The city has a tropical monsoon climate, with hot and humid weather throughout the year. 

The monsoon season in Kollam starts in June and lasts until September, bringing heavy rainfall to the region. The average annual rainfall in Kollam is around 2,700 mm (110 inches), with the highest rainfall occurring in July and August. 

The winter season in Kollam is from December to February, with relatively cooler and drier weather. The summer season in Kollam is from March to May. 

Overall, Kollam has a warm and humid climate throughout the year, with heavy rainfall during the monsoon season and warm temperatures during the rest of the year.