This was taken on our awesome North America road trip a few years ago; snapped from the car while travelling along the I-90 (I think) towards Seattle. We were actually headed to Vancouver but wanted to stop and experience Seattle for a couple of hours. On the way, seeing these clouds meandering down the mountains was unexpected and very cool.
Shot on iPhone 4, tweaked in Lightroom with additional b/w processing in Silver Efex Pro 2.
1/1000 sec | f/2.8 | ISO 80