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Science | Edge Cases Science | Earth, Water, Atmosphere & Celestial World
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Frost Flowers
How Sea Ice Can Grow Salty Crystal Gardens From Water Vapour
Wait, What? Sea Ice Can Grow “Flowers” That Were Never Liquid Water Droplets
On newly formed polar sea ice, delicate white structures can rise like feathers, ferns or petals. They look as if the ocean has frozen upward into tiny plants.
But the ice crystals themselves can grow mainly from water vapour depositing directly as solid ice.
water vapour → ice crystal, without becoming a liquid droplet first.
Then something even stranger happens: the crystals can become coated with concentrated salty brine from the young sea-ice surface.
The scientific job here is precise: frost flowers own vapour-deposited ice crystals growing above young sea ice and their interaction with concentrated surface brine. They do not duplicate Brinicles, which own sinking cold brine and downward tubular ice growth beneath sea ice.
Big Question: How can water vapour make a fresh ice crystal above sea ice, and how can that nearly pure ice become coated with salts from the ocean below?
Quick Answer
Frost flowers form most readily over very young sea ice when the ice surface is much warmer than the cold air immediately above it. Water vapour supplied near the warm ice surface becomes supersaturated relative to ice and deposits onto tiny surface irregularities, building branching crystals. The National Snow and Ice Data Center defines frost flowers as ice crystals formed when water vapour becomes solid directly on the sea-ice surface.
Young sea ice also contains concentrated brine because growing ice rejects much of the salt. That brine can wet the frost-flower crystals by capillary action and surface contact. Field and laboratory studies show that frost flowers can therefore carry concentrated sea-salt components even though their ice skeleton grew from vapour.
National Snow and Ice Data Center — Frost Flowers →
What You Will Learn
- What deposition means in a phase change.
- Why young sea ice can supply abundant water vapour.
- Why cold air favours supersaturation with respect to ice.
- How branching ice crystals grow.
- Why sea ice rejects salt while freezing.
- How concentrated brine reaches frost-flower surfaces.
- Why a frost flower can be both vapour-grown and salty.
- How frost flowers differ from snowflakes and brinicles.
- How wind can turn salty surfaces into aerosols.
- Why frost flowers matter to polar atmospheric chemistry.
- Why their role as an aerosol source requires careful evidence.
- How satellites and radar can respond to frost-flower-covered surfaces.
Part 1 — Deposition Skips the Liquid Phase
Water has three familiar phases: solid, liquid and gas. In deposition, water molecules move directly from vapour into an ice crystal.
This is the reverse of sublimation.
gas → solid = deposition
solid → gas = sublimation
Frost on a cold surface and many features of snow-crystal growth use the same phase-change route.
Part 2 — Young Sea Ice Creates a Strong Temperature Gradient
New sea ice may be only centimetres thick. The ocean beneath is near its freezing point, while polar air above can be tens of degrees colder.
The upper surface of the thin ice therefore remains relatively warm compared with the air just above it. Water vapour supplied near the surface encounters rapidly colder air.
Because colder air can sustain a lower equilibrium vapour pressure over ice, the local air can become supersaturated with respect to ice.
Part 3 — Supersaturation Drives Crystal Growth
When vapour pressure exceeds the equilibrium value over ice, water molecules are more likely to join an ice surface than leave it.
Tiny crystals begin on irregularities at the sea-ice surface and grow upward into the cold air.
The shape depends on temperature, humidity and vapour supply, producing feathery, dendritic or blade-like structures.
Part 4 — Why They Look Like Flowers
Ice crystals grow anisotropically: different crystallographic faces grow at different rates under different environmental conditions.
Edges and tips can collect vapour more efficiently than flat sheltered regions, amplifying branching. This instability produces ornate structures rather than simple blocks of ice.
Part 5 — The Ocean Salt Mostly Does Not Enter the Ice Lattice
As seawater freezes, the crystal lattice of ice incorporates water molecules far more readily than dissolved ions.
Salt becomes concentrated in remaining liquid brine channels and surface slush.
NSIDC — Science of Sea Ice and Brine Rejection →
Part 6 — So Why Are Frost Flowers Salty?
The ice skeleton can grow from atmospheric water vapour while its surface becomes wetted by concentrated brine from the sea ice below.
