Including hail, downdraughts and cloudbursts
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.
In the diagram on the right, streamlines of air in which condensation occurred are shaded. The surface areas affected by rain and hail are shown by, respectively, grey and black shading. Heights are shown in thousands of feet. Precipitation formed in air which entered the storm near position H. As shown, the precipitation was carried across relative to the storm to around 13-15,000 feet, whereupon it fell and re-entered the strong updraught near posit-ion O. Some precipitation particles reached altitu-des of 30,000 feet or more and grew into large hail-stones before falling again, forward of the strong updraught, near position H’. The storm moved from left to right, with rain on its left flank and a squally ‘gust front’ (shown as a cold front) on its right flank. Behind the storm, chilled air reached the ground.
The diagram on the right shows a vertical section through a typical severe hailstorm (moving from right to left) and is also taken from Ludlam’s 1961 article in Weather. Compare this diagram with the three-dimensional model above
The paths of the air are drawn as if the storm was stationary. They are, therefore, relative streamlines. The dashed lines are trajectories of small hailstones. The thick full line shows the trajectory of a large hailstone.
To some extent, the features shown on this vertical section occur also in vigorous cumulo-nimbus systems which do not produce large hail. Students can look out for mamma, the udder-like cloud feature that hangs under the anvil and other parts of the cloud. How are mamma formed? Students can also observe gust fronts and measure the temperature drop that occurs when a storm passes. It is often several degrees Celsius. Perhaps, with the help of someone who has a car, they can map areas of rain and hail relative to moving storms.
In severe storms, downdraughts may be as strong as 30-40 m/s and reach the ground as ‘down-bursts’. These are dangerous, as the diagram above shows. Downbursts spread out near the ground. An aeroplane that flies into such an outflow first encounters an increasing head-wind (at 1 and 2), which adds to the speed of the speed of the air flowing over the aircraft’s wings and thus increases lift. At 3, however, the strength of the downdraught begins to reduce the altitude of the aircraft; and at 4 and 5 the aircraft experiences both a tail-wind (which reduces air speed and lift) and a downward force from the downdraught. Over the years, there have been many air disasters caused this way, especially in North America.
On 15 August 1952, the village of Lynmouth in North Devon was devastated by a torrent of water which poured off Exmoor; 34 people died. On 29 May 1920, in and around the Lincolnshire town of Louth, 22 people died when water from a storm over the Wolds caused the River Lud, normally a small stream, to rise 5 m above its normal level. In Dorset and Somerset, there have been similar occurrences; and in all cases, severe storms caused the havoc. When such storms occur in the British Isles, the wind in the upper troposphere is typically from the south-west, with the wind in the lower troposphere from a north-easterly point (and pressure low to the south and south-west). If this flow is lifted orographically, the storm may become stationary and deposit several inches of rain in a short time. Thus, it is places below slopes that face northwards or north-eastwards that are most at risk.
To frighten away the evil spirits that caused hail, primitive tribes used to shoot arrows into storm clouds; and Christians have tried to exorcise these spirits by ringing church bells (a dangerous practice because of lightning strikes on bell towers). Not only arrows, but also cannon-balls, artillery shells and rockets have been fired into storm clouds, but all to no avail. Though there is some evidence that cloud seeding may help to reduce the size of hailstones, there is nothing we can yet do to prevent the formation of severe storms.