Why Are Hailstones Layered? Reading the Storm in the Ice

TL;DR: Why are hailstones layered? Because a hailstone is built in coats rather than cast in one go. Every band records a different pocket of the cloud it grew through, clear where the water spread and froze slowly, milky where it froze on landing and locked tiny air bubbles in. Cut one open and you are reading the storm.

Key takeaways

• Why are hailstones layered? Because every pass through a different part of the cloud glazes on another coat of ice.
• Clear bands form where the water freezes slowly and the air escapes, milky bands where it freezes on contact and the air is sealed in.
• The National Severe Storms Laboratory notes that a hailstone needs liquid water to grow, so below roughly minus 40 degrees it stops growing.
• The Met Office puts hailstones between 5mm and over 50mm across, with most smaller than 25mm.
• The heaviest hailstone officially recorded in Britain weighed 190g and fell on Horsham in Sussex on 5 September 1958.

Last updated: 7 September 2026

Why are hailstones layered like the rings of an onion?

Why are hailstones layered? Because the stone is assembled one collision at a time, and the conditions change between collisions. Each pass through a different part of the cloud adds a fresh coat of ice, and that coat sets clear or milky depending on how fast the water froze.

So a cut hailstone looks like an onion, or a tree in cross-section. The rings are not decoration. They record where the stone has been, in order, from the frozen embryo at the centre to the last coat before it fell.

How does hail form inside a storm cloud?

It starts very small. The Met Office describes strong updraughts and downdraughts carrying droplets up and down through a thundercloud and freezing them at the top, where it is coldest. The updraught holds the stone aloft, and every second up there is another chance to collect ice.

The National Severe Storms Laboratory adds a limit. A hailstone needs liquid water to grow, and very high in the storm, below roughly minus 40 degrees, there is none left. The growing happens in the middle of the cloud, and the updraught decides how long a stone stays there.

The journey is messier than a simple lift and drop. Winds inside a thunderstorm are not purely vertical, so a rotating updraught carries the stone sideways too, through pockets differing in temperature and liquid water. Ask why are hailstones layered and the answer is that variety.

What makes one layer clear and the next one milky?

Speed. Why are hailstones layered in two finishes? Because water can freeze onto the stone in two ways. Freeze it slowly and the air works its way out, leaving glassy clear ice. Freeze it on landing and the air has nowhere to go, so it stays as a haze of bubbles.

If the water freezes instantaneously when colliding with the hailstone, cloudy ice will form as air bubbles will be trapped in the newly formed ice. However, if the water freezes slowly, the air bubbles can escape and the new ice will be clear. NOAA National Severe Storms Laboratory

Atmospheric scientists call the two modes wet growth and dry growth, and the Space Science and Engineering Center at the University of Wisconsin sets them side by side. In wet growth the air is below freezing but not brutally cold, so a droplet spreads over the surface and hardens slowly into a clear band. In dry growth it freezes where it lands, and the gaps between the frozen beads stay full of air.

Can you count the layers to read the storm?

The rings do record the storm, but they do not count trips to the top of it. A popular explanation says each ring marks one journey up and down the cloud. The National Severe Storms Laboratory puts it more carefully.

The layers, however, do not occur simply due to the hailstone going through up and down cycles inside a thunderstorm. NOAA National Severe Storms Laboratory

Why are hailstones layered so unevenly, then? Because storms are not tidy. The National Severe Storms Laboratory ties the banding to the stone meeting different temperature and liquid water conditions, so one falls almost plain while another arrives banded like an agate.

How big does hail get in the UK?

Size is a separate question from why are hailstones layered, and the British answer is modest. The Met Office puts hailstones between 5mm and over 50mm across, says most are smaller than 25mm, and notes that the largest come from multicell storms, far more common in the United States.

British hail also has a season, and not the obvious one. It is most common in western regions in winter, most frequent in spring across eastern England and south-east Scotland, and the most damaging storms arrive in summer, though rarely.

The record shows what this country can still produce. The Tornado and Storm Research Organisation lists the heaviest hailstone officially recorded in Britain as a 190g stone that fell on Horsham in Sussex on 5 September 1958.

Does this change how you read a storm on the ground?

In a quiet way, yes. Why are hailstones layered? Because a storm is not one condition but many, changing minute by minute. The water it leaves on a site behaves the same way. A single reading tells you little; the rise and the fall are where the information sits.

That instinct runs through everything we do. At AQUAIOT we watch water all day: a downpour filling a Smart Water Butt that empties before the next one, non-contact level monitoring on flood-risk assets with the AQUAIOT Radar, or continuous sewer and drainage monitoring when a storm outruns the network. Our round-up of UK water facts has more.

Frequently asked questions

The questions that tend to follow on from why are hailstones layered, answered briefly.

Why are hailstones layered instead of being solid clear ice?

Why are hailstones layered rather than uniform? Because the stone grows in stages and the freezing conditions change between them. Water that freezes slowly lets its air escape and sets clear, while water that freezes on contact traps the air as bubbles and sets milky white.

Is hail just frozen rain?

No. Sleet is the frozen raindrop, a small hard clear pellet. Hail grows by collecting supercooled water that freezes onto its surface while an updraught keeps it airborne, which is why it gets so much bigger and carries layers. The National Severe Storms Laboratory treats them as separate types of frozen precipitation.

What is graupel, and how is it different from hail?

Graupel, also called snow pellets or soft hail, forms when supercooled droplets freeze onto a snow crystal. The pellets stay under about 5mm and fall apart when handled. Graupel is often the embryo a hailstone starts from, before the coats of ice begin.

How many layers can a hailstone have?

There is no fixed number. Each band records a change in the temperature or liquid water around the stone, so a stone that met many changes carries many bands, while one that met little variety falls nearly plain. The number of bands is not a count of trips up and down the storm.

By Gianbattista Porru, Digital and IoT lead at AQUAIOT.