Due to its location close to the equator, Singapore experiences a warm and tropical climate year-round. Rainfall is common and it's always a good idea to pack waterproofs in anticipation of a daily rainstorm. The high humidity is almost legendary with little variation in temperature or humidity month-on-month.
However, monsoon rains do create fluctuation in the volume of precipitation with the wettest months between November and January. Rain showers tend to fall in the late afternoon and evening, and winds tend to be stronger during the first half of the monsoon season.
It's something of a misnomer but February to August is considered the 'dry season' but don't be fooled as rain is a common feature of the weather even at this time of the year.