Hail is one of the three main forms of precipitation, the others being rain and snow.
Precipitation of all types is caused as part of the natural water cycle. Water vapor enters the atmosphere by means of evaporation, transpiration and perspiration. The sun causes evaporation from bodies of water on the earth’s surface, it also draws moisture from plants by transpiration, and from animals and humans by perspiration.
The capacity of air to hold water vapor depends on its temperature. When it is warm it can hold a great deal and the vapor remains invisible, but when it cools sufficiently the vapor will condense to form water droplets and hence clouds, or fog if this occurs at or near ground level. It is the water droplets in a cloud which eventually give rise to precipitation.
When air is heated rapidly it expands, grows lighter, cools and rises, and this usually occurs as the result of the sun’s heat, although sometimes it can happen when air is forced to rise above a steep cold front. The faster the air rises, the taller the clouds will grow and the taller they grow the colder the upper levels of the cloud will be.
In the upper levels of a very tall cloud, where the temperature is way below zero degrees Celsius, water droplets can become supercooled. This supercooling can cause ice droplets to form around particulates in the upper atmosphere, such as particles of dust, smoke, salt or anything else. It is these supercooled water droplets which eventually form hail.
As a droplet’s weight increases it falls through the cloud, but as the cloud is formed by rapidly rising currents of air, the droplet will be swept up again into the upper reaches of the cloud where a new layer of ice will form around the original droplet. This process can be repeated many many times, and the stronger the updrafts of air in the cloud, the more often this will happen. By this means the frozen water droplet becomes a hailstone.
With each new layer of ice round the hailstone, its mass and its diameter will increase, and this process will continue until the hailstone is so heavy that it is no longer lifted by rising currents of air and falls to the ground.
Hailstones are most often merely millimeters in diameter. However, in very unstable air in which rising currents can lift hailstones back up into freezing conditions a considerable number of times, hailstones can become the size of golf balls, tennis balls or even grapefruit. This is when they are particularly dangerous, flattening crops, breaking windows and damaging vehicles.
If a hailstone can be collected, and a section cut through it, the evidence of its formation will be clear in its many layers of ice, like the layers of an onion.
So hail is formed essentially like other forms of precipitation, as part of the water cycle, but in the case of hail it is the strong rising currents of air within a cloud which cause ever more layers of ice to form, thus causing a hailstone to grow from a frozen minute water droplet to a sizable and dangerous projectile.