as high altitude porters, guides, cooks and other accessories to tourism and alpinism.
Bhote language and culture extend north into Tibet proper, with the international border following the Himalayan crest in eastern Nepal. In central and western Nepal the border mostly follows lower (~6,000 meter) ranges tens of kilometers north of the highest peaks, the watershed between the Ganges and Brahmaputra river basins.
Situated south of the Mountain Region, the Hill Region (Pahar in Nepali) is mostly between 1,000 and 4,000 meters in altitude. This region begins at the Mahabharat Lekh (Leser Himalaya) where a fault system called the Main Boundary Thrust creates an escarpment 1,000 to 1,500 meters high, rising to the crest of this range generally at about 2,000, occasionally up to 3,000 meters. The abrupt south-facing slope is nearly uninhabited, thus an effective buffer between languages and culture in the Terai and Hill regions. Northern slopes of the Mahabharats are gentler and moderately well populated.
North of this range, valleys as high as 2,000 meters are densely populated by rice-growing, Nepali-speaking Hindus and by Newar merchants who also speak Newari. The increasingly urbanized Kathmandu and Pokhara valleys are part of this region. Hillsides up to about 3,000 meters are instead occupied by indigenous “janjati” ethnic groups natively speaking highly localized Tibeto-Burman languages and dialects. There are Magars and Kham Magars west of Pokhara, Gurungs south of the Annapurnas, Thamangs north of Kathmandu, Rai and Limbu further east. Upland staple crops are maize, millet, barley and potatoes rather than rice. Temperate and subtropical fruits are grown as cash crops. Marijuana is grown and processed into hashish, or was until international pressure forced the government to stop being the middleman operating government monopoly stores in urban centers. There is increasing reliance on animal husbandry with elevation, using land above 3,000 meters for summer grazing and moving herds of sheep and goats to lower elevations in winter. Except for the rice-growing lower valleys, the hills are in chronic food deficit. Many menfolk are employed in the Terai, in India or overseas to earn cash for imported grain. The Hill region ends dramatically where the main Himalayan Range abruptly rises thousands of meters into the realm of perpetual snow.
The Terai or Madhesh region begins at the Indian border and includes the northermost part of the flat, intensively farmed Gangetic Plain called the Outer Terai. This is culturally an extension of northern India with Hindi, Awadhi, Bhojpuri and Maithili spoken more than Nepali, however it was annexed to Nepal by conquest and by treaty with the British.
Rivers including the large Koshi, Narayani (called the Gandak in India), and Karnali meander across the Terai after breaking through the ranges to the north. These and smaller rivers rising south of the main Himalaya are prone to flooding during the summer monsoon.
The Outer Terai ends at the first range of foothills called the Siwaliks. This range has a forested alluvial belt along its base, marshy with springs fed by groundwater percolating down from higher elevations. Before the use of DDT the alluvial zone was dangerously infested with malaria. Nepal’s rulers used it as a defensive frontier called the char kose jhadi (twelve kilometer forest).
Beyond the alluvial belt, the Siwaliks rise as high as 1,000 meters, steepest on their southern flanks because of faults. This range is composed of poorly consolidated, coarse sediments that quickly absorb rainfall. This is unsuited to agriculture so there is very little population. However in several places north of the Siwaliks there are dun valleys or the Inner Terai. Among these are Surkhet, Dang and Deukhuri in western Nepal and the Rapti Valley (Chitwan) in central Nepal. These valleys were also malarial and lightly populated until DDT was used to suppress mosquitos, but they had significant agricultural potential that was exploited to some degree by the Tharu ethnic group who were resistant to malaria. After DDT was used to suppress malaria in the 1950s, farmers from the hills began settling in these valleys to the detriment of the Tharus.
The Terai ends and the Hills begin at a higher range of foothills called the Mahabharat Range.
Nepal can be divided into three major river systems
from east to west: the Koshi River basin, the Narayani River basin
(India’s Gandak River), and the Karnali River basin. All ultimately
become major tributaries of the Ganges River in northern India. After
rising inside Tibet or in the Transhimalayan Ranges along the border,
they cross the main Himalaya and Lesser Himalaya in deep gorges and then
deposit their heavy loads of sediments and debris on the plains,
thereby nurturing them and renewing their alluvial soil fertility. Once
they reach the Tarai Region, they often overflow their banks onto wide
floodplains during the summer monsoon season, periodically shifting
their courses. Besides providing fertile alluvial soil, the backbone of
the agrarian economy, these rivers present great possibilities for
hydroelectric and irrigation development. To date Nepal has not made
extensive use of its hydrolelectric resourses. All of them make use of
the natural elevation differences, diverting water from the river and
running it through turbines further downstream. The latest one is the
Kali Gandaki hydroelectricity project a few km north of Tansen. Building
dams in Nepal has remained a disputed issue, mainly because of the high
risk of earthquakes in the region. None of the river systems support
any significant commercial navigation facility. Rather, the deep gorges
formed by the rivers represent immense obstacles to establishing the
broad transport and communication networks needed to develop an
integrated national economy. As a result, the economy in Nepal has
remained fragmented. Because Nepal’s rivers have not been harnessed for
transportation, most settlements in the Hill and Mountain regions remain
isolated from each other. As of 1991, trails remained the primary
transportation routes in the hills.
The eastern part of the country is drained by the Koshi River, which
has seven tributaries. It is locally known as the Sapt Kosi, which means
seven Kosi rivers (Tamur, Likhu Khola, Dudh, Sun, Indrawati, Tama, and
Arun). The principal tributary is the Arun, which rises about 150
kilometers inside the Tibetan Plateau. The Narayani River or Gandak
drains the central part of Nepal and also has seven major tributaries
(Daraudi, Seti, Madi, Kali, Marsyandi, Budhi, and Trisuli). The Kali
Gandaki, which flows between the Dhaulagiri Himal and the Annapurna
Himal (Himal is the Nepali variation of the Sanskrit word Himalaya), is
the main river of this drainage system. The river system draining the
western part of Nepal is the Karnali. Its three immediate tributaries
are the Bheri, Seti, and Karnali rivers, the latter being the major one.
The Maha Kali, which also is known as the Kali and which flows along
the Nepal-India border on the west side, and the (West)Rapti River also
are considered tributaries of the Karnali.
