What makes hailstones grow inside thunderstorms?

Introduction

Hailstones are balls or lumps of ice that form inside strong thunderstorms. They grow through cycles of freezing, accretion, and circulation inside the storm’s updrafts. Understanding why hailstones grow requires looking at how clouds, temperature, liquid water, and air motion interact in severe convective storms.

What is hail and how is it different from other frozen precipitation?

Hail is solid ice that forms within thunderstorms and falls to the ground when it becomes too heavy for storm updrafts to hold. It differs from sleet and snow: sleet forms from raindrops freezing while falling through a below-freezing layer, and snow forms when ice crystals grow directly by vapor deposition. Hailstones begin as ice particles inside a convective cloud and grow by collecting liquid water and sometimes by colliding with other ice particles.

Basic conditions needed for hail growth

Several environmental factors must align for hail to form and grow significantly:

1. Strong convective updrafts. Powerful upward air currents inside a thunderstorm lift ice particles and keep them suspended long enough to grow.

2. Mixed-phase cloud regions. Hail grows where the cloud contains both supercooled liquid water (liquid water below 0°C) and ice particles. This mixed-phase environment promotes freezing onto nuclei.

3. Sufficient liquid water content. A higher concentration of supercooled droplets provides more material to freeze onto an existing ice embryo and enlarge it.

4. Temperature profile with freezing layers. The vertical temperature structure of the atmosphere determines where liquid droplets are available and where freezing is possible. Hail growth typically occurs in parts of the storm that are below freezing but still contain liquid droplets.

The core mechanism: accretion and riming

The principal physical process that makes hailstones grow is called accretion, and a key subtype of accretion is riming. Riming happens when a frozen particle—an ice embryo, graupel pellet, or small hailstone—collides with supercooled liquid droplets. Those droplets freeze on contact, producing a rime layer of opaque, often bubbly ice.

When riming is continuous and heavy, an initially small particle can rapidly gain mass. Each collision adds a thin coating of frozen water. With repeated passes through regions of plentiful supercooled droplets, the particle can grow into a pea-sized hailstone or even much larger forms depending on how long it remains aloft.

Recycling inside the storm: updrafts, downdrafts, and trajectories

Hailstones rarely follow a simple, straight path. Instead, they move in complex trajectories set by the storm’s internal circulation. Strong updrafts can lift a growing hailstone back into higher parts of the cloud where it encounters fresh supplies of supercooled water. Downdrafts and gusts can push the stone into different temperature zones or cause it to fall temporarily before the updraft lifts it again.

Each time a hailstone cycles upward into a region rich in supercooled droplets, it can accumulate additional rime or layers of clearer ice. This repeated cycling—sometimes called the “recycling” process—lets hailstones spend far longer inside the thunderstorm than they would under simple free-fall, enabling substantial growth.

Two growth regimes: wet growth and dry growth

Hailstones grow in two main modes depending on the local conditions at the point of accretion:

1. Wet growth occurs when the heat released by freezing is not removed quickly, so the newly accreted water does not freeze solid immediately. This produces clear, often concentric ice layers because trapped water freezes slowly and expels air bubbles. Wet growth tends to create denser, transparent ice.

2. Dry growth happens when collisions freeze almost instantly, trapping air and creating opaque, rime-like layers. Dry growth produces a more porous, white appearance and generally occurs at colder temperatures or when freezing is rapid.

The alternation of wet and dry growth as a hailstone travels through different parts of the cloud produces visible layers—clear and opaque—that reveal its history, similar to tree rings.

Role of temperature and supercooled water concentration

Temperature affects both the amount of supercooled liquid water in the cloud and the freezing rate after collisions:

– Near 0°C, clouds often contain abundant supercooled droplets because water can remain liquid well below freezing in tiny droplet form. This favors wet growth and relatively smooth, clear layers on hailstones.

– At much colder temperatures, supercooled droplets are scarcer and freezing is faster, favoring dry growth and rime accumulation.

The vertical temperature profile controls where in the storm these conditions exist. Large hail tends to form in storms with a deep layer of supercooled water and strong updrafts that lift particles through that layer multiple times.

Aggregation and collision-coalescence among ice particles

While riming is the dominant growth process for hail, collisions between ice particles also matter. Two small particles can collide and stick, forming a larger aggregate. If those particles are wet or partially melted at the point of collision, they are more likely to cohere and form a larger hailstone.

