There are two monsoon seasons in the Bintulu : the northeast moonsoon ( November to March ) and the southwest moonsoon ( May to September ) . The calm period between these two moonsoons is known as the transitional period . In the coastal region , maximum rainfall occurs in the month of January , while minimal rainfall occurs from the period June to August . Rainfall is more evenly distributed in the inland areas . The annual rainfall of the Bintulu region is about 3 @,@ 750 mm ( 148 in ) annually . The mean daily hours of sunshine at Bintulu is about 5 @.@ 0 to 5 @.@ 5 hours . Bintulu receives on average 14 to 15 mJ / m2 of radiation throughout the year . Bintulu 's relative humidity is 85 % .