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was 73% (actually higher than that of Sydney by 6%.) By using Parramatta's surface data and taking a composite pseudo-adiabatic chart with Rathmines data from 860 mbs (Fig. VII.) we find condensation at about 3700 feet when surface temperature has reached only 84°F or 28.9°C. and this would just surmount the inversion; with the temperature continuing to 90.8°F. unlimited instability was assured. The closeness of the Picton temperatures to those of Parramatta give some justification for assuming somewhat similar conditions in the Camden-Liverpool area. Parramatta's maximum temperature applied directly to Rathmines pseudo-adiabatic diagram is barely sufficient to surmount the inversion. Instability, however, would have followed very easily with only a slight additional mechanical lift or even if the Rathmines inversion were not real. The thunderstorm, however, did not develop over Parramata, and did not approach within seven miles of Parramata. It may be that not only was the inversion at Rathmines real, but there was probably a much greater inversion over the Parramatta area but no inversion to southward. Of course there may be other possibilities such as that the actual conditions at Parramatta were less favourable than in the composite adiabatic diagram. There was a second thunderstorm some two hours later over the northern suburbs (see Fig.I) but this was not severe and missed Parramatta by 8 miles.
Summarising, it appears that the following primary predictions or estimates would have been necessary:-
(1) With Rathmines pseudo-adiabatic chart as the basis, a maximum temperature of at least 97°F. over inland suburbs was to be predicted.
(2) This would mean a temperature of about 85°F. at Sydney
(3) Obviously a sea-breeze would have been forecast, but even though the temperature as about 76°F. at 9 a.m.; a maximum of 85°F, under a sea breeze would not normally be expected - an estimate would have been about 82°F.
(4) A possible increase in humidity. This did not actually occur at the surface but another 10% may have been predicted. Generally, however, if we expect an increase in humidity with the sea breeze, we are inclined to lower our estimate of maximum temperature.
