Page:1947SydneyHailstorm.djvu/19

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to the likelihood of thunderstorm formation inland but not on the coast. This actually occurred, and the thunderstorms move Eastwards to the coast. Thus, with this kind of analysis of conditions, a forecast of probable thunderstorms would have been justifiable. The uncertainty factor in this analysis is, of course, evident but the procedure is still practically helpful.

Several interesting matters are raised by this article. They are ―

(1) The influence of sea-breezes on the uplift of the overlying air and, hence on convectional cloud and thunderstorm formation in certain favourable cases;

(2) The maximum size of hailstones;

(3) The vertical currents associated with hailstones and with the pressure rise in thunderstorms; and

(4) The modification of present technique in forecasting convectional cloud from adiabatic diagrams.

Regarding the first matter, there is little information available, but the general physical effects and nature of ? sea-breezes are of first class importance in sub-tropical lands, because of the climatic weather and even probable radii effects. It is not improbable that moist sea air arriving early over the land might with diurnal heating, be a decisive cause of thunderstorms, or that its arrival might conceivably cause sufficient uplift of upper air to touch off a thunderstorm.

The maximum size of hailstones is discussed in a paper by Belham and Relf in 1937 (Q.J.R. Met. Soc.) There the relations between terminal velocity and diameters are deduced from values of the drag coefficient obtained from observations on a sphere towed by aeroplanes. Values of terminal velocities are calculated for various mean specific gravities, and it is concluded that an upper limit of 1.5 lbs, is set by aerodynamical considerations to the mass attainable by spherical hailstones. This follows from the conclusion that under natural conditions there is a sudden and very large increase in the terminal velocity when the

(diameter