Introduction to Climate of Sri-Lanka
Due to location slightly North (6-10o) of the Equator in the Indian Ocean, the climate of Sri-Lanka is tropical, is relatively wet, and has two seasons. From May to October, the winds over Sri-Lanka are Westerly to North-Westerly (i.e winds come from the west). While from December to March, the winds over Sri-Lanka are North-Easterly. The wind speeds rises in the land surfaces in the months of June to September and from December to February. Regionally the average wind speeds are high in the South-Eastern and North-Western extremes of the island and in the mountain passes.
The island is humid all over Sri-Lanka and the relative humidity is over 65% usually. In the coasts, it rises to 90% in the wetter seasons. There are dry desiccating effects due to mountain effects in the windier months and the humidity drops in the eastern region from June to September. There is a weaker dessicating effect on the western mountain slopes.
Sri-Lanka receives 1,800 mm of rainfall on average annually distributed unevenly ranging from 500 to 5000 mm/year. The rainfall follows a bimodal climatology with the main rains from September to December and subsidiary rains from April to June. The Eastern and Western Hill Slopes garner orographic (mountain induced) rainfall from December to March (North-Easterly winds) and May to October (Westerly wind) respectively.
The North-East receives cyclonic rainfall from November to December from the storms and cyclonic systems that are steered from the Bay of Bengal by the North-Easterly winds towards Sri-Lanka. Once they make landfall, these systems can dissipate quickly unless they are particularly strong.
The mean annual temperature of the country is 27 oC with lower temperatures in the mountains that rise to 2500M.Temperature drops during December and January and increases from April to September. The mean daily range is approximately 6 oC. These temperatures are more moderate than what is typical for such latitudes during the Northern Hemisphere summer from June to August. We can attribute this to the cooling effect of the oceans during the warmest months.