Hail forms when ice droplets, snowflake clusters, or floating debris enter thunderclouds and act as ice nuclei. Ice droplets form nuclei when collections of ice droplets group together. Snowflakes form nuclei when several snowflakes clump together. Debris forms nuclei when wind blows tiny bits of matter into storm clouds.
The coming together of ice droplets or snowflakes occurs as frozen bits of water pass through freezing thunderclouds. Once the nuclei form, ice continues to collect on top of the mass sort of like adding snow to a snowball in your hand.
As growing hailstones become bigger, they eventually reach sizes and weights that allow them to fall from clouds to earth.
Although hail may potentially form in just about any thundercloud, there are certain conditions thunderclouds must meet in order to allow hailstones to form and to grow. These conditions increase the likelihood that hail will become strong enough to complete successful falls to earth.
The best conditions to encourage hailstone success include:
* Cumulonimbus clouds – Vertical clouds that reach high into the atmosphere where the air is much colder than lower atmospheric air.
* Tornados or similar storms that swirl and push ice particles upward and downward, thus, allowing them to grow.
* Storm conditions must be extremely cold inside thunderstorms and underneath thunderstorms in order to enable hailstones to freeze firmly.
As long as hailstones remain inside thunderstorms, they continue to grow and, in fact, even after they become icy and firm, they still become bigger each time additional ice or snow surrounds them. In addition, two or more hailstones may conjoin or freeze to one another. When hailstones conjoin, they gain the potential to form much larger stones.
Another method in which hailstones grow in size is by accumulating additional layers when thunderstorms blow them in upward directions. As they move upward, they pass through additional freezing thunderstorm tiers. Moreover, when hailstones exit the upward movement and begin to fall downward, they can accumulate even more growth as they pass through the storm again. The constant forces of wind pushing hailstones upward and downward helps the stones reach sizes that are heavy enough for them to fall and reach earth.
Even though hail has the potential to form in just about any thundercloud that is high enough in the atmosphere, some hailstones do not make it to earth; and stones that do make it to earth may be smaller than they were when beginning their fall. Decreasing their size typically occurs when hailstones mix with rain or snow as gravity pulls the stones through thunderstorms and down to earth.
When rain falls from the clouds, you might want to get your umbrella, and when snow falls, you might start thinking about snowball fights or building snowmen. When hail falls, however, you may well decide to get indoors, if you are not inside already, because hailstones can be as small as raindrops and as large as golf balls or tennis balls.