A thunderstorm-generated gravity current was observed to propagate for at least
45 km above the ocean immediately off Sydney. On this occasion, bushfire smoke
made the current visible. Wind speeds within the head are likely to have been
at least 20 
(40 knots). The height of the nose of the current,
when approximately 10 km from the end of the north-south runway at Sydney
Airport, was at least 800 m. The comparatively unremarkable nature of the
thunderstorm that created it is further evidence, in addition to common
observations, that large gravity currents occur from time to time in this area.
Gravity currents are known to be hazardous to aircraft. If aircraft had been landing to the north using the north-south runway at around 1600, they would have been at an altitude of about 500 m when 10 km from touch-down, and hence would have flown through the head of the current. However, controllers at Sydney Airport appeared to be directing aircraft to use the east-west runway exclusively at the time. This precluded any aircraft from flying anywhere near the most vigorous stage of the gravity current.
We are grateful to John James of the New South Wales Branch of the Australian Bureau of Meteorology for the radar and meteorological data and a for a useful discussion. Thanks are also due to Peter Tate of Sydney Water for the data from ORS1 and to Vic Cimadori of the Caltex Kurnell refinery for data on the chimney height. Russel Morison (UNSW) provided a spare roll of film in the nick of time. This work was supported by an ARC small grant and ARC large grant A89030563.