Polonnaruwa sits in Sri Lanka's dry zone, with a markedly hotter and drier climate than the central hill country. The best months are December to March, when daytime temperatures are around 30°C and the humidity is bearable; April and May, before the southwest monsoon, are the hottest and most uncomfortable.
The northeast monsoon brings rain from October to December, but rarely enough to wash out a visit — afternoons may end in a thunderstorm but mornings stay clear. Start your day at the site at first light: the ruins are at their best in the cool dawn light, and most of the tour buses arrive after nine.