It's embarrassing that it took me so long to realize that clouds act the same as water during long exposures - that is, the become smooth and formless. Anyhow, that's how I got this. I was testing out my new neutral density filter (the Tiffen 52mm 0.9 Neutral Density filter)  and it was working pretty well. It's not going to really let you do long exposures in broad daylight, but this is a 30 second exposure at f/25 directly facing the sunset, so I'd say it's approximately good enough for that.

Fwoooooosshhhh. That's probably what those clouds sound like.

Original post can be found at kylecameronphotography.com