It is well known that thunderstorms can create gravity currents when they generate large outflows of cold air (Simpson 1987; Fulton et al 1990; Manasseh & Middleton 1995). Because the cold air is generated by the evaporation of precipitation, these horizontally-propagating currents are often invisible. If they pass over arid land they may entrain dust and are then visible; these phenomena are known as `haboobs' (Simpson 1987) in North Africa and parts of Europe.
A gravity current has a well-known structure consisting of a head and a
following flow. The airflow within the head consists of a rapid feeder flow
near the ground and in the direction of propagation of the current, which
sweeps vertically upwards at the nose of the current and brings air to the top
of the head. The head may have turbulent eddies on top of it caused by
shear-flow instabilities, which may take the form of Kelvin-Helmholtz billows.
Gravity currents can be hazardous to aircraft; the instantaneous wind speed
change encountered in flying through a gravity current can be as much as
30-35 
(60-70 knots) and aircraft have undergone structural
failure while flying through the billows (Simpson 1987) although the major loss
of aircraft and life in gust fronts has been associated with the loss of lift
due to the wind shift.
The event occurred on 11 April 1994; times quoted are in Eastern Australian
Standard Time, which is 10 hours in advance of Universal Time.
The dry-bulb temperature at Sydney Airport was
C and the
relative humidity was about 41%.
Winds at Sydney Airport were west-south-westerly at about
5-7 during the early afternoon of 11 April.
Figure 1 is a map of the region of interest.
Fires were lit by the Sydney Water Board in forested areas around Cordeaux Reservoir as a routine hazard-reduction measure. These burn-offs occurred along two lines as shown in figure 1 and made a plume of smoke that was advected to the east by the prevailing wind.
Figure 1: Map of the region of interest. Latitude and longitude are in decimal
degrees. The lines emanating from UNSW and marked with the time are the lines
of sight along which photographs referred to in the text were taken.