Cloud Forming From the top of Dakota Ridge in Boulder, Colorado on Tuesday morning:
What initially looked like a rain storm dumping on a lake turned out to be exactly the opposite! This picture at sunrise shows a cloud being formed over a small lake east of Boulder, Colorado. The picture was taken from the top of Dakota Ridge.
There's an interesting article on cloud formation at Wikipedia. It makes me wonder if the cloud formation was accelerated by particulate matter from the nearby smokestack.