Climate of Sri Lanka
Being located in the tropics, Sri Lanka which consists mostly of flat to rolling coastal plains, with mountains rising only in the south-central part has a tropical climate with two distinct seasons of wet and dry. The rainfall pattern is influenced by monsoon winds from the Indian Ocean and Bay of Bengal and there are two monsoons viz. northeast monsoon (December to March) and southwest monsoon (June to October).
Due to the location of Sri Lanka within the tropics between 50 550 and 90 510 North latitude and between 790 42' and 810 53' East longitude, the climate of the island could be defined as tropical.
The central part of the southern half of the island is mountainous with heights more than 2.5 km. The core regions of the central highlands contain many complex topographical features such as ridges, peaks, plateaux, basins, valleys and escarpments. The remainder of the island is flat except for several small hills in the lowlands. These topographical features strongly affect the spatial patterns of winds, seasonal rainfall, temperature, relative humidity and other climatic elements, particularly during the monsoon season.
Rainfall in Sri Lanka has multiple origins. Monsoonal, convectional and expressional rains account for a major share of the annual rainfall. The mean annual rainfall varies from under 900mm in the driest parts (southeastern and northwestern) to over 5000mm in the wettest parts (western slopes of the central highlands).
Sri Lanka's position between 50 and 100 north latitude endows the country with a warm climate including moderate ocean winds and considerable moisture. The mean temperature ranges from as low as 15.80oC in Nuwara Eliya in the Central Highlands (where frost may occur for several days in the winter) to the hottest temperature of 290oC in Trincomalee on the northeast coast (where temperatures may reach 370oC). The average yearly temperature for the country on a whole ranges from 260oC to 280C. Day and night temperatures may vary from 4 to 7. January is the coolest month, causing people, especially those in the highlands, to wear coats and sweaters. May, the hottest period, precedes the summer monsoon rains.
Regional differences observed in air temperature over Sri Lanka are mainly due to altitude, rather than to latitude. The mean monthly temperatures differ slightly depending on the seasonal movement of the sun, with some modified influence caused by rainfall. The mean annual temperatures in Sri Lanka is manifested largely as standardized temperatures in the low lands and rapidly decreasing temperatures in the highlands. The mean annual temperature varies between 26.50oC to 28.50C in the lowlands up to an altitude of 100m to 150m with an annual temperature of 27.50oC. In the highlands, the temperature falls quickly as the altitude increases. The mean annual temperature of Nuwara Eliya, at 1800m above the sea level, is 15.90oC. The coldest month with respect to mean monthly temperature is generally January, and the warmest months are April and August.
The mean annual temperature varies from 270oC in the coastal lowlands to 160oC at Nuwara Eliya, in the central highlands (1900m above mean sea level).
of the country and the Southwest and Northeast monsoons regional scale wind regimes. The climate experienced during the 12 month period in Sri Lanka can be characterized into 4 climate seasons as follows.
-First Inter Monsoon Season - March - April
-Southwest Monsoon Season - May - September
-Second Inter Monsoon Season - October - November
-Northeast Monsoon Season - December - February
First Inter-Monsoon Season (March - April)
During this season the typical weather conditions are: thunderstorm-type rains, particularly during the afternoon or evening. The distribution of rainfall during this period shows that the entire South-western sector at the hill country receives 250mm of rainfall, with the localized area on the South-western slopes experiencing a rainfall in excess of 700mm (Keragala 771mm). Over most parts of the island, the amount of rainfall varies from 100 to 250mm with the notable exception being the Northern Jaffna Peninsula (Jaffna - 78mm, Elephant Pass - 83mm).
Southwest Monsoon Season (May - September)
Windy weather during this monsoon eases off the warmth that prevailed during the 1st inter monsoon season. Southwest monsoon rains are experienced at any time during the day or night; sometimes occasionally mainly in the Southwestern part of the country. Amount of rainfall during this season varies from about 100mm to over 3000mm. The highest rainfall received in the mid-elevations of the western slopes (Ginigathhena - 3267mm, Watawala- 3252mm, Norton - 3121mm). Rainfall decreases rapidly from these maximum regions towards the higher elevation, in Nuwara Eliya it drops to 853mm. The variation towards the Southwestern coastal area is less rapid, with the Southwestern coastal belt experiencing between 1000mm to 1600mm of rain during this five-month long period. The lowest figures are recorded from Northern and Southeastern regions.
Second Inter-Monsoon Season (October - November)
Thunderstorm-type of rain, particularly during the afternoon or evening, is the typical climate during this season. But unlike in the inter monsoon season, the influence of weather system on the land with effects like depression and cyclones in the Bay of Bengal is common during the second inter monsoon season. Under such conditions, the whole country experiences strong winds with wide spread rain, sometimes leading to floods and landslides. The second inter monsoon period which is from October - November is the most evenly balanced distribution of rainfall in Sri Lanka. Almost the entire island receives in excess of 400mm of rainfall during this season, with the Southwestern slopes receiving higher rainfall in the range of 750mm to 1200mm.
Northeast Monsoon Season (December - February)
The dry and cold wind blowing from the Indian land-mass will create a comparatively cool, but dry weather condition over many parts making the surrounding pleasant and comfortable weather except for coldness to a certain extent during the morning hours. Cloud-free skies provide days full of sunshine, and a pleasant and cool night. During this period, the highest rainfall figures are recorded in the North, Eastern slopes of the hill country and the Eastern slopes of the Knuckles/Rangala range. The maximum rainfall is experienced in the Kobonella estate (1281mm), and the minimum is in the Western coastal area around Puttalam (Chilaw - 177mm) during the same period.