The whole archipelago is tropical, with temperatures at sea level always between 21˚C and 33˚C, although cooler in the mountains. In theory, the year divides into a wet and dry season, though it’s often hard to tell the difference. Very roughly, in much of the country, November to April are the wet months (Jan and Feb the wettest) and May through to October are dry. However, in northern Sumatra, this pattern is effectively reversed. The peak tourist season, but not necessarily the best time to visit, is between mid-June and mid-September and again over the Christmas and New Year season. This is particularly relevant in the major resorts, where prices rocket and rooms can be fully booked for days, and sometimes weeks, on end.