How Snowflakes Form: Ice Crystals, Shapes, and Weather Conditions

How Snowflakes Form Inside Clouds

A snowflake begins high in a cold cloud, often as a crystal too small to see without a microscope. Water vapor gathers on a tiny frozen particle and changes directly from gas to solid ice. This process is called deposition. As more molecules join the crystal, its flat faces, corners, branches, and familiar six-sided pattern begin to develop.

The full journey depends on more than freezing air. A growing crystal passes through pockets with different temperatures, moisture levels, droplets, and air currents. Each pocket alters the rate and direction of growth. The final shape records that journey through the cloud.

From Water Vapor to an Ice Crystal

Ice nucleation begins the process

Clouds contain microscopic aerosol particles carried by wind and rising air. Mineral dust, biological material, and certain other particles can provide surfaces on which ice starts to form. A particle able to help water freeze is called an ice-nucleating particle.

Many cold clouds also contain supercooled water droplets. These droplets remain liquid even though their temperature is below 0°C (32°F). Liquid water does not always freeze at the ordinary freezing point because a droplet may lack a suitable surface on which an ordered ice structure can begin.

At very low temperatures, near −38°C (−36°F), small cloud droplets can freeze without the help of an ice-nucleating particle. This is called homogeneous freezing. At warmer subfreezing temperatures, ice commonly begins through heterogeneous nucleation, with a particle assisting the first stage of freezing.

Water vapor deposits onto the ice

Once an ice particle exists, nearby water vapor can attach to it. The vapor changes directly into solid ice rather than first becoming liquid. This adds layers of water molecules to the crystal surface.

The air must contain enough water vapor for growth to continue. Meteorologists describe this condition using supersaturation with respect to ice. Supersaturated air holds more vapor than can remain in equilibrium over an ice surface at the same temperature. The excess vapor moves toward the crystal and freezes onto it.

Ice crystals can grow while droplets shrink

A mixed-phase cloud contains both ice crystals and supercooled liquid droplets. At the same temperature, the equilibrium vapor pressure over liquid water is higher than it is over ice. Water molecules therefore tend to leave the droplets and deposit on nearby ice crystals.

This cloud process is commonly called the Wegener–Bergeron–Findeisen process, or the ice-crystal process. The ice crystals gain mass while some liquid droplets evaporate. When the crystals become heavy enough, gravity begins pulling them downward through the cloud.

Why Snowflakes Have Six Sides

Ordinary atmospheric ice usually forms a crystal structure called hexagonal ice, also written as ice Ih. Within this structure, water molecules connect through hydrogen bonds in an ordered arrangement. That molecular order produces two main kinds of crystal surfaces: basal faces at the top and bottom, and prism faces around the sides.

A young snow crystal often begins as a tiny hexagonal prism. Depending on which surfaces grow faster, the prism may become a thin plate, a long column, a hollow tube, or a more elaborate branched crystal.

The six arms of a stellar snow crystal are not coordinated by a central control point. They grow separately, yet all six experience nearly the same temperature and vapor supply because the crystal is small. When conditions change, each arm responds at about the same time. This shared environment creates sixfold radial symmetry.

Perfect symmetry is uncommon in natural snowfall. Collisions, uneven vapor flow, riming, partial melting, and damage during descent often produce irregular crystals. The clean, balanced forms seen in close-up photography are a beautiful part of snowfall, though they are only one part of its natural variety.

How Temperature Changes Snowflake Shape

Temperature affects how quickly molecules attach to the basal and prism faces of ice. When growth along the prism direction dominates, crystals stretch into columns or needles. When outward growth along the basal plane dominates, they spread into plates and stars.

The ranges below describe common laboratory and atmospheric patterns. They are approximate because humidity, air pressure, ventilation around the crystal, and its earlier growth history also influence the result.

Plates and stellar crystals

A plate develops when the crystal expands outward much faster than it thickens. At low vapor supply, it may remain a simple hexagonal plate. With more available vapor, the corners grow faster and spread into six arms. Side branches may then appear along each arm.

Large stellar dendrites often develop near −15°C (5°F) when the air is moist enough for rapid deposition. The word dendrite comes from a term associated with tree-like branching. These are the broad, fern-like forms commonly used as the visual symbol for snow.

Columns and needles

Columnar crystals grow mainly along their central axis. They may be short and solid, long and hollow, or narrow enough to resemble needles. Needle growth is often favored near −5°C (23°F) when the vapor supply is high.

A hollow column forms when the edges of a crystal face receive vapor more readily than its center. The edges advance while the center lags, leaving an open cavity. Some columns later grow plates at one or both ends, producing capped columns or bullet rosettes.

Mixed and transitional forms

A crystal rarely remains in one temperature band throughout its entire descent. A column may enter a region favorable to plates, causing flat caps to grow from its ends. A plate may later develop branches when it reaches air with greater ice supersaturation.

These transitions create scrolls, cups, capped columns, split plates, and other compound forms. Their structure can reveal changes in the cloud layer through which they traveled.

How Humidity Controls Detail and Branching

Temperature helps select the basic habit, while the supply of water vapor affects how fast and how elaborately the crystal grows. Meteorologists often discuss that supply in terms of ice supersaturation rather than ordinary surface-level relative humidity.

When supersaturation is low, vapor arrives slowly. Crystal faces have time to remain smooth and even, producing small plates, solid prisms, or simple columns. When supersaturation is higher, growth becomes faster. Corners and small surface projections receive vapor before flatter recessed areas do.

A tiny projection therefore grows faster than the surface around it. As it extends, it intercepts still more vapor. This self-enhancing process is called a branching instability. Six main arms emerge from the corners of a hexagonal plate, followed by smaller side branches when conditions support continued rapid growth.

