Explainer: The making of a snowflake | Science News for Students

Explainer: The making of a snowflake

Here’s the science behind a snowflake’s striking shape
Feb 14, 2019 — 6:40 am EST
a girl playing in a snowdrift

Snowflakes take a long journey down from the clouds in which they form. But it takes more than cold air and moisture to create them.

SbytovaMN/iStockphoto, adapted by L. Steenblik Hwang

Snowflakes come in an infinite range of shapes and sizes. Many appear to be two-dimensional works of art. Others look like a matted cluster of fraying ice strands. Most come as individuals, although some can fall as multi-flake clumps. What all have in common is their source: clouds that usually hover at least a kilometer (0.6 mile) above the ground.

a composite image showing how snowflakes clump together when they collide to create much larger compound snowflakes
When snowflakes collide, their branches can tangle. This can create a compound flake. This often leads to whoppers (like those in first and third rows) by the time the flakes land.
Tim Garrett/Univ. of Utah

In winter, the air up there can be very cold — and will get chillier the higher you go. To form snowflakes, those clouds need to be below freezing. But not too cold. Snowflakes form from the moisture in a cloud. If the air gets too cold, a cloud won’t hold enough water for anything to precipitate out. So there has to be a balance. That’s why most flakes develop at or just below freezing — 0º Celsius (32º Fahrenheit). Snow can form in cooler environments, but the colder it gets, the less moisture will be available to make a snowflake.

In fact, a cloud’s air has to be supersaturated with moisture for a flake to form. That means there is more water in the air than would normally be possible. (The relative humidity can reach 101 percent during supersaturation. That means there is 1 percent more water in the air than it should be able to hold.)

When there is too much liquid water in the air, a cloud will try to rid itself of the excess. Some of that excess can flash freeze into crystals, which then lazily meander to the ground.

Or that’s the simple answer. The details aren’t quite that straightforward.

Cold water alone won’t a snowflake make

One more thing is needed to turn cloud moisture into a flake. Scientists call it a nucleus (NOO-klee-uhs). Without something to glom onto, water droplets can’t freeze. Even when the air temperature is well below freezing, water droplets will remain liquid — at least until they have a solid object onto which they can attach.

Usually, that will be something like a pollen grain, dust particle or some other airborne bit. It could be smoglike aerosols or the volatile organic compounds released by plants. Even tiny soot particles or microscopic metal bits spewed in a car’s exhaust could become the nuclei around which snowflakes crystalize.

Indeed, when the air is very clean, it can be very difficult for a cloud’s moisture to find a nucleus.

Near the ground, any object can prove a suitable freeze-onto zone. That’s how we get rime ice to form on the branches of trees, light poles or vehicles. Different from frost, rime ice develops when supercooled water droplets freeze onto subfreezing surfaces. (In contrast, frost forms when moisture collects on surfaces in liquid form, and then freezes.) 

High in a cloud, there have to be some tiny floating particles for snow crystals to develop. When the right conditions do emerge, supercooled drops of water will latch onto these nuclei (NOO-klee-eye). They do it one by one, building an ice crystal.

How the flakes shape up

a composite image showing the microscopic crystal structure of three very different snowflakes
Snowflakes come in an endless variety of shapes and sizes — but all have six sides.
Kenneth Libbrecht

To understand what’s behind a snowflake’s intricate and complex shape, scientists turn to chemistry — the action of atoms.

A molecule of water, or H2O, is made of two hydrogen atoms bound to an oxygen atom. This trio combines into a “Mickey Mouse” pattern. That’s due to polar covalent (Koh-VAY-lent) bonds. The term refers to three atoms that each share electrons with one another, but unevenly.

The oxygen’s nucleus is larger, so it has more pull. It yanks more strongly at the negatively charged electrons that they share. This brings those electrons a little closer. It also gives the oxygen a relative negative electric charge. The two hydrogen atoms end up a tad positive, in terms of charge.

Alone, the structure of a water molecule resembles a wide V. But when multiple H2O molecules find themselves close to one another, they begin to pivot so that their electrical charges pair up. Opposite charges attract. So a negative hydrogen aims itself towards a positive oxygen. The shape that tends to result: a hexagon.

That’s why snowflakes have six sides. It stems from the hexagonal — six-sided — structure of most ice crystals. And hexagons team up. They link with other hexagons, growing outward.

That’s how a snowflake is born.

Each hexagon contains a lot of empty space. This explains why ice floats on water; it’s less dense. Warmer H2O molecules in the liquid phase are too energetic to settle into a rigid hexagon. As a result, the same number of H2O molecules occupy 9 percent more space as solid ice than they do as liquid water.

