Friday, August 14th 2020, 1:37 pm - The science behind hail formation
If you haven't been in a major hailstorm you don't know how dangerous it can really be. The deadliest of these storms dates back to April 26,1888, when according to World Meteorological Organization (WMO) data, 246 people were killed along with 1,600 head of cattle. It happened in Moradabad, India.
Hail is one of costliest types of adverse weather. It means billions of dollars a year in the Canada and the U.S. alone and it can cause damage to homes, cars, aircrafts, crops and other structures, while in some cases being deadly for humans and animals.
In order for hail to form you need a thunderstorm to be present, but not just any kind of thunderstorm -- it has to be one in which updrafts are strong enough to carry raindrops upward into extremely cold areas of the atmosphere where they can freeze and become ice pellets.
The process of hailstone formation and growth implies that the original ice pellets or small hailstones collide with super cooled water droplets. These very pure droplets are in the liquid state but will freeze upon contact with ice crystals, frozen raindrops or some other nuclei in the cloud known as condensation nuclei.
If the updrafts within the atmosphere are strong enough, they will continue to lift the hailstones to the top of the cloud where they can encounter more super cooled water and continue growing. This process is known as accretion.
How large a hailstone gets before it falls to the ground will depend on how strong the updraft with in the cloud is capable of lifting it and keeping it suspended. The general rule is the stronger the updraft, the larger the hailstone.
*Hailstone captured in July 2020 in Alberta. Courtesy: Kyle Brittain*
One way to know how many times a hailstone has travelled up and down the cloud is to cut it in half and count the number of concentric circles in it's interior (they resemble the inside of an onion). If there are few or no circles present, it is most likely a hailstone that was in balance in the updraft. Each time a hailstone travels below the cloud freezing level it begins to melt, but then once pushed upward by the updrafts, it will have more water freeze on to it, making it larger and occasionally irregular in shape.
WHAT DETERMINES SIZE AND SPEED OF HAIL
Once the hailstone grows large enough it's weight will help the force of gravity overcome the upward force of the updraft, thus making it fall to the ground.
How fast it falls will depend on the structure of the cloud. While falling, the air friction, updrafts within the cloud and even the collision with other hailstones and rain drops can slow it down. On occasions strong surface winds will cause the hail to move almost horizontally near the ground impacting one side of structures or cars more than the others.
There are no set values for how fast a hailstone can fall from a cloud because there are several factors that can influence it's speed. As a reference, a hailstone 0.4 inches in diameter will fall at about 25 km/h, while one 3.2 inches in diameter weighing near 1.5 pounds, will acquire a speed of 172 km/h.
Hailstones falling from a cloud rarely reach terminal velocity because of the mentioned friction and collisions during their cloud to surface journey.
Depending on the severity of the thunderstorm, and thus the strength of the updrafts in the cloud, hail can be smaller or larger. Normally, hailstones vary between the size of a pea and a marble, but vigorous thunderstorms can produce hailstones the size of a golf or tennis ball.
A common comparison with very large hail is the one made to a baseball. Baseball-sized hail can cause serious injuries to humans and animals. It is also responsible for destroying crops, making holes in roofs, walls, windows and windshields, as well as denting vehicles.