The Mentawai Islands are located only 1 degree south of the Equator. Their tropical climate does not alter much during the year. The average temperatures are between 26° and 28° Celsius. It often rains at the end of the day, and the rainfall’s monthly average only varies by 30 per cent during the whole year. The rainiest months are supposed to be October, November and early December.
It is almost impossible to predict what the weather will be like during your stay, especially as the global warming induces a unstability that is often greater than the usual seasonal variability.
What is for sure: it will rain, usually at the end of the day, one or two hours, about 4 times a week. You will get lot of sun, too.
Very roughly, one can say that from October to early January, the rains tend to be stronger and heavier. But it rains not more often than the rest of the year and the sky tends to be more often blue.
From February to October, the rains tend to be a little lighter and shorter, but not less often, and the sky is rarely perfectly blue.

The best season for the surf is from mid-April to mid-September, when the swell is at his best.