Page:1947SydneyHailstorm.djvu/10

31. It seems that the origin of the storm was definitely localised convection inland. It drifted seaward under a light westerly component above and additional energy was generated by the seabreeze. Temperatures at Rathmines and Sydney were sufficiently close to justify the application of Sydney's readings to Rathmines radiosonde. As a further point of interest a composite pseudo-adiabatic chart for 9 a.m (Fig. VI.) has been constructed using temperature and humidity for the surface and then joining up with Rathmines data from 860 millibars. To completely overcome the inversion at 8000 feet by auto-convection a surface temperature of 36 degrees centigrade (97°F.) was necessary. The maximum temperature reached at the Weather Bureau was 80.5°F or 29.6°C. Applying this temperature,condensation occurs at 880 mbs. or 4000 feet but ascent would be arrested at the inversion (actually about 7800 feet). We must remember, of course, that the thunderstorm did not originate over the city but rather some 20 miles inland and drifted eastward. On the other hand conditions at Sydney should have been such that the lifting of the air and the activity of the storm could be maintained. In this case some other means in addition to thermal convection were necessary to lift the condensation beyond the inversion. We are again assuming the inversion was real - if not the temperature curve might have been taken as straight from 770 mbs. to 700 mbs. We would then have had condensation to unlimited height and a thunderstorm certain.

Returning, however to the actual pseudo-adiabatic chart with its inversion, and examining Mascot's pilot balloon observations, a light NNW wind at 8 a.m. up to 2000 feet (probably 3000 feet) was replaced at 2 p.m. by north-east up to probably 3000 feet; possibly it may have been deeper during the forenoon. The question of how much an overlaying mass of compressible fluid is lifted by an invading wedge below is probably not definitely known. It is reasonable to assume, however, that 3000 feet (possibly 4000 feet) of north-east sea-breeze would lift the upper air at least from 7800 to 9500 feet. In this case the condensing mass would have been carried along the wet adiabat to a condition of instability beyond 9500 feet or 720 mbs.

In regard to the conditions over the core inland locality where the storm originated we have since obtained from observers some useful data. Both Picton and Parramatta had a maximum temperature of approximately 91°F. (32.8°C.) At Parramatta the 9.a.m. relative humidity .... was 73%