Hail is solid precipitation formed inside powerful convective clouds, usually cumulonimbus. It can reach the ground when surface temperature is well above 0 °C because the crucial processes occur several kilometres higher, where air is below freezing. Not every thunderstorm produces hail that survives to the ground, and the affected swath can be narrow even within one storm.
From a supercooled droplet to a hailstone
Updraughts carry water droplets into very cold parts of the cloud. Some remain liquid below 0 °C —these are supercooled droplets— and freeze when they collide with small ice embryos. A hailstone grows by collecting more droplets and ice particles while it remains in a favourable region of the storm.
Its layers are not an exact clock recording a fixed number of trips through the cloud. Clear or opaque ice depends partly on how much liquid water is available and how quickly it freezes. The stone eventually falls when the updraught can no longer support it or its path carries it out of the growth zone.
What controls hailstone size
Larger hail generally requires an updraught capable of supporting heavier particles, plentiful supercooled liquid water and enough time for growth. Storm organisation and motion, changes in wind with height and melting on the way down also matter. Size cannot be inferred simply from how dark the sky looks or how long rain lasts.
Comparisons with coins or fruit are approximate and differ between countries. For a useful report, measure only after danger has passed, using a ruler and without exposing yourself to lightning or another storm cell.
What a forecast can tell you
Meteorological services assess favourable ingredients and monitor development with radar, satellites and detection networks. A thunderstorm warning mentioning hail communicates likelihood and expected impact; it does not promise hail in every town or street. If your location misses the hail, that does not make the warning wrong: convection can vary greatly over a few kilometres.
Check current AEMET warnings and local authority instructions. Radar is useful for tracking cells, but it should not be treated as permission to delay leaving an outdoor activity.
How to protect yourself
- At home: go indoors, keep away from windows and skylights, and close shutters without going outside once the storm has arrived. Secure awnings, pots and loose items beforehand.
- In a vehicle: keep doors and windows closed, slow down smoothly and stop only in a legal, safe place if visibility is lost. Do not shelter beneath trees or stop in flood-prone low ground.
- Outdoors: seek a substantial building or an enclosed hard-top vehicle. If no immediate shelter exists, protect your head and neck and leave exposed ground promptly.
- Afterwards: avoid broken glass, slippery ice, unstable branches and fallen power lines. Do not climb onto a wet roof to inspect damage.
Hail comes from a thunderstorm that may also produce lightning, gusts and intense rain. If you hear thunder, also follow our outdoor thunderstorm safety guide.
This article is educational and does not replace a live warning or emergency instruction.