Turbulent 'mammatus' clouds boil over Seattle

Mammatus clouds appear near Seattle's Space Needle on Jan. 3, 2012.

Clouds have been absent for a few days around Seattle so it makes sense when they returned they'd want to put on a show.

Seattle and other parts of the Puget Sound region were treated to a display of "mammatus clouds" as rain approached Thursday afternoon.

The clouds and are usually a signal of turbulent, unstable air, and are usually seen after a strong storm has passed, although in Thursday's case, it was before any rain arrived.

Their formation is somewhat unique in that they are caused by sinking motion -- typically associated with drying weather as sinking air warms and dries as it does so.

Check the University of Illinois atmospheric sciences site for a more detailed explanation of how mammatus clouds form.

Here is some time lapse video of the clouds as they approached South Everett, courtesy Jonathan Cooper of Seattle Image Photography:

And here is one from Daron Johnson, also of South Everett

And finally one from Greg Johnson -- who has 2 cameras focused on the

skies over Skunk Bay

(near Hansville):