Depending on the temperature, these hexagons join with each other and grow in different ways. Sometimes, they make needles. Others may form branch-like dendrites. All are beautiful. And all have their own unique story of crystal growth.

Snowflake structure has been a scientific curiosity since Wilson Alwyn “Snowflake" Bentley attached a microscope to his camera in 1885 and became the first person to photograph them.

These short-lived crystals still enthrall scientists. To better capture their shape and movement, Tim Garrett at the University of Utah in Salt Lake City recently built a better snowflake camera. He’s been using it to get an inside view of the  variety of flakes that fall.

a diagram showing how temperature and humidity changes impact snowflake shape
This diagram shows how temperature and humidity affect the shape of a snowflake. Note the six-sided shape. It’s instrumental in how the crystals form and grow. The largest flakes tend to occur at temps close to freezing. As temperatures drop, flakes with fewer branches become more common. Scientists are still probing how temperature and humidity affect a flake’s shape.
Kenneth Libbrecht

Snowflakes by the numbers

1. A typical snowflake may contain 1,000,000,000,000,000,000, or one quintillion water molecules. That’s a million times a million times a million! Those building blocks can configure themselves in a virtually infinite array of patterns. So it stands to reason that no two snowflakes that you encounter will ever be exactly the same.
 

2. Snowflakes tend to be less than a coin’s width in diameter. But once in a while, true whoppers form. In January 1887, a Montana rancher reported snowflakes “larger than milkpans.That would make them some 38 centimeters (15 inches) across. As that was back before portable home cameras, this number can be challenged. But snowflakes larger than 15.2 centimeters (6 inches) do sometimes develop. Biggies tend to form when temps are near freezing and the air humid. A snowflake’s size also reflects other factors. These include wind speed and direction, dew point — even how electrified different layers of the atmosphere are. But nobody has ever really conducted measurements when gigantic flakes were flying.

3. Most snowflakes fall at roughly a walking pace — between 1.6 and 6.4 kilometers (1 and 4 miles) per hour.

4. With the cloud in which flakes form usually one to two kilometers (0.6 to 1.2 miles) up, each crystalline wonder may drift anywhere from 10 minutes to more than an hour before reaching the ground. Sometimes, they get carried back up, and it takes several tries for them to reach the ground.


Power Words

(more about Power Words)

aerosol     A group of tiny particles suspended in air or gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.

array     A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.

atmosphere     The envelope of gases surrounding Earth or another planet.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.

cloud     (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

compound     (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

develop     To emerge or come into being, either naturally or through human intervention, such as by manufacturing.

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

electric charge     The physical property responsible for electric force; it can be negative or positive.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

exhaust     (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.

factor     Something that plays a role in a particular condition or event; a contributor.

frost     What results when liquid water freezes as it comes in contact with a surface that has a below-freezing temperature.

hexagon     A geometric shape that has six equal sides. It takes its name from the Greek word for six.

hover     To above the ground, maintaining a relatively slow and predictable or intentional position.

humidity     A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

link     A connection between two people or things.

meander     A bend in a stream or to move slowly and with no straight path in mind.

microscope     An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.

moisture     Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nucleus     Plural is nuclei. (in physics) The central core of an atom, containing most of its mass.

organic      (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

particle     A minute amount of something.

pollen     Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.

precipitation      (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

prism     A triangular wedge of glass or another clear substance that can bend the components of white light into a rainbow-like succession of colored bands.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

rime ice     A coating of tiny, opaque ice crystals formed when supercooled water freezes rapidly on contact with an object.

smog     A kind of pollution that develops when chemicals react in the air. The word comes from a blend of “smoke” and “fog,” and was coined to describe pollution from burning fossil fuels on cold, damp days. Another kind of smog, which usually looks brown, develops when pollutants from cars react with sunlight in the atmosphere on hot days.

soot     Also known as black carbon, it's the sometimes oily residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.

supercooled     An adjective for a liquid or a gas that has been slowly cooled to below its freezing point without it becoming a solid.

unique     Something that is unlike anything else; the only one of its kind.

volatile     Chemical that easily evaporates.

Citation

Book: R. R. Rogers and M. K. Yau.  A short course in cloud physics. American Meteorological Society. June 3, 2009. ISSN 0731-8871.

Website: SKYbrary. Supercooled water droplets.

Website: American Physics Society. Physics when it’s cold outside: Snowflake symmetry. Physics Central.

News article: W. Broad. “Snowflakes as big as frisbees?The New York Times. March 20, 2007.

News article: M. Cappucci and I. Livingston. “Wintry precipitation types can be confusing. Here’s what they are and how they form.The Washington Post. November 14, 2018.