Snow and Ice

Snow is frozen water that falls from clouds as ice crystals or flakes. It is one of the most beautiful kinds of precipitation and one of the most powerful forces in shaping the landscape. Roughly 10% of Earth's land surface is permanently covered in snow or ice, including Antarctica, Greenland, the Arctic, and the high peaks of major mountain ranges. Ice in glaciers and polar caps holds approximately 70% of the world's fresh water. As the climate warms, what is happening to all this snow and ice is one of the most important questions in modern science.

  • Snowflake symmetrySix-sidedAlways, from water's crystal structure
  • Largest snowflake everApproximately 38 cmReported in Montana, 1887
  • % of Earth's land coveredApproximately 10% permanentlyPlus more seasonal snow
  • % of fresh water in iceApproximately 70%Mostly Antarctica and Greenland
  • Greenland ice melts per yearApproximately 250 billion tonnesAdding to sea level rise
  • Glacier movementFew cm to several m per dayDepends on slope and ice thickness

How snowflakes form

Snowflakes form in cold clouds, where water vapour turns directly into ice without going through the liquid stage. Each snowflake starts with a tiny ice crystal that grows around a speck of dust. As the crystal grows, water vapour deposits onto its six points, producing the famous six-sided pattern.

The exact shape depends on the temperature and humidity at every moment of the snowflake's growth. Different conditions produce different shapes: needles, plates, hollow columns, branched stars. Each snowflake takes a slightly different path through the cloud, encountering different conditions, so each ends up with its own unique pattern.

Why every snowflake is unique

The saying that "no two snowflakes are alike" is, in practice, true. A typical snowflake contains around 1018 water molecules, each arranged in a slightly different position. The number of possible arrangements is so vast that no two snowflakes have ever been found to be exactly identical, even though trillions of them form in every snowstorm.

The pattern is always six-sided because of the crystal structure of ice. Water molecules naturally arrange themselves in a hexagonal lattice when frozen. Each new water molecule joining the crystal slots into a position that maintains the six-fold symmetry. The result is a vast variety of shapes that all share the same underlying six-sided pattern.

Other forms of frozen precipitation

  • Sleet: rain that freezes into small ice pellets as it falls through a layer of cold air below the cloud.
  • Hail: ice balls that form in thunderstorms when raindrops are carried up and down through freezing layers, adding more ice each time.
  • Freezing rain: rain that stays liquid until it touches a freezing surface, then instantly freezes into a thick layer of ice. Dangerous because it makes everything slippery.
  • Graupel: small soft pellets of icy snow, halfway between snowflakes and hail.
  • Frost: water vapour that freezes directly onto cold surfaces (windows, leaves, cars) without ever being a cloud.

Glaciers and ice sheets

When snow falls and does not melt year after year, it gradually compacts into ice, forming glaciers in mountain valleys and huge continent-wide ice sheets in Greenland and Antarctica.

Glaciers are not static; they flow slowly downhill under their own weight, like a very slow-moving river of ice. Speeds vary from a few centimetres per day in cold dry conditions to several metres per day in fast-moving outlet glaciers. As they flow, glaciers scrape away rock and soil, carving deep U-shaped valleys, smoothing mountain ridges and depositing piles of debris where they end.

The largest single chunk of ice on Earth is the Antarctic Ice Sheet, which covers approximately 14 million square kilometres (about the size of the United States and Mexico combined) and is up to 4.8 km thick in places. The Greenland Ice Sheet covers about 1.7 million square kilometres and is up to 3.4 km thick.

Fact If all the ice in Antarctica melted, global sea levels would rise by approximately 58 metres: enough to drown London, New York, Shanghai and most of the world's coastal cities. If all the ice in Greenland melted, sea levels would rise by another 7 metres. The good news is that complete melting would take many thousands of years, but even partial melting is already raising sea levels by approximately 3.5 mm per year and accelerating.

Avalanches: the danger of snow

An avalanche is a fast-moving mass of snow sliding down a mountainside. Avalanches typically happen on steep slopes after fresh heavy snow, when the new snow has not yet bonded to the snow below. A trigger (a skier, a falling rock, a loud noise) can set off a slab of snow that quickly turns into an unstoppable wave.

Large avalanches can reach speeds of over 300 km/h and contain millions of tonnes of snow and debris. They are one of the major causes of death in mountainous areas. Modern ski resorts use controlled explosives to deliberately trigger small avalanches before they can build up into dangerous large ones.

Did you know? Inuit and other Arctic peoples have many separate words for different kinds of snow, although the famous claim that they have "100 words for snow" is exaggerated. They do, however, recognise different types of snow with high precision (powder snow, packed snow, melted-and-refrozen snow, etc.), each with its own name. This makes practical sense for people whose lives depend on understanding the snow around them.
Deeper dive: why melting Arctic ice does not raise sea levels (but Antarctic ice does)

One of the most common confusions about climate change is the role of melting ice in rising sea levels. The answer depends on where the ice is.

The Arctic Sea Ice floats on the Arctic Ocean. Like an ice cube floating in a glass of water, it is already displacing its own weight in water. When it melts, the resulting water takes up almost exactly the same volume as the ice did, so the sea level does not rise.

The Antarctic and Greenland Ice Sheets, on the other hand, sit on land. When this ice melts and flows into the sea, it adds new water to the ocean. This is why those ice sheets matter so much for sea level. Antarctica alone holds enough ice to raise global sea levels by 58 metres if it all melted. Greenland holds another 7 metres.

Other contributors to rising sea levels include:

  • Melting mountain glaciers: smaller but still significant. All the world's mountain glaciers combined hold enough ice to raise sea levels by about 0.5 metres.
  • Thermal expansion: warmer water takes up more space. This currently accounts for about a third of all sea level rise.
  • Changes in stored water: humans pumping out groundwater for irrigation and drinking has measurably added to sea level rise over the last century.

Currently, sea levels are rising at approximately 3.5 mm per year, and the rate is accelerating. By 2100, projections suggest a rise of 30 to 100 cm depending on how much we reduce greenhouse gas emissions. That may not sound like much, but combined with storm surges it would put many coastal cities at risk.

For more, see rain and the water cycle.