Asperatus Clouds Over New Zealand . ... Speculation therefore holds that asperatus clouds might be related to lenticular clouds that form near mountains, or mammatus clouds associated with thunderstorms, or perhaps a foehn wind -- a type of dry downward wind that flows off mountains. Such a wind called the Canterbury arch streams toward the east coast of New Zealand's South Island . ... NASA Web Site Statements, Warnings, and Disclaimers . NASA Official: Jay Norris. ...