I created this album specifically for photos of Lenticular Clouds. I was excited to see some yesterday (Christmas Day 2006) for the first time over the western Sierra Nevada... But I hear they are quite common on the east side.
Here's a definition of a lenticular cloud courtesy of wikipedia.org
"Lenticular clouds, technically known as altocumulus standing lenticularis, are stationary lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction.
Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves. Under certain conditions, long strings of lenticular clouds can form, creating a formation known as a wave cloud."