The first photograph below was taken on a late-afternoon flight from Bangkok back to Hong Kong on 17 July 2004. Pileus cloud was seen above a towering cumulus which rose to the altitude of the flight (more than 30,000 feet). Pileus cloud is normally very short-lived. It occurs when a vigorously rising cumuliform cloud encounters a hitherto invisible layer of moisture. Such clouds are very similar in appearance to lenticular clouds formed over mountains (see second photograph below taken over Lantau Island on 10 April 2004). Indeed they both arise through a rather similar mechanism – a layer of humid airflow forced to rise on encountering the towering cumulus (for pileus clouds) or mountains (for lenticular clouds).