|CHAPTER I - AREA AND CLIMATE
|Location and relief of Sri Lanka
As Wadia, an eminent geologist has shown, until the separation of the island sub-continent during Miocence times (10-25 million years ago), the geological history of the island would have been the same as that of Gondwanaland – super continent which included Antarctia, Australia, Madagascar, along with the subcontinent. Adam’s Bridge across the Palk Strait and shallow Gull of Mannar were created through this process of gradual separation from the South Asian land-mass..
Sri Lanka, which ranks 118th in area among the 184 member family of nations, is a small country. Sri Lanka is located well within the tropics, between the northern latitude of 50 55' and 90 51' and the longitude of 790 41' to 81 0 53'. It has an area of 65610 km2. The maximum length of the island is 432 km and the maximum width is 224 km.
Sri Lanka’s territorial sea extends to a distance of 12 nautical miles from its coastline. Although Sri Lanka’s land area is relatively small, its marine economic zone comprising some 230,000 km2 is nearly four times its land area. Sri Lanka has a coastline of some 1790 km.
The land-mass of Sri Lanka is a teardrop or pear-shaped, elongated island, with a wider area in the south that narrows forwards the north. The south central area is mountainous, with enchanting scenery that has attracted many a visitor from foreign lands over the centuries.
Some of the high peaks in the central highlands rise about 2000m above the sea level. Among these, Piduruthalagala being the highest at 2524m.
The central mountain region is surrounded by a plain, rising above the sea level up to about 300 m. This plain spreads over two thirds of the total area of the island. It is narrow and somewhat ragged in the south west but broad and flat in the north and east. It is characterized by residual rocks, which carry local names such as Ethugala, lbbagala and Sigiriya.
The core area of the central mountains is a complex of peaks, plateaus, basins, ridges, valleys and escarpments. One of the most developed escarpments is the World’s End that marks the edge of the southern mountain wall.
Sri Lanka receives an average some 2000mm of rain annually, amounting to about 130 billion cubic meters of water. The surface water that remains, after evapotranspiration and seepage, drains in to the sea through the well-organized system of natural river basins. Sri Lanka has a total of 103 distinct natural stream basins.
The Mahaweli occupies a unique position among the rivers of Sri Lanka, both in terms of its length as well as its basin size. It is longer than the combined length of any other two rivers in the island.
Sri Lanka is also well known for its numerous waterfalls, some 54 distinctive waterfalls have been recorded. The most prominent of the waterfalls include Diyaluma, Dunhinda, St.Clair and Devon. The tallest waterfall is Bambarakanda, the height is 241m.
There are four rainfall seasons during the year. These are :-
Rainfall is of three types- monsoonal, convectional and depressional. Monsoon rain occurs during the two monsoon periods, namely, the South - west and North - east, and is responsible for nearly 55% of the annual precipitation. Convectional rain occurs during the intermonsoon periods, mainly in the afternoon or evening and is likely to be experienced anywhere over the Island. Depressional rain also occurs during the intermonsoonal periods, particularly during the second intermonsoon (October to November).
The annual average rainfall varies from below 1000 mm (39'') over a small region in the arid parts of the North - west and South - east of the Island to over 5000 mm (197'') at a few places in the Kegalle and NuwaraEliya districts (on the South - western slopes of the central hills).
Rainfall during the South - west monsoon is mostly over the South - western parts of the Island. At the beginning it occurs in the South- western low country. As winds strengthen, it spreads gradually to the interior, with considerable heavy rain in the hill country from June to August. South - west monsoon rainfall exceeds 3000 mm (118'') at a few places in the Kegalle and NuwaraEliya districts.
During the North - east monsoon, the eastern half of the Island receives from about 200 mm (8'') to over 1200 mm (47'') of rain. The higher rainfalls are experienced over the Rangala range of hills.
During the intermonsoon periods, winds are generally light except for the sea breeze which develops from about noon and is also responsible for the thundershowers that occur in the afternoon or evening. These showers may occur anywhere over the Island. Another source of rain during these periods is depressional activity.
Depressions are responsible for a good part of the precipitation during the intermonsoon period, October to November. Rainfall during this period is therefore widespread and exceeds 500 mm (20'') at many places. Taking the Island as a whole, this is the rainiest period of the year.
Conditions are similar during the other intermonsoonal period, from March to April, but the rainfall is less, mainly because of less depressional activity.
The mean annual temperature over Sri Lanka, which lies within latitudes 6 deg N and of approximately 10 deg N and longitudes of approximately 80 deg E and 82 deg E, is about 27.5 degC over the lowlands. The oceanic influence (the maximum width of the Island being only 225 kilometers) helps to reduce the temperature in lowlands by sea breeze. The highlands in the central region enjoy a cooler climate with a mean temperature of 18 deg C.
Temperature decreases at a steady rate of about 6.5 deg C for each 1,000 metre rise. Thus, at Kandy, which is 488 metres above mean sea level, the mean annual temperature is about 24.5 deg C; at Diyatalawa, (1,250 metres above mean sea level) the mean annual temperature is about 20.2 deg C, and at Nuwara-Eliya, where the elevation is 1895 metres, the mean annual temperature is about 15.8 deg C.
A noteworthy feature in many parts of Sri Lanka is the small variation in the mean monthly temperatures throughout the year. On average, the mean temperature of 25.0 deg C during the coolest months, November to February, is only 2.4 deg C lower than that during the warmest months April and May.
Higher temperatures are experienced generally in the Northern, North - central and Eastern regions of the Island and range between 33.3 deg C and 34.7 deg C, on average.
Lower temperatures are experienced during the early hours of the day, a little before dawn. Along the coast, these temperatures occur during December and January and range between 21.0 deg C and 24.2 deg C, on average. At NuwaraEliya the average maximum temperature is 22.8 deg C in April (highest) and the average minimum temperature is 9.4 deg C in January (lowest). The minimum temperature at NuwaraEliya falls below the freezing point (0 deg C) only very occasionally.
Diurnal variation of temperature, i.e., the rise to a maximum early in the afternoon and the fall to a minimum shortly before dawn, is well marked. Its magnitude depends on the season. There is a gradual increase in the range with altitude as well as with distance from the sea.
Relative humidity varies generally from about 70 percent during the day to about 90 to 95 percent at night. In the dry zone, however these values are lower by about 5 percent, while in the driest areas in the North - west and South - east relative humidity drops to about 60 per cent.
During intense thunderstorms, hail is experienced occasionally. It occurs mainly in the hill - country but there have been reports of the occurrence of hail in low - country stations too.
Ground frost occurs in Nuwara - Eliya on a few days of the year during the months of January and February.
Statistical analysis of air temperature and rainfall data collected by the Department of Meteorology over a period of more than 100 years have shown an increasing trend in the annual mean air temperature over the entire Island, particularly during the more recent period, 1961 - 1990. This increase was found to be approximately 0.16 deg C per decade. Rainfall trends were found to be some what complex; there were decreasing trends over most of the island except for some isolated areas in the north-western province, where an increasing trend was indicated. Thunder activity, in addition to showing an increasing trend, was found to be positively correlated with air temperature.