No, it's not the subject line from junk mail, but an obscure reference to an atmospheric condition.
A friend sent me this link to a set of spectacular photographs of mammatus clouds - which are the best I have seen.
Within the same week, I read this news release from the Geological Society of America, talking about a new form of ash plume over an Ecuadorean volcano. Although the article does not explicitly say it, they are describing volcanic mammatus.
I would bet there are some slight differences in the dominant wavelengths because of density differences between regular cloud and an ash-steam mix, but the "hundreds of meters" wavelength looks about right for the Nebraska photos.