Capillary forces, contact with slush and brine migration can spread saline liquid over the microscopic crystal branches.
vapour builds the crystal framework; brine supplies much of the salt coating.
Part 7 — Salt Chemistry Changes as Brine Gets Colder
As concentrated brine cools, different salts do not all remain dissolved equally. Some mineral salts precipitate at particular temperatures, changing the relative abundance of ions remaining in solution.
Field studies in Greenland found frost flowers and brine enriched in sea-salt components and showed that low-temperature salt fractionation can alter their chemistry.
Atmospheric Chemistry and Physics — Frost Flowers, Brine and Sea-Salt Aerosols →
Part 8 — Frost Flowers Are Tiny High-Surface-Area Landscapes
A flat square metre of sea ice has roughly one square metre of geometric top surface. Add thousands of microscopic branches and the true ice–air interface becomes much larger.
Large surface area matters because chemical exchange, condensation and evaporation happen at interfaces.
Electron-microscope studies have observed brine coating tiny frost-flower fingers, exposing saline liquid over a greatly enlarged surface.
Atmospheric Chemistry and Physics — Brine on Frost Flowers Under Electron Microscopy →
Part 9 — Wind Can Mobilise Sea Salt From the Ice Surface
Polar sea-salt aerosol does not come only from breaking ocean waves. Wind can loft saline particles from snow, blowing snow, slush and young sea-ice surfaces.
Frost flowers can be part of that saline surface environment. However, modern evidence cautions against treating intact frost flowers as the sole or automatic source of every observed polar sea-salt aerosol.
Part 10 — Why Atmospheric Chemists Care
Sea-salt aerosols contain chloride, bromide and other ions. In polar air, chemical reactions involving halogens can influence ozone and mercury chemistry.
The exact chain from sea-ice surface to airborne reactive chemistry is complicated. Frost flowers are scientifically valuable because they sit at the boundary between ocean, ice and atmosphere where those transfers occur.
Part 11 — Frost Flowers Change How the Surface Looks to Instruments
The rough, crystalline surface changes how electromagnetic radiation interacts with sea ice. NSIDC notes that frost flowers can dramatically alter the electromagnetic signal of the surface.
This matters for remote sensing because a satellite may interpret brightness or radar return differently when frost flowers cover young ice.
Part 12 — Frost Flowers Are Not Snowflakes
| Frost flowers | Snowflakes |
|---|---|
| Grow attached to sea-ice surface | Grow suspended in clouds |
| Often associated with young sea ice and steep temperature gradients | Associated with atmospheric cloud microphysics |
| Can become coated in concentrated brine | Usually not coated with sea-ice brine while forming |
| Remain part of the surface until broken or altered | Fall through the atmosphere |
Part 13 — Frost Flowers Are Not Brinicles
Both involve young sea ice and brine, but their claimed mechanisms point in opposite directions.
- Frost flower: water vapour deposits upward into cold air above sea ice.
- Brinicle: cold dense brine sinks below sea ice and freezes surrounding seawater into a downward tube.
Part 14 — Follow One Frost Flower
- Thin young sea ice forms over relatively warm seawater.
- Much of the salt is rejected into brine.
- The ice surface remains warmer than very cold air above.
- Water vapour enters the near-surface air.
- The air becomes supersaturated relative to ice.
- Vapour deposits onto a tiny ice nucleus.
- Branches grow upward.
- Concentrated brine wets parts of the crystal surface.
- Low-temperature salt fractionation changes the brine chemistry.
- Wind, snow and ageing modify or destroy the delicate structure.
How Do We Know?
- Time-lapse field observations record growth on new sea ice.
- Humidity and temperature measurements establish supersaturation conditions.
- Chemical sampling measures salts in flowers and brine.
- Electron microscopy reveals brine-coated microstructures.
- Aerosol measurements test whether saline particles are released nearby.
- Remote-sensing measurements quantify changes in electromagnetic response.
Observation vs Inference
- Observation: branching ice crystals form above very young sea ice.
- Measurement: surrounding air can be supersaturated relative to ice.
- Measurement: frost-flower surfaces can contain concentrated sea salts.
- Inference: vapour deposition builds the ice while brine contact supplies salts.
