The Island of Sri Lanka is located between six and ten degrees north of the equator. Obviously the average temperature is quite high in Sri Lanka and fluctuates between 27° and 29° Celsius. As everywhere, sea-winds exert a moderating influence. Temperatures in the central mountain region are lower than normal.
Day and night temperatures may vary by 4 °C (7.20 °F) to 7 °C (12.60 °F). January is the coolest month, especially in the highlands, where overnight temperatures may fall to 5 °C (41 °F).
Nuwara Eliya, a mountain resort town, frosts on some nights in December and January. As for humidity it is till 90% at night and 70% during the night. May, the hottest period, precedes the summer monsoon rains.
There are four rainy periods in Sri Lanka.
From May until September, Southwest monsoon rain. In this period central province (Kandy, Nuwara Eliya),North central province( Anuradapura, Polonnaruwa, Sigiriya) and Eastern provinces and coastal areas are more friendly for tourists.
Inter-monsoon showers come in October and November, northeast monsoon rains from December till February. In this period western/southern coastal areas (from Kalpitiya to Tangalle) are ideal for white sandy beach lovers.
Again inter-monsoon rain in March and April.
For a beach holiday on the southwest coast, you must go from November until April.
If you plan to visit east coast, you must visit during March to September.
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% all year, rising to almost 90 % during the monsoon season in June. Anuradhapura experiences a daytime low of 60% during the inter monsoonal month of March, but a high of 79% during the November and December rains. In the highlands, Kandy’s daytime humidity usually ranges between 70 and 79%.