There are two main seasons in the Mentawai: the dry season, which runs from around February to October, and the wet season, which takes up November to January. While the wet season is certainly rainier, one will experience plenty of refreshing tropical showers in the dry season as well. Average air temperatures range between 72°F (22°C) to 90°F (32°C). Water temp runs a pretty constant 80°F (27°C). Wetsuits are never necessary, but tropical wax is always required!
The Mentawai's position in the Indian Ocean put them in the ultimate spot to pick up phenomenal amounts of swell. Add that swell to the ideal geography of the islands and you end up with the seemingly endless array of perfect set-ups on offer. In no other place on earth can you find such an abundance of world class waves in such a small area breaking with such consistency. There are set-ups for every type of wind and swell condition. Clean, head high waves are virtually guaranteed even if there is no obvious groundswell running.
While great waves can be found in the Mentawai all year 'round, most people choose to go during the dry season between March and November when the Indian Ocean is at its most active. Whatever month you go, and whatever the wind, swell, and tide conditions happen to be, somewhere in the Mentawai will be working and very likely pumping.
Perhaps the best thing about the islands is that the perfection isn't limited to the dozen or so famous breaks you've seen in the surf mags and movies. There are plenty of other lesser known and even yet to be discovered breaks that would be considered a dream wave in any surfer's book.