Presumably mammatus cloud is created by rolling turbulence. It always seems to occur at the trailing edge of a storm, much like the turbulence in seawater behind a breaking wave. Here you can see the back of a storm as it recedes, with the turbulent mammatus cloud patterns getting smaller as the leading edge travels away from the viewer. You can see a similar pattern from underwater when a breaking wave passes overhead.


QF1549 landing in Canberra by RobGeraghty, on Flickr

[ranmar850]

Often associated with rain developing, over our way--you see stratus developing mammatus, likely to get wet. Good captures.

[eaglem]

Interesting and strange cloud formations

[ZombieArmy]

Around here you'll see them preceding severe weather.

[RobG]

I guess YMMV. Different places will get different clouds. Where I am, mammatus cloud seems to be in the turbulent wake of a storm front.

[reh321]

I have to admit I've never been out there observing such things - I'm more likely to be under cover or looking for cover - but the photos are interesting anyway.