A banner cloud is a stationary cloud forming exclusively on the lee side of sharp, isolated peaks (Matterhorn, Peña Telera). Unlike normal orographic clouds that form on the windward side, banner clouds are generated by pressure drop in the lee vortex.
Pressure reduction in the lee vortex cools air below the dew point, forming a cloud that "flies" horizontally from the summit. It is one of the most photogenic cloud phenomena and its presence indicates strong winds at the peak.