Sri Lanka's position between 5 and 10 north latitude endows the country with a warm climate, moderated by ocean winds and considerable moisture. The mean temperature ranges from a low of 15.8° C in Nuwara Eliya in the Central Highlands (where frost may occur for several days in the winter) to a high of 29° C in Trincomalee on the northeast coast (where temperatures may reach 37° C). The average yearly temperature for the country as a whole ranges from 26° C to 28° C. Day and night temperatures may vary by 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.

The rainfall pattern is influenced by the monsoon winds of the Indian Ocean and Bay of Bengal and is marked by four seasons. The first is from mid-May to October, when winds originate in the southwest, bringing moisture from the Indian Ocean. When these winds encounter the slopes of the Central Highlands, they unload heavy rains on the mountain slopes and the southwestern sector of the island. Some of the windward slopes receive up to 250 centimeters of rain per month, but the leeward slopes in the east and northeast receive little rain. The second season occurs in October and November, the intermonsoonal months. During this season, periodic squalls occur and sometimes tropical cyclones bring overcast skies and rains to the southwest, northeast, and eastern parts of the island. During the third season, December to March, monsoon winds come from the northeast, bringing moisture from the Bay of Bengal. The northeastern slopes of the mountains may be inundated with up to 125 centimeters of rain during these months. Another intermonsoonal period occurs from March until mid-May, with light, variable winds and evening thundershowers.

Humidity is typically higher in the southwest and mountainous areas and depends on the seasonal patterns of rainfall. At Colombo, for example, daytime humidity stays above 70 percent all year, rising to almost 90 percent during the monsoon season in June. Anuradhapura experiences a daytime low of 60 percent during the intermonsoonal month of March, but a high of 79 percent during the November and December rains. In the highlands, Kandy's daytime humidity usually ranges between 70 and 79 percent.