Hailstones are made up of alternating layers of clear ice and opaque ice because they repeatedly pass through areas of the cloud with different physical conditions before falling. Each cycle inside the storm adds a new layer, so their internal structure works like a record of the path the stone took inside the cloud.
What each layer reveals when a hailstone is cut
When a hailstone is cut in half, concentric rings similar to those in a tree cross-section can be seen. These layers are not merely decorative: each one corresponds to a specific moment in the growth of the stone and provides information about the temperature, the water available and the type of ice that formed.
The layers can be distinguished with the naked eye because ice formed under different conditions looks different:
- Clear ice: forms when water droplets settle slowly and the trapped air can escape, producing dense, transparent ice.
- Opaque or white ice: appears when droplets freeze very quickly and trap small air bubbles, which scatter light and give it that milky appearance.
How each layer forms inside the cloud
Hail originates inside storms called cumulonimbus clouds, where updrafts and downdrafts coexist. An ice embryo begins to fall due to gravity, but if the updraft is strong enough, it pushes the stone back upward. On each ascent and descent, the stone passes through regions with different temperatures and liquid water content:
- Upper zone of the cloud, with very low temperatures and water vapor: there, growth is slow and clear ice forms.
- Middle and lower zone, with less extreme temperatures and many supercooled droplets: on impact with the stone they freeze almost instantly and trap air, producing opaque layers.
- If the stone rises again, a new clear layer is added to it.
This process can repeat several times, which explains why some stones show several visible layers while others show only one or two.
What information can be extracted from these layers
Studying hailstone cross-sections is a common tool in meteorology because it allows part of the storm's history to be reconstructed:
- Number of layers: indicates how many up-and-down cycles the stone went through.
- Color of the layers: reveals whether growth during that stage was slow (clear) or fast (opaque).
- Chemical composition: it sometimes contains impurities or aerosols that provide clues about the air the stone passed through.
In this way, a simple cut in half can show how the stone moved from a dry, cold area to a humid area with droplets, and then returned to higher areas, all within a single storm.
Why not all hailstones show visible layers
Not all stones show clearly defined rings. This happens when:
- The hailstone forms and falls in a single cycle, resulting in a fairly homogeneous piece.
- Growth is so fast that a single type of ice predominates.
- The stone partially melts as it falls and then refreezes, erasing the previous structure.
That is why hailstones with clearly differentiated layers are the ones that provide the most information about the internal workings of the cloud that generated them.
