Monsoons produce two rainy seasons. The southwest monsoon lasts roughly from May to September. During this period, the southwestern part of the island, including Colombo, receives much of its average annual rainfall of 100 inches.
The northeast monsoon, from about October or November through February, provides the northern and eastern parts of the island virtually all their average annual rainfall of 60 inches. Monsoon showers can become torrential in the Colombo area. December through March generally proves the driest period.