Sri Lanka has a typically tropical weather pattern with distinct dry and wet seasons. The island has two monsoons â€“ the south west monsoon that falls in May/ July which brings rain to the southern, western & central regions and the northeast monsoon that falls in October/ November which brings rain to the north and east of the island. Since most tours are confined to the South, central and western regions, the traveller need only worry about the south western monsoons that fall between May and July.
During monsoons it may rain continuously for several days in a row, shower for a couple of hours then clear up, rain only at night or not rain at all for weeks, then switch to torrential downpours. If you're travelling at this time, expect rain, but don't worry unduly about it. The temperature will be mid 20's to early 30's in Celsius.
Most beach resorts are on the west coast where the sea is calmest between November and March.
The temperature ranges between 27- 32 degree Celsius on the island with it being hottest around the April- July period. Arguably the best months to travel to the island are between October and March though it's very safe to say that you will not be disappointed if you travel during any other time of the year either. The temperature in the hilly central regions is much cooler than in the lowlands. (So make sure you pack a jumper if you are travelling to the hills)