Page:1947SydneyHailstorm.djvu/20

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41

diameter of a hailstone reaches a certain value. Taking a hailstone of s.g. 0.6 at 10,000ft., the terminal velocity increases steadily from 88 f/s for diameter 1.5" to 153 f/s for 4.5", then there is a rapid increase to 168 f/s at 4.8" when it jumps suddenly to a value of 380 f/s for somewhat larger stones, The sudden increase is due to the change in the nature of the turbulence flow at this stage. Thus it is concluded that the maximum spherical hailstone is likely to be about 5 inches in diameter and weigh about 1.5 lbs. Possibly the "record" hailstone is that is noted by Potter and found in Nebraska. It weighed 1.5 lbs and was 5.4" in diameter, being formed in concentric spheres around a single centre, proving it to be a single hailstone.

The vertical currents associated with hailstones have just been touched on, but the calculation of vertical currents from the pressure rise in thunderstorms has been used by the author of the article by Newman in his article under comment to estimate the vertical currents involved in the thunderstorm, He uses formulae given by Levine and Buell. In this Bulletin for July 1946 Loewe in an article on "A remarkable pressure variation" quotes reasons for doubting the decisive effect of vertical accelerations on the pressure change and further indicates that in the calculations of Levine and Buell have not taken into account the deceleration effects, so that their formulae are unreliable.

A more important matter is that the present technique (parcel method) for forecasting the tops of convectional cloud from tephigrams greatly overestimates these heights (Convection in Theory and Practice - M.O. S.D.T.H. No. 102 Petterssen) by amounts in the order of 4,000 feet. Strictly virtual temperatures should replace dry bulb temperatures, and virtual saturated adiabatics replace the ordinary saturated adiabatics in evaluating convective processes on the tephigram. As the virtual saturated adiabatic laps rate is steeper than the ordinary saturated-adiabatic rate, the use of the latter, which is present practice, results in an estimated cloud top which would be higher than that derived from the more correct procedure, which already gives tops too high. Hence especially in low latitudes or with moist air, estimated cloud tops may apparently be some thousands of feet too high, materially affecting the forecasting of thunderstorms. Forecasters should keep this in mind. Some checks against figures provided by planes would be valuable.