Hail, decoded. Ice, falling out of a summer sky.

Hail is the sky's least polite surprise. One minute it is raining, the next it is rattling off the roof in hard little stones, and a storm that looked ordinary has suddenly dented cars and shredded gardens. This page is the short version of how that happens, what size actually counts as dangerous, and the one decision that matters most when hail is forecast: get inside and stay away from the windows.

WHAT IT ACTUALLY IS.

A thunderstorm, making ice in midair.

Hail forms inside a thunderstorm, not in winter weather. Strong updrafts carry raindrops above the freezing level, where they freeze onto small ice pellets, and the pellets are tossed up and down through the cloud, growing a new layer of ice on each trip, until they are too heavy for the updraft to hold and fall to the ground. That is why hail arrives with thunder and lightning rather than snow: it is a warm-season storm's product, built by the same updrafts that make the storm tall and strong.

HOW BIG IT GETS.

Pea-size is a nuisance. Quarter-size is severe.

Hail is measured by everyday objects because the numbers alone do not land: pea-size is about a quarter inch, marble-size about a half inch, golf-ball-size about one and three-quarter inches, and the largest hailstones on record are softball-size and larger. One inch — about the size of a quarter — is the severe threshold: a Severe Thunderstorm Warning is issued when hail of one inch or larger, or winds of 58 mph or stronger, are expected or occurring. Below that size hail is mostly noise and dented leaves; at and above it, it breaks windshields, damages roofs, and can injure anyone caught outside.

WHAT TO DO ABOUT IT.

Get inside, away from the glass, and wait.

The safest action during hail is to move indoors and stay away from windows and skylights, since large hail can break glass and send shards inward. If you are driving when hail starts, pull over somewhere safe, away from traffic, and stay in the vehicle with the windows up — do not stop under an overpass, where stopped cars block traffic and create a crash risk for everyone behind you. Brzzy's forecast, live radar, and severe weather alerts turn the same kind of ground truth into a plain-language answer before the storm arrives: go now, or get inside before the sky starts throwing ice.

What is hail?

Hail is ice that falls from a thunderstorm. Strong updrafts carry raindrops above the freezing level, where they freeze onto ice pellets and grow in layers until they are too heavy to stay aloft, then fall as hard stones of ice.

How does hail form?

It forms inside a thunderstorm with strong updrafts. Raindrops are carried above the freezing level and freeze onto small ice pellets; the pellets are tossed up and down through the cloud, adding a layer of ice on each trip, until their weight overcomes the updraft and they fall to the ground.

What size hail is considered severe?

Hail one inch in diameter — about the size of a quarter — is the severe threshold. A Severe Thunderstorm Warning is issued when hail of one inch or larger, or winds of 58 mph or stronger, are expected or occurring. Smaller hail is usually a nuisance; quarter-size hail and larger can break glass and damage roofs and cars.

Is hail dangerous?

Yes, when it is large. Quarter-size hail and larger can break windshields and windows, damage roofs and crops, and injure people and animals caught outside. The risk is why the safest action is to move indoors away from windows, and never to watch a hailstorm from an open porch or a car with the windows down.

Does Brzzy warn me about hail?

Brzzy shows your local forecast with hourly updates, live radar, severe weather alerts, GIF forecasts, and activity ratings that weigh the sky for your plans. Brzzy is free to download on iPhone.