An intimidating looking thunderstorm rolled across Sydney, Australia yesterday afternoon, bringing with it flooding rainfall and intense lightning according to the Australian Bureau of Meteorology.
The awesome cloud everyone took a picture of is called a shelf cloud. Basically, the cold air descending out of a thunderstorm flows out ahead of the storm (called an "outflow"). As the outflow moves away from the thunderstorm, it takes on a half-dome/bubble shape as it rides along the surface.
As warm, moist air rides up along the colder, denser air in the outflow, it condenses along the top and creates a striking shelf cloud.
[Image by James Brickwood/Sydney Morning Herald]