Why do frost needles grow up out of the ground?
― They aren't falling from above, they're pushed up from below
On a winter morning, thin columns of ice crunch under your foot when you step on the ground. They didn't fall from the sky. It's thought that water sitting in the soil is carried upward through fine gaps and keeps freezing just above the surface, building the column up. Ice is added at the bottom end of the column, so the column grows as if it were being pushed up from underneath.
On a clear winter morning, if you walk across a corner of the schoolyard or beside a field, you hear a crunch under your feet.
Crouch down and look, and you'll find thin ice needles standing packed together. They're a few centimetres tall, white, and some are slightly bent.
Look closely and you'll notice a small crumb of soil perched right on top of each needle. That's the strange part.
There are only two reasons frost needles form
Between grains of soil there are gaps far thinner than a human hair. Water has the property of climbing up these narrow paths on its own.
The water that climbs up freezes in the cold spot just above the ground surface. Because that freezing spot stays put, the ice that formed earlier gets pushed upward.
While these two things happen at the same time, the column keeps growing. The reason soil sits on top of the column is that the very first patch of ground surface that got lifted is carried along, still acting as a "ceiling." Let's look at this step by step.
Water climbs higher through narrower paths
If you dip the tip of a thin glass tube into water, the water climbs up the tube on its own. The narrower the tube, the higher it climbs. This happens because of a balance between the force of water molecules pulling on each other and the force pulling water toward the tube's wall. The gaps between soil grains act as the same kind of narrow tube.
On a winter morning, this "climbing water" becomes the raw material for the ice. As the water at the surface freezes and disappears, more water is drawn up from the damp soil below to fill the gap. The ground below is often warmer than 0°C, so that water hasn't frozen yet. In other words, the supply can keep coming up from below almost indefinitely.
Ice is newly formed at the "bottom end" of the column
Look at the right side of Figure 1. The fact that soil sits on top of the column is proof that ice hasn't simply piled up from above. If it had fallen from the sky, the soil would end up underneath the ice.
What actually happens is this. First, the water right at the ground surface freezes into a thin layer of ice. That ice receives the water climbing up from below and adds new ice to its own underside. Because the freezing spot stays fixed just above the ground, the ice that formed earlier has nowhere to go but up. The crumb of soil that was at the ground surface gets carried up along with it.
The needle forms a straight vertical line because both the path the water climbs and the direction it gets pushed are up and down. It's thought that a needle can grow several centimetres in a single night, and under the right conditions can reach nearly 10 centimetres.
Gaps that are too wide or too narrow both fail to work well. Sand grains are large, so the gaps between them are wide, and that weakens the force pulling water up high. Clay, on the other hand, has gaps so fine that water can barely flow through. It's thought that soil with a grain size in between — like farm soil or Kanto loam — grows frost needles well.
When the same thing happens on a larger scale underground, layers of ice form and slowly push the whole ground surface upward. This is called "frost heave," and in cold regions it's thought to be a cause of road pavement buckling. A frost needle is a small-scale example of the same thing.
Summary
Frost needles are not ice that fell from the sky. Water in the soil climbs up through fine gaps and keeps freezing just above the ground surface. Because that ice is added to the bottom end of the column, the part that formed earlier gets pushed upward. The soil perched on top of the needle is solid proof of that.
Frost needles aren't ice that fell from above.
They're ice built by water pushing up from below.
Even on the same winter morning, the water droplets and ice that form on grass work by a completely different mechanism. The story of water vapour in the air turning to liquid water on a cold surface is explained in "Why is the morning grass wet even when it hasn't rained?," and the reason ice crystals form hexagons is explained in "Why are snowflakes hexagonal?." For the mystery of why ice floats on water, see "Why does ice float on water?" too. The story of how the same "water in fine gaps" that lifts frost needles also hardens sand is explained in "Why does dry sand crumble but wet sand hold its shape?." For the mechanism of ice that grows downward instead — the opposite of a frost needle — see "Why do icicles form not on the coldest day, but on a "slightly milder" one?."
- Pick a morning after a clear, calm night, when the lowest temperature was around 0°C. The time before the sun comes up is your best bet.
- Look for ground with fine soil, like a flower bed or a field. You'll rarely find any near a sandbox or beside asphalt.
- Gently pinch up one needle and check whether a crumb of soil sits on top. If it does, that's proof it was pushed up from below.
Please don't enter someone else's garden or field. Also, ground where frost needles have melted becomes muddy and slippery. Watch your footing while you observe.
Want to know more? ― Terms, formulas, and how this connects to textbooksWe label each section from junior-high science up to university-level specialist courses, so you know exactly what level you're reading
- Jr. HighCovered in junior-high school science
- High SchoolCovered in high-school "Basic Physics / Basic Chemistry"
- High School+High-school advanced content, or textbook sidebar material
- UniversityNot taught in high school — content from university specialist courses (geotechnical engineering, cryospheric science)
- ResearchNot yet settled as "established fact" even at university — what researchers are actively studying now
Jr. HighTerms: this phenomenon has names
- Frost needle (shimobashira): A thin column of ice that rises from the ground. It forms when water in the soil keeps freezing near the surface.
- Capillary action: The phenomenon where water climbs on its own through a narrow tube or gap.
- Frost heave: When layers of ice form underground and push the whole ground surface upward.
