How Hail Forms

Wondering, How Does Hail Form?

With costly hailstorms in Texas, it is worth investigating how hail is formed.  Hail accompanies severe thunderstorms.  In thunderstorms, fast currents of air move upward.  These are called updrafts.  There are also downward currents of air called downdrafts.  Inside the updrafts, water vapor and rain are pushed into high cumulonimbus clouds.  At a certain height, the temperature drops below freezing.  The super cooled water vapor and rain condense and freeze.  These frozen tiny ice pieces become the center of the hailstones.

The force of gravity will eventually push down the tiny frozen particles.  As gravity pulls these particles to the ground, ice particles collide with rain and debris, getting bigger. The updraft continues to pull the particles back upward, freezing them as they get larger.

The hailstone continues this up-and-down cycle within the thunderstorm until either: a downdraft sends the hail to the ground, or the hailstone grows to a size in which it is too large for the storm's updrafts, and it falls to the ground.

For a thunderstorm to produce dime-sized hail, the updraft speed would need to be 37 mph. For golfball-sized hail, the updraft needs to be around 56 mph. Baseball-sized hail requires strong winds blowing at least 100 mph.

The Size of Hail

The world's largest hailstone by diameter and weight was found July 23, 2010, near Vivian, South Dakota. The hailstone measured 8.0” in diameter, 18 ½” in circumference, and weighed 1.9375 pounds. The largest hailstone by circumference occurred on June 22, 2003, in Aurora, Nebraska. The hailstone was 7.0 inches in diameter and 18.75 inches in circumference.

The size of hail is often compared to everyday objects. This makes it easy to relay a hail storm report to forecasters and the public.  Weather reporters do not say "marble-sized hail," since marbles come in various sizes.

Hailstorm Safety

Most significant hail damage to property and crops comes from hailstones at least 1 inch in diameter.  If a thunderstorm is expected to produce hail 1 inch in diameter or bigger, the National Weather Service will issue a severe thunderstorm warning.  When a severe thunderstorm warning is issued, stay inside.  Bring pets inside. Secure your belongings and hunker down until the storm passes.

If you live an area where hailstorms are frequent, make sure your property is insured.