These cloud-waves rarely occur because you need almost perfect atmospheric conditions.  Much like ocean waves, the air on the tops of these clouds is moving faster than the bottom of the clouds.  The clouds near the surface are cooler than above and the wind speeds are light, like fog.  Over the low clouds is a warmer and faster-moving layer of air creating the crest, like we see in the ocean.

Rita Casserly sent in this picture.

Wave Cloud

Photo by: Lonnie Follis, near Livingston

Wave Cloud

Photo by: Kathleen Dimmel, Navasota