Hail Formation
A raining down of hailstones as large as golfballs--or even baseballs, will quickly destroy crops, maim or kill animals and damage roofs. How do such large hailstones materialize out of thin air?
A clue to the formation of hailstones is seen when a hailstone is cut in half. Most show an onion-like layering of alternating clear and opaque ice. This layering demonstrates that the stone is built up by coatings of ice successively frozen onto the surface of the hailstone. Each coating evidences a buildup in an environment different from that in which the coat below formed. Clear ice forms in the part of the cloud where water is copious; ice with many trapped air bubbles, hence opaque, forms in drier portions.
Important to the buildup of hailstones is the existence in thunderhead clouds of supercooled water: water that remains in liquid form well below normal freezing temperature. Supercooled water droplets colliding with a foreign object--a piece of dirt, an embryonic hailstone--will freeze to it and thereby increase its size.
Much of the water in the upper part of a tall thunderhead cloud is supercooled. Once a hailstone starts to form in this part of the cloud, it can rapidly grow as it moves either upward or downward through the supercooled water.
Violent updrafts and downdrafts in thunderheads can carry hailstones, water droplets, (and airplanes) upward and downward at speeds to 180 mph (300 km/h). A single trip through a tall thunderhead can cause a hailstone to grow about one-half inch, so several trips are necessary to build up a large hailstone.
In the north, large hailstones are rare because the lack of strong ground surface heating does not favor the buildup of the gigantic thunderstorms common in some temperate climates. It would be interesting to know how the largest hailstone found by an Alaskan or Yukoner would compare with those found by residents of Potter, Nebraska in 1928. Hailstones there were about the size of large grapefruit with an average diameter of 5 1/2 inches. The biggest weighed a pound and a half.
