Geography of Nepal - Climate

Climate

Nepal has tremendous variation in climate. Its latitude is about the same as that of Florida so Terai land up to 500 meters (1,640 ft) has a fully tropical climate, with a subtropical zone extending up to 1,200 meters (3,937 ft) which is the lower limit of frost in winter. Warm temperate climates prevail from 1,200 to 2,400 meters (3,937 to 7,874 ft) where snow occasionally falls. Then there is a cold zone to 3,600 meters (11,811 ft) (treeline), a subarctic or alpine zone to 4,400 meters (14,436 ft) and fully arctic climate above that. Precipitation generally decreases from east to west with increasing distance from the Bay of Bengal, source of the summer monsoon. Eastern Nepal gets about 2,500 mm (98.4 in) annually; the Kathmandu area about 1,400 mm (55.1 in) and western Nepal about 1,000 mm (39.4 in). This pattern is modified by adabiatic effects as rising air masses cool and drop their moisture content on windward slopes, then warm up as they descend so relative humidity drops. Annual precipitation reaches 5,500 mm (216.5 in) on windward slopes in the Annapurna Himalaya beyond a relatively low stretch of the Mahabharat Range. In rainshadows beyond the high mountains, annual precipitation drops as low as 160 mm (6.3 in), creating a cold semi-desert.

Furthermore the year is divided into a wet season from June to September—as summer warmth over Inner Asia creates a low pressure zone that draws in air from the Indian Ocean—and a dry season from October to June as cold temperatures in the vast interior creates a high pressure zone causing dry air to flow outward. April and May are months of intense water stress when cumulative effects of the long dry season are exacerbated by temperatures rising over 40 °C (104 °F). Seasonal drought intensifies in the Siwaliks hills consisting of poorly-consolidated, coarse, highly permeable sediments that do not retain water, so hillsides are covered with tropical scrub forest that is extremely drought-tolerant. In fact outside marsh and riparian zones, most of Nepal's native vegetation is adapted to withstand drought, although less so at higher elevations where cooler temperatures mean less water stress.

The monsoon is preceded by a buildup of thunderstorm activity in the hills that helps farmers irrigate rice seedbeds. Sustained rain on average arrives in early June as rising temperatures over Inner Asia creates a low pressure zone that draws in air from the Indian Ocean, but this can vary up to a month. Significant failure of monsoon rains historically meant drought and famine while above-normal rains still cause flooding and landslides with losses in human lives, farmland and buildings. The monsoon also complicates transportation with roads and trails washing out while unpaved roads and airstrips may become unusable and cloud cover reduces safety margins for aviation. Rains diminish in September and generally end by mid-October, 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 marked by occasional, short rainfalls in the lowlands and plains and snowfalls in the high-altitude areas. Precipitation varies from year to year but increases markedly with elevation. Adequate snowfall in the Himalaya is important for sufficient spring and summer meltwater for irrigation in the lower hills and valleys. At lower elevations, winter rainfall is needed for the success of winter crops such as wheat, barley and vegetables. In this season the Himalaya function as a barrier to cold air masses from Inner Asia, so southern Nepal and northern India have warmer winters than would otherwise be the case. April and May are dry and hot, especially below 1,200 meters (3,937 ft) where afternoon temperatures may exceed 40 °C (104 °F).