On 5th May 2005, a group of storm chasers were chasing a possible touchdown for a tornado outside Seminole, Texas. As they approached the site, hailstones started falling from the sky. Exhilarated by the golf ball sized hailstones falling outside, one storm chaser braved the elements and brought one into the car. However, their exhilaration was soon turned into dread when the hailstones got larger and larger until they were the size of tennis balls. Pounding the car, the hailstones broke through a window, spraying broken glass into the eyes of the passengers. Luckily, the storm-chasers escaped this storm relatively unscathed suffering minor cuts and lacerations. their story can be found here. However, some are not so lucky. Earlier this month, 40 people were killed in an hour long hailstorm in the Min region of China. Houses collapsed and farmland was destroyed affecting up to 300,000 people in the region.
Yet, these hailstones do not account for the largest hailstones recorded. The largest hailstone to have been preserved in the US was found in South Dakota in 2010 and weighed in at an impressive 1.94 lb and measured 8 inches in diameter. However, this is not the heaviest stone ever recorded that honour goes to a hailstone weighing a staggering 2.25lb from in Bangladesh in 1986. Surprisingly, hail is most frequent over Kericho in Kenya with an average of about 50 days of hail per year, the record is 165 days in 1965.
For a hailstorm to form, the first thing that is required is a very strong convective cell, causing a strong updraft to force air up into the higher atmosphere. As moist air is forced into higher altitudes, it’s temperature will decrease, it condenses and forms acumuliform cloud. In circumstances where the updraft is particularly strong and there is enough moisture in the air, a cumulonimbus cloud will form.
Normally, cumulonimbus clouds produce short bursts of very heavy rain over a localised area. However, when a large portion of the cloud is cooler than 0C, the conditions are right for hailstones to form. This requires a droplet of super-cooled water (liquid water that is below its freezing point) droplet to freeze onto a condensation nucleus. This little hailstone is then pushed up into the higher reaches of the cloud until its weight exerts a greater force downwards compared to the buoyancy force that had initially pushed the hailstone upwards, at this point the hailstone begins to fall towards the Earth until the force exerted by the updraft is stronger than the weight of the hailstone at which point ascent begins again and the hailstone rises again.
During this ascent and descent through the cloud the hailstone acts as a condensation nuclei for other super-cooled droplets, hence increasing in size. The hailstone will also collide with other hailstones in the cloud and begin to coalesce producing bigger and bigger hailstones until the weight of the hailstone exceeds the force exerted by the cloud at all points within the cloud, resulting in the hailstone falling to the Earth. This process in which hailstones form is similar to raindrops, however the colder temperatures in the clouds means that hailstones do not melt when descending to Earth.
If you are caught in hailstorm, what should you do? The first thing you should do is seek shelter to protect your head, hailstones can cause severe concussion if they hit you upon the head. Trees should be a last resort in severe hailstorms, as there is the possibility that the hail may knock off any branches and any lightning that is occurring will be attracted towards the tree. In the case of large hailstones, stay away from windows, especially they are cracked by the hail. If you are in a car and severe hail starts to fall, stop your vehicle in a safe place and try to stay away from the windows as best you can, protecting your eyes at all times. As long as you’re well sheltered, hail is a magnificent a beautiful weather type that shows the great power of mother nature at work.