Nepal has a great deal of 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-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 sub arctic 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 vegetation 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 southland east facing 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 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. 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 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. The post monsoon season lasts until about December.
After the post monsoon, 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 melt waters, which are critical for irrigation in the lower hills and valleys where agriculture predominates. Winter rain is also indispensable for the success of winter crops, such as wheat, barley, and numerous vegetables.
Rainfall is ample in the eastern portion of the Terai (which receives from 70 to 75 inches [1,800 to 1,900 millimeters] a year at Biratnagar) and in the mountains, but the western portion of Nepal (where from 30 to 35 inches a year fall at Mahendranagar) is drier. Because warm rain-bearing monsoon winds discharge most of their moisture as they encounter the Annapurna range, rainfall is quite heavy (about 100 inches) in the Pokhara Valley. The average annual rainfall is about 55 inches, most of which falls in the period from June to September.
In Kathmandu Valley, average temperatures range from 50 F (10 C) in January to 78 F (26 C) in July, and the lowest and highest temperatures recorded have been 27 and 99 F (-3 and 37 C). At Pokhara the temperature ranges from 40 F (4 C) in January to approximately 100 F (38 C) in June, just before the monsoon. In winter, temperatures during the day rise to 70 F (21 C), creating pleasant conditions, with cool nights and warm days.
• Summer Season: The hot weather season begins in March and continues till the end of May. Due to the northward march of the Sun, the high temperature occurs in all parts of the country. During this season, temperature exceeds 30 o C in the Terai. Towards North, altitude increases and it remains warm in midland hills and valleys. It is cool in the Himalayan region. Days become sunny, windy and dusty and it gets hazy in some areas. Most parts of Nepal are dry in this season. However because of great heat, precipitation in the form of rain and sleet occasionally occurs from local convective storms in April and May.
• Rainy Season: It begins in June and continues up to the end of September. During this season, monsoon blows from south-east direction and bring rain to the country. The regional distribution of rainfall varies from one to the other area. It depends on the oceanic distance and local topographic features of the place. Pokhara Valley located in the central Nepal receives more than 400 cm rainfall. Similarly, southern slopes of Mahabharat and Churia hills in East Nepal receive more than 200 cm rainfall. In general, rain diminishes from Pokhara Valley towards west and north. In the north, there exists a larger rain shadow area beyond the great Himalayan Ranges in the central and western parts of Nepal. It receives poor rainfall less than 50cm. During rainy season it gets sultry and humid. Temperature slightly drops down in rainy season due to frequent rainfall.
• Winter Season: It includes the month of October, November, December, January and February. During winter season, the sun shines over the southern hemisphere and the temperature becomes very low all over the country. It is about 15 o C temperature in the Terai. The Himalayan zone above 3000m, records the temperature below freezing point. The higher mountain areas are quite cold. On the whole, winter is generally a dry season. The monsoons blow from land to the sea during this season. However westerly wind cause light rain in the western Nepal. Clear sky, foggy or frosty and cold morning are the general features of this season.
Required Clothing: Lightweight and tropical clothes with umbrella are advised for June to August. Between October and March, lightweight clothes are worn in Kathmandu, with a coat for evenings and warm clothing for the mountains.