Why corners grow first

Water molecules move through the air by diffusion. A protruding corner reaches farther into the surrounding vapor field than a flat area does. It collects molecules more readily and advances faster. The same effect occurs at the tip of a growing branch.

Air movement around a falling crystal also affects vapor delivery. A crystal tumbling through a cloud may receive slightly different airflow across its surface. This can disturb its balance and produce arms of unequal length.

Common Snow Crystal Shapes

Researchers classify snow crystals by habit, branching pattern, internal structure, and the effects of collisions or frozen droplets. Natural specimens often combine more than one category.

Snow Crystal and Snowflake: The Difference

A snow crystal is a single, ordered crystal of ice. A snowflake may be one snow crystal or an aggregate made from several crystals joined together. In everyday speech, both are usually called snowflakes.

Large flakes seen during mild snowfall are often aggregates. Individual crystals collide as they fall and stick together. Their branches interlock, and a thin liquid-like layer on the ice surface can help them adhere when temperatures are close to freezing.

Cold, dry snowfall tends to contain smaller and more separate crystals. Snow near 0°C often falls in larger clumps because aggregation is easier. Wind, turbulence, and the number of crystals in the cloud also affect collision rates.

Riming, Graupel, and Other Changes During Descent

Riming coats crystals with frozen droplets

A falling snow crystal may collide with supercooled cloud droplets. The droplets freeze onto its surface almost immediately. This process is called riming.

Light riming leaves scattered rounded beads on the original crystal. Heavy riming can hide its branches and turn it into a soft, opaque pellet called graupel. Graupel often looks like a tiny white foam ball and can crumble when pressed.

Aggregation joins crystals together

During aggregation, two or more snow crystals collide and remain attached. The resulting flake has no single crystal lattice extending through the entire structure. It is a cluster of separate crystals held together by interlocking branches, frozen contact points, or both.

Melting changes the edges

A snowflake entering above-freezing air begins to melt around its thinnest parts. Sharp tips round off first. Continued melting changes the flake into a wet ice cluster and eventually into a raindrop.

Snow can sometimes reach the ground when the measured surface temperature is slightly above 0°C. The outcome depends on the depth and temperature of the warm layer, the humidity of the lower air, the flake’s size, and its falling speed. Evaporation and sublimation can cool the surrounding air enough to slow melting.

Why Snowflakes Rarely Look Identical

The phrase “no two snowflakes are alike” describes probability rather than a rule that can be checked for every crystal ever formed. Very simple prisms can look almost identical under ordinary magnification. Exact agreement in their complete molecular arrangement, surface details, defects, and growth history would be extraordinarily unlikely.

Each crystal follows its own route through a cloud. One may spend several seconds in a moist layer near −15°C, then pass through drier air. Another may enter the same layers in a different order. Even a small difference in vapor supply can change where a branch begins or how far it grows.

Crystal defects add more variation. Dislocations, impurities, collisions, and uneven surface attachment can alter later growth. By the time a flake reaches the ground, its form contains the combined effects of nucleation, deposition, temperature changes, moisture changes, airflow, riming, aggregation, and melting.

Weather Conditions That Favor Different Snow Types

Cold and dry clouds

Low vapor availability limits crystal growth. Snow from cold, dry air often consists of small plates, columns, prisms, or fine grains. The snowpack may feel loose and powdery because the crystals contain little liquid water and do not readily bond on contact.

Moist dendrite-producing clouds

Cloud layers near −12°C to −18°C can support broad plates and dendrites when the vapor supply is high. Meteorologists sometimes call this layer the dendritic growth zone. Strong lift through the zone can keep crystals suspended while they collect vapor and grow.

The depth of that zone and the time crystals spend inside it affect snowfall texture and accumulation. Many branched crystals can form low-density snow because their arms create open spaces after landing.

Temperatures near freezing

Snow near the melting point is wetter and more adhesive. Crystals aggregate easily, producing large flakes. Once on the ground, wet snow packs tightly and usually contains more liquid-equivalent water per unit depth than dry, fluffy snow.

Strong wind and turbulent clouds

Turbulence increases collisions among crystals and droplets. It can encourage aggregation and riming, though it may also break fragile dendrites. Snow reaching the surface after a turbulent descent may contain fragments, compact clusters, and heavily rimed particles rather than clean stars.

Snowflakes Compared With Other Frozen Precipitation

Snowflakes grow through ordered crystal deposition inside cold clouds. Ice pellets come from refreezing liquid drops, while hail develops in powerful thunderstorm updrafts. Their different formation paths explain why only snow crystals display the familiar plates, columns, and six-armed dendrites.

Observing Snowflakes Safely

Fresh crystals are easiest to examine on a cold, dark surface. A sheet of black paper, fabric, or metal can be chilled outdoors before snowfall begins. Placing warm material beneath a flake causes its smallest branches to melt within seconds.

A hand lens reveals plates, columns, needles, aggregates, and riming. Light from the side makes edges and surface patterns easier to see. Catching several flakes during one storm may show changes as different cloud layers pass overhead.

Surface observations cannot reconstruct every part of a crystal’s route, yet they can reveal the conditions that dominated its growth. Delicate dendrites point to moist plate-forming air, needles suggest a different temperature range, and rounded white pellets indicate strong riming. Each surviving crystal is a small physical record of temperature, water vapor, and motion inside the cloud.

References

• NOAA NESDIS – How Do Snowflakes Form?
  (Explains ice-crystal growth, facets, branching, and sixfold symmetry.)
• UCAR Center for Science Education – Snowflakes
  (Describes cloud formation, crystal shapes, aggregation, and the effects of temperature and water vapor.)
• California Institute of Technology – Morphogenesis on Ice: The Physics of Snow Crystals
  (Examines snow-crystal morphology, supersaturation, branching, and temperature-dependent growth.)