- Segregated ice: A solid layer of ice that forms by pushing soil grains aside. A frost needle is thought to be a form of this that has broken through to the surface.
Jr. HighHigh SchoolChecking with a formula: how high can water climb through a narrow gap?
Let's model the gap between soil grains as a thin tube with a radius of 0.01 millimetres and calculate. All we need is the balance between water's surface tension and its weight.
| In symbols | h = 2 × γ ÷ ( ρ × g × r ) |
| In words | Height climbed = 2 × surface tension ÷ (density of water × strength of gravity × radius of the gap) |
| Where it comes from | The balance between the surface tension pulling water up along the edge of the gap and the weight of the lifted column of water (Jurin's law, assuming water wets the gap's walls well) |
| Surface tension of water (near 0°C) | about 0.073 newtons per metre |
| Density of water | about 1000 kilograms per cubic metre |
| Strength of gravity | about 9.8 metres per second per second |
| Radius of the gap | 0.00001 metres (0.01 millimetres) |
| Symbol | Meaning and unit |
| γ (surface tension) | How strongly the water surface tends to shrink [newtons per metre] |
| ρ (density) | Weight per cubic metre [kilograms] |
| g (strength of gravity) | The increase in falling speed per second [metres per second per second] |
| r (radius of the gap) | The width of the narrow path between soil grains [metres] |
| Lifting side, base value | 0.073 × 2 = 0.146 |
| Weight side, base value | 1000 × 9.8 = 9800 |
| Factoring in the gap's narrowness | 9800 × 0.00001 = 0.098 |
| Height climbed (metres) | 0.146 ÷ 0.098 ≈ 1.49 |
| Converted to centimetres | 1.49 × 100 = 149 |
For a gap with a radius of 0.01 millimetres, water could in theory climb up to 149 centimetres. Since frost needles are only a few centimetres tall, this is far more than enough lifting power to supply the material. In real soil, the paths twist and turn, so water doesn't move nearly this fast. Even so, this shows that "water arriving from below" is a perfectly reasonable idea.
High SchoolHigh School+Why does ice grow at the "bottom end"?
High SchoolWhen water turns to ice, it releases heat. That heat escapes upward toward the cold air. In other words, the spot that can shed heat most easily is right beneath the ice that has already formed. That's why the ice keeps growing in that exact spot.
High School+There's another factor: it's thought that a very thin film of water remains on the surface of soil grains even below 0°C, without fully freezing. Water from below is drawn through this film toward the freezing surface. The ice grows by pushing soil grains aside, and in doing so lifts whatever sits above it. Depending on conditions, that pushing force is thought to reach the equivalent of several kilograms per square centimetre.
UniversityFrost heave from a geotechnical engineering perspective
In geotechnical engineering, the phenomenon where ice layers form underground and push soil upward is called "frost heave," and it's always taken into account when designing structures in cold regions. The process that draws water toward the freezing surface is called "frost suction," explained by a pressure difference across the thin water film. It's thought to occur strongly when three conditions line up: fineness of the soil, water supply, and speed of cooling — and it can be suppressed by inserting a water-cutting layer. A frost needle is this same phenomenon breaking through to the surface, where the only thing left to push against is the air.
📖 For the derivation of the formula and further reading: Capillary action (Japanese Wikipedia) ／ Frost heave (Japanese Wikipedia)
ResearchWhat's still not fully understood
- The thickness of the thin water film. How thin the water film between the ice and soil grains gets at a given temperature is difficult to measure, and reported values still vary.
- Predicting the amount of frost heave. Accurately predicting in advance how much the ground will rise for a given soil type is still said to be far from simple.
- The link to climate change. How changes in winter cold snaps and snowfall affect the way ground freezes and how frost heave occurs is still being studied region by region.
In other words, everything in this article is "the best explanation we have so far." Even the small ice needles under your feet still have parts that researchers are actively working on.
How this maps onto textbooks (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Jr. High | Science: states of matter / forms of water | Water turning to ice, freezing temperature |
| High School | Basic Physics / Basic Chemistry (surface tension, freezing and heat) | The calculation of water climbing a narrow gap, why ice grows at the bottom |
| High School+ | Advanced / sidebar topics (interfacial water, pressure in narrow tubes) | The unfrozen thin water film, the lifting force |
| University | Geotechnical engineering / cryospheric science (frost heave, frost suction) | Ice layers and cold-region design |
| Research | Cryospheric science / cold-region civil engineering (ongoing) | Water film thickness, predicting frost heave amounts, link to climate change |
| ― | Everyday connections | Flower-bed soil lifts, sandboxes don't; winter road damage |
- Japan Meteorological Agency explanatory materials on weather observation (on observing frost, frost needles, and minimum temperature).
- Technical materials from the Civil Engineering Research Institute for Cold Region (寒地土木研究所) and others, on frost heave in cold-region roads and layers used to suppress it.
- Sections on surface tension, capillary action, and states of matter in high-school "Basic Physics" and "Basic Chemistry" textbooks.
- Taber, S., "The Mechanics of Frost Heaving," Journal of Geology, 1930. (A classic study showing the mechanism by which ice layers grow.)
※This article is a general-audience science explainer. The figures given are approximations meant to help you understand the mechanism. How readily frost needles form varies greatly with soil type, moisture, and cooling conditions. Frozen ground, and ground right after it thaws, can be slippery. Please watch your footing carefully and follow any warning signs posted by local authorities or facilities.