- Boundary: finding salty frost flowers does not prove that intact frost flowers are the dominant aerosol source in every polar environment.
Common Misconceptions and Better Models
| Misconception | Better model |
|---|---|
| Frost flowers are frozen splashes of seawater. | The ice crystals can grow primarily by vapour deposition. |
| If the crystal came from vapour, it cannot be salty. | Concentrated brine can later coat the vapour-grown ice. |
| They are small snowflakes. | They grow attached to a young sea-ice surface. |
| They are miniature brinicles. | Brinicles grow downward around sinking brine beneath the ice. |
| Every polar sea-salt aerosol comes from frost flowers. | Snow, slush, brine and open water can also contribute. |
| The crystals are chemically pure because ice excludes salt. | The ice lattice may be relatively pure while its surface brine is highly saline. |
Checkpoint Questions
- What is deposition?
- Why does young sea ice favour frost-flower growth?
- What does supersaturated with respect to ice mean?
- Why does sea ice create concentrated brine?
- How can a vapour-grown crystal become salty?
- Why is high surface area chemically important?
- How can frost flowers influence remote sensing?
- How are they different from snowflakes?
- How are they different from brinicles?
- Why should aerosol-source claims be made cautiously?
Answer Key
Open after attempting the questions
- Direct change from gas to solid.
- Warm thin ice beneath very cold air produces strong vapour and temperature gradients.
- Vapour pressure exceeds the equilibrium value over ice.
- Growing ice rejects most dissolved salts.
- Surface brine can wet the branches.
- It provides more interface for chemical exchange.
- They roughen and chemically alter the electromagnetic surface.
- They grow attached to sea ice rather than in clouds.
- They grow upward from vapour; brinicles grow downward around sinking brine.
- Several other sea-ice and ocean surfaces can generate saline aerosol.
Primary Science Bridge
- water can change between solid, liquid and gas;
- temperature affects phase changes;
- salt changes the behaviour of water;
- crystals can grow in patterns;
- observed appearance does not always reveal the pathway that formed it.
Secondary and JC Bridge
| Core idea | Higher-resolution route |
|---|---|
| Condensation/frost | Vapour pressure and deposition |
| Solutions | Brine concentration and salt fractionation |
| Crystal growth | Diffusion-limited branching |
| Surface area | Interfacial chemistry |
| Atmosphere | Sea-salt aerosols and halogen chemistry |
| Measurement | Microscopy and remote sensing |
Deep Science Window — Morphological Instability
A protruding crystal tip can intercept more diffusing water vapour than a recessed surface. That makes the tip grow faster, which makes it intercept still more vapour. This positive feedback helps explain dendritic crystal shapes.
Deep Science Window — The Ice–Ocean–Atmosphere Interface
Frost flowers matter because three reservoirs meet in centimetres: ocean salt, solid sea ice and atmospheric water vapour. Small structures can therefore connect phase physics, surface chemistry and climate measurements.
Evidence Boundaries
- Frost flower ≠ frozen splash.
- Vapour-grown ice ≠ salt-free surface.
- Salty surface ≠ salt incorporated uniformly in the ice lattice.
- Frost flower ≠ brinicle.
- Observed sea-salt aerosol ≠ automatic proof of one source.
- Beautiful structure ≠ biological flower.
Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK
KNOW: deposition, supersaturation, brine rejection, capillary wetting, salt fractionation.
CONNECT: warm young ice to vapour supply, cold air to crystal growth, and sea-ice brine to surface salinity.
EXPLAIN: why a crystal formed from vapour can become extremely salty.
APPLY: predict how warmer air, older ice or strong wind would alter frost flowers.
CHECK: separate crystal formation from later chemical coating.
Teaching Guide for Parents, Tutors and Teachers
The useful contradiction is “made from vapour, yet salty.” Teach those as two consecutive mechanisms instead of trying to make one mechanism do both jobs.
- Review solid–liquid–gas phase changes.
- Add deposition as gas → solid.
- Build the warm-ice/cold-air gradient.
- Explain supersaturation and branching.
- Add salt rejection from sea ice.
- Show how brine coats the crystal.
- Finish with polar chemistry and evidence limits.
Safety boundary: use photographs, controlled classroom frost observations or published datasets. Polar sea ice is a hazardous field environment and should never be treated as a casual observation site.