The picture in today's "Earth Sciences Picture of the Day" (EPOD) interested me immediately with its title "Waves in the Sky." It's not freaque wave, but an interesting picture nevertheless.
While it is not extremely rare, it is considered as unusual clouds. Another similarly intriguing one is given here as the Kelvin-Helmholtz wave cloud shown below. (In EPOD's description, Helmholtz was misspelled.)
It is usually being formed at the interface of two layers of air mass with a warm layer on top a cold layer generally led to instability and Kelvin-Helmholtz wave cloud. Actually the EPOD picture shows better that the two air mass layers.
Apparently there is no equivalent freaque wave in the sky. But to the extent that freaque waves sometimes being described as "a hole in the sea," it is not hard to look up in the sky and find a hole in the cloud -- though not of the freaque wave kind. There are freaque waves in the stock market, but probably not in the sky!