Collisions can also break fragile particles into fragments that act as additional embryos—each fragment can then collect more supercooled water and grow. Thus, interactions among many particles create a dynamic environment where sizes and numbers evolve rapidly.

Why some hailstones grow very large

Several factors combine to produce very large hailstones (golf-ball size and larger):

1. Extremely strong, sustained updrafts — these keep hailstones aloft for long periods so they can undergo many growth cycles.

2. High liquid water content — more supercooled droplets mean more material available to freeze onto the hailstone each pass.

3. Favorable storm structure — organized storms with strong inflow and a steady supply of humid air feed the updrafts and maintain the mixed-phase region.

4. Efficient recycling — hailstones that avoid being ejected into warm or dry parts of the storm continue to recirculate through growth zones.

Large hail therefore indicates a particularly energetic storm with the right thermodynamic and kinematic environment.

What hailstone structure tells us

When hailstones are cut open, they often show concentric layers of clear and opaque ice. These layers record changes in growth mode and environmental conditions during the stone’s life:

– Clear layers point to wet growth at temperatures near freezing with slower freezing and clearer ice formation.

– Opaque layers indicate rapid freezing and riming in colder or more droplet-rich parts of the cloud.

By studying these internal layers, meteorologists can infer aspects of the storm’s vertical structure and the hailstone’s flight path through it.

Why not every thunderstorm produces hail

Hail formation requires a specific combination of atmospheric properties. Many thunderstorms lack one or more of the necessary ingredients:

– Weak updrafts: small convective cells may produce rain but not keep ice particles aloft long enough to grow.

– Lack of supercooled liquid water: high-altitude, cold clouds may be primarily ice crystal clouds without enough liquid droplets for accretion.

– Warm lower layers: if the entire cloud column is too warm, any ice melts before it can grow and reach the ground as rain.

Thus, hail is most common in vigorous, deep convective storms where dynamics and microphysics combine favorably.

Practical examples and observed patterns

Regions with frequent strong instability and wind shear—ingredients that favor organized thunderstorms—tend to experience more hail. For example, parts of the U.S. Great Plains, southeastern Australia, northern India, and central Asia report frequent hail during their convective seasons. However, hail can occur anywhere that supports strong thunderstorms and appropriate cloud microphysics.

Storms that move slowly or have rotating updrafts (supercells) are often particularly efficient hail producers because the updraft can remain concentrated and sustained, giving hailstones prolonged growth periods.

Scientific study and measurement

Meteorologists study hail using radar, storm intercept observations, and laboratory experiments. Dual-polarization Doppler radar helps identify hail by detecting signatures that differ from rain. Field researchers sometimes use instrumented aircraft, probes, or hail pads to sample hailstones and measure size distributions, internal structure, and fall speeds.

Laboratory experiments and cloud-model simulations reproduce riming and accretion processes to test how variables like droplet size, temperature, and updraft speed control hail growth. These studies inform forecasting and improve understanding of severe-storm hazards.

Consequences and safety

Large hail can damage crops, vehicles, buildings, and windows, and pose a risk to people and livestock. Forecasting hail and issuing warnings helps reduce damage and protect lives. The same storm properties that encourage hail often produce other hazards—heavy rain, strong winds, and sometimes tornadoes—so hail is often one sign of an especially powerful thunderstorm.

Summary of the growth process

1. A small ice embryo forms in a thunderstorm.

2. The embryo collides with supercooled liquid droplets, which freeze onto it (riming), or with other ice particles (aggregation).

3. Strong updrafts keep the particle aloft for repeated passes through droplet-rich zones, allowing continued growth.

4. Alternating conditions produce layers of clear and opaque ice, showing the history of wet and dry growth.

5. When the stone becomes too heavy, it falls to the ground as hail.

Conclusion

Hailstones grow inside thunderstorms through repeated accretion of supercooled water and collisions with other particles while being held aloft by strong updrafts. The balance between storm dynamics (updraft strength and recycling), cloud microphysics (supercooled liquid content and temperature), and particle interactions determines hail size and structure. Observing hailstone layers and storm behavior helps meteorologists understand and forecast these impactful weather events.