A very rare roll cloud formed at the foot of Mount Salak. .
This cloud only happens when there are two opposite wind directions that frontally hit each other:
The cold gust from the top of Mount Salak and the warm breeze from Jakarta bay. They collide, and as the result, the heavy cold air from Mount Salak sinks to the ground pushing the warm moist breeze from Jakarta bay up, forcing the moisture to condense as it gains more altitude and becomes colder, creating a cloud that rolls horizontally.
Quite an awesome sight.