It was reported in The Times newspaper on 15 April 1986 that a hailstorm lashing Dhaka, the capital of Bangladesh, had killed nearly 50 people and injured more than 400. The storm had brought winds of about 60 mph and hailstones weighing up to 2 lb (nearly 1 kg). Houses had been flattened, communications disrupted and the windscreens of more than 700 cars shattered. In such conditions, an umbrella was no use whatsoever; even a riot shield may not have provided adequate protection! According to Dick File, in Weather Facts (Oxford University Press, 1991), this storm (which struck on 14 April 1986) killed 92 people and produced hailstones that weighed 1.02 kg.
The heaviest hailstones to fall on the United Kingdom did so at Horsham, West Sussex, on 5 September 1958 and weighed 140 g. They were almost the size of a tennis ball. When they hit the ground, they were travelling at speeds in excess of 100 mph (50 m/s). If you find this surprising, do a little calculation, using the formula:
V2 = u2 + 2as where u is the initial speed, v the terminal speed, a the acceleration (in this case, due to gravity) and s the distance travelled. For a hailstone falling from a height of 500 m through still air, v = 100 m/s! The impact of a missile the size of a tennis ball travelling this fast is much more serious than that of a cricket ball hit for six.
Should you ever get the chance, collect some large hailstones and cut them in half. You may find a layered structure, with alternate layers of clear and opaque ice (as in the picture on the right, which shows a section of a hailstone viewed by transmitted light). The layers are acquired in different parts of the storm clouds. As hailstones fall, they collect tiny water droplets, which flow around them and freeze. If no air is trapped, the ice is clear.
The storm which struck the Wokingham area of Berkshire on 9 July 1959 produced hailstones more than 2.5 cm in diameter. This storm was studied in detail by Professor Frank Ludlam of Imperial College and his team of co-workers, who produced a striking three-dimensional model of the airflow with-in the storm and explained how large multi-layered hailstones may form in such weather systems.