Mountain wave cloud (also known as lenticular cloud, i.e. lens shaped) was observed on the hilltop of Lantau on 12 August 2003. The picture was taken at around 2:20 p.m. that afternoon at the Hong Kong International Airport looking south-southeast towards Lantau with a setting of 1/160s, F8.0 and ISO-100 speed. At the time winds were light to moderate southerly, with showers just passing over Lantau an hour earlier.

Lenticular clouds are evident of wave motions in the airflow over and to the lee of mountains, where the moisture in the air condenses into water droplets as a result of the upward motions. They are more often observed in the cool season when the air is more stable. However, in this case, it was a little unusual that lenticular clouds were observed on a summer day. It is believed that a weak stable layer ("inversion" in weathermen's terminology) at around the hilltop level might have contributed to the cloud formation.