Weather
Jakarta basically has just two seasons: the "dry season" and "rainy season." Temperatures are almost exactly the same warm conditions all year round.
Historical Weather Patterns for Jakarta
As noted above, there is almost no variation in high or low temperatures during the year. Average highs and lows vary by just a couple of degrees throughout the year.
The rainy season runs from around around October to March, when the monsoons arrive from the north. This isn't an entirely bad time to visit. The rains usually only last an hour or two of drenching downpours in the afternoon, leaving the rest of the time to explore with the sights pretty much to yourself.