There are four rainfall seasons during the
year. These are :-
1. The South - west monsoon period (May
2. The Intermonsoon period following the
South-west monsoon (October to November)
3. The North - east monsoon period
(December to February)
4. The Intermonsoon period following the
North- east monsoon (March to April)
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 Nuwara Eliya 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 Nuwara Eliya 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 deg C 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 Nuwara Eliya 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 Nuwara Eliya 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 per
cent, 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.