Hail the size of grapefruit shattered car windows in Johnson City, Texas, in May 2024. In June, a storm chaser found a hailstone almost as big as a pineapple. Even larger hailstones have been documented in South Dakota, Kansas and Nebraska. Hail has damaged airplanes and even crashed through the roofs of houses.

How do hailstones get so large, and are hailstorms getting worse?

As an atmospheric scientist, I study and teach about extreme weather and its risks. Here’s how hail forms, how hailstorms may be changing, and some tips for staying safe.

Hail begins as tiny crystals of ice that are swept into a thunderstorm’s updraft. As these ice embryos collide with supercooled water – liquid water that has a temperature below freezing – the water freezes around each embryo, causing the embryo to grow.

Supercooled water freezes at different rates, depending on the temperature of the hailstone surface, leaving layers of clear or cloudy ice as the hailstone moves around inside a thunderstorm. If you cut open a large hailstone, you can see those layers, similar to tree rings.

The path a hailstone takes through a thunderstorm cloud, and the time it spends collecting supercooled water, dictates how large it can grow.

Rotating, long-lived, severe thunderstorms called supercells tend to produce the largest hail. In supercells, hailstones can be suspended for 10-15 minutes or more in strong thunderstorm updrafts, where there is ample supercooled water, before falling out of the storm due to their weight or moving out of the updraft.

Hail is most common during spring and summer when a few key ingredients are present: warm, humid air near the surface; an unstable air mass in the middle troposphere; winds strongly changing with height; and thunderstorms triggered by a weather system.

Hailstorms can be destructive, particularly for farms, where barrages of even small hail can beat down crops and damage fruit.

As hailstones get larger, their energy and force when they strike objects increases dramatically. Baseball-sized hail falling from the sky has as much kinetic energy as a typical major league fastball. As a result, property damage – such as to roofs, siding, windows and cars – increases as hail gets larger than the size of a quarter.

...continued