Wave Clouds

Wave clouds (or gravity waves): These form when stable air moves over a raised land feature, such as hills or mountains, and is forced upward. Gravity then causes the air to fall back down, and it begins to oscillate, creating that ripple effect. 

This visible satellite image was taken at 400 pm PDT 9 Sep 2000. The cloud lines oriented north to south are wave clouds. They are the result of west-to-east wind flowing over the Cascade mountains of Washington and the Rocky Mountains of Montana and Alberta.