Tuesday, October 1

Climates of Nepal with glimpse on Post Monsoon

Nepal is a small landlocked country located between world’s two most populous countries China to the north and India to the east, west and south with a total land area of 147,181 sq km. Nepal stretches from 26º 22’ to 30º 27’ North latitude and from 80º 04’ to 88 º 12’ East longitudes. The country looks roughly rectangular in shape with the length from east to west of about 885 km and width ranging from 130 to 260 km. The elevation of the country increases from about 60 m above mean sea level in the south to more than 8800 m above mean sea level in the north. The highest peak Mt. Everest, known as Sagarmatha to the Nepalese, rises to 8848 m and lies at the northern part of east Nepal . The country is divided into three broad ecological regions, i) the mighty Himalayas in the north, ii) Hills and Valleys in the middle, and iii) Terai, an extension of Indo-Gangetic plain, in the south. Mountains, Hills and the Terai regions are homes to 7 %, 46 % and 47 % of Nepal’s population respectively. Terai is a low-lying plain, highly vulnerable to floods during the monsoon. Northwards, a series of complex valleys breaks up the simple pattern of parallel east-west mountain ranges, and one of these, the valley of Kathmandu, contains the capital of Nepal. Further to the north rises the Himalayas, the world's greatest mountain range.

Climate of Nepal is based upon monsoon with great deal of variation in climate. Altitudinal variation within a country ranges climate from tropical to arctic. 

Seasons or Climate of Nepal can be categorized as;

Pre-monsoon or Hot weather Season :                     March-May
Summer(southwest) Monsoon or Rainy Season:       June-September
Post-monsoon:                                                        October and November
Winter (Northeast) Monsoon:                                  December-February

Traditionally, 6 types of Nepali Seasons

Spring -     Basanta           Chaitra-Baisakh (~April-May)    
Summer -  Grishma            Jestha-Asar (~June-July)    
Monsoon - Barsha            Shrawn-Bhadra (~Auguste- September)    
Autumn -  Sharad              Ashoj-Kartik (~October-November)    
Winter -    Hemanta          Mangsir –Poush (~December-January)    
Windy -     Sisir                 Magh-Falgun (~February-March)  

As described in Wikipedia, the free encyclopedia,( http://en.wikipedia.org/wiki/Geography_of_Nepal), Nepal has tremendous variation in climate. Its latitude is about the same as that of Florida of the USA 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.

Fig.1; Nepal Map
(source: Wikipedia, the free encyclopedia)

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.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.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).

Below is the first day temperature plot of Post Monsoon 2013;

Fig.2: 1 Oct 2013, Temperature Plot
(Data Source: mfd.gov.np)

And the rainfall statistics for the October in Kathmandu as stated in mfd.gov.np, normal this month is 51.2 mm and the highest 24hrs amount ending at 8:45 AM ever recorded in this month on 20/1987 is 124 mm.

The post monsoon can also be called an autumn. This period is considered as retreating of summer monsoon and is transitional months when the pressure and upper wind systems undergo gradual change. The intertropical convergence zone (ITCZ) now moves towards equator and the temperature starts falling down. Also, autumn sees the southward swing of the Equatorial Trough and the zone of maximum convection, which lies just to the north of the weakening easterly jet. The break-up of the summer circulation systems is associated with the withdrawal of the monsoon rains, which is less clearly defined than their onset. By October, the easterly trades of the Pacific affects the Bay of Bengal at the 500mb level and generate disturbances at their confluence with the equatorial westerlies. Occasional showers with thunderstorm and wind directions undergoes a change i.e. winds start blow from west. 

Fig.3: Mean sea level isobars -October
(Source:
Ways of the Weather, P A Menon, National Book Trust, India)

Weakening of low pressure system over the Indian subcontinent as a whole is the most important climatic characteristics feature. But the low pressure center is found over the Bay of Bengal where water surface temperature reach a maximum in this season. This is the major season for Bay of Bengal cyclones and it is these disturbances, rather than the onshore north-easterly monsoon, that cause the October/November rainfall. A number of cyclone form over the southern part of the Bay. Most of these cyclones curve round to the north of central low and reach the east coast from the west.  Sometimes these cyclones becomes very violent with formation of eye in Indian Ocean. There is also correspondence between the oscillation of ITCZ and the areas of rain incidence. 
Fig.4: Normal Upper Winds- October
(Source; 
Ways of the Weather, P A Menon, National Book Trust, India)

During October, the westerly jet re-establishes itself south of the Tibetan Plateau, often within a few days, and cool season conditions restored over most of south and East Asia. Temperatures decreases all over and the nights become cool. By November sub-zero temperatures are reached over high terrain. Hence, this is really the cool season.

Rather than Nepal mostly south India gets rainfall in this season. Further, rain occurrence is confined to periods when lows, depressions, cyclones or easterly waves move over or across the peninsula. Hence this season is one of retreating south west monsoon

Reference for Further Reading:
  1. Ways of the Weather, P A Menon, National Book Trust, India
  2. Atmosphere, Weather and Climate, Seventh Edition, Roger G. Barry and Richard J. Chorley, Routledge, London and New York
  3. General Climatology, Fourth Edition, Howard J. Critchfield, Prentic-Hall, India
  4. Climatology, D.S. Lal, Sharda Pustak Bhawan, Allahabad, India