This is a rotor cloud, and it's formed by the mountains behind it.

You'll notice that most visible rotors tend to form in long lines that parallel the mountain barrier that's creating them.

Here's a time-lapsed example:

As mountain waves travel downwind of the peaks, rotors form underneath the crests of the ascending wave. These rotors can create moderate to severe turbulence.

Check out the video below to see where they typically form:

So how can you avoid them? If there's any moisture in the atmosphere, the ascending part of the rotor will form a ragged cloud as it rises and cools. Maneuver to avoid the visible moisture, and treat rotors like a thunderstorm: don't fly through them.

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