Snow in bands

The snow falls from long, narrow bands of cloud, sometimes only 10 to 20 km wide. Just one town can get 1 metre while a place 30 km away gets none, and thundersnow, thunder and lightning in a snow band, is not unusual.

When the lakes freeze

The season runs from late autumn, when the lakes hold the most summer heat, until the lakes ice over or the air warms. Warm autumns and less lake ice can lengthen the season, and many scientists expect lake-effect snow to increase for a while in a warming climate before eventually turning to rain.

It is not only the Great Lakes

The same process works elsewhere: the Great Salt Lake in Utah, the Sea of Japan (which buries Hokkaido and Japan's west coast), the Black Sea, the Baltic Sea and the Caspian. Over salt water it is often called sea-effect snow.

Record-setting snow

A Tug Hill observer in New York reported 77 inches (1.96 m) of snow in 24 hours in January 1997, a US record. In November 2014 a lake-effect band dropped more than 6 feet of snow on parts of Buffalo in three days, and in December 2022 Buffalo airport recorded about 50 inches in a blizzard that combined lake-effect snow and 70 mph winds.

The poles

Antarctica's interior has an average yearly temperature of around −55 °C (−67 °F) at Vostok Station, where the coldest air temperature ever measured on Earth, −89.2 °C (−128.6 °F), was recorded in 1983. The centre of Greenland's ice sheet averages about −30 °C. It never melts, so every snowfall stays.

High mountains

Temperature falls about 6.5 °C for every 1,000 metres you climb, so above a certain height the air is always below freezing: the permanent snowline. Mount Kilimanjaro is on the equator, and its summit is still capped with ice, as are the tropical Andes of Ecuador and Peru.

But the poles are dry

The coldest places get surprisingly little snow. Very cold air holds almost no water vapour, so the interior of Antarctica receives only a few centimetres of water-equivalent a year, less than many deserts. It is a polar desert. The ice there is thousands of years of snow that never melted.

Can it be too cold to snow?

Not exactly. Snow can fall at any temperature below freezing, and ice crystals even fall from clear skies in the Arctic ("diamond dust"). But the heaviest snowfalls usually occur near −10 to 0 °C, when the air still holds a lot of moisture. In extreme cold the flakes are small and dry.

Snow in surprising places

Snow falls on Hawaii's Mauna Kea and Mauna Loa most winters, on the Atlas Mountains and in the Sahara (the town of Ain Sefra, Algeria, had snow in 2016 and 2018), in the Australian Alps, and occasionally in Florida.

Snow cover of the Earth

Seasonal snow covers roughly 46 million square kilometres of the Northern Hemisphere land in midwinter, about half of all its land, and shrinks to a tiny fraction by late summer. Snow reflects up to 80 to 90 percent of sunlight, helping to keep the planet cool.

From snow to ice

1. Snow piles up in layers each year.
2. Its weight packs the flakes into firn, a grainy old snow.
3. Over decades to thousands of years, firn is squeezed into solid glacier ice, which slowly flows downhill toward the sea.

Antarctica

• About 14 million km² of ice, nearly twice the size of Australia
• Ice up to about 4.8 km thick
• Holds roughly 90% of the world's ice, enough to raise the sea about 58 metres if it all melted
• Ice cores reveal about 800,000 years of climate history

Greenland

• About 1.7 million km² of ice
• Ice more than 3 km thick in places
• Enough water to raise the sea about 7 metres if it all melted
• It is currently losing ice faster than it gains snow

Sea ice at the poles

Sea ice is frozen seawater, and it floats, so its melting does not raise sea level much. Arctic sea ice shrinks every summer: its smallest recorded extent was about 3.4 million km² in September 2012, roughly half of the 1980s average. Antarctic sea ice grows and shrinks dramatically each year.

Ice sheets, ice caps and glaciers

An ice sheet covers more than 50,000 km² (only Greenland and Antarctica). An ice cap is smaller and dome-shaped, as in Iceland and Arctic Canada. A glacier is a river of ice, usually in mountain valleys.

Why it matters for snow

Snowfall feeds the ice and melt and calving remove it. If more is lost than gained, the ice sheet shrinks and the sea rises. Bright snow also reflects sunlight, so when it melts the darker ground or sea absorbs more heat and speeds the warming (the ice-albedo feedback).

Slab avalanches

The most deadly kind. A cohesive layer of snow (the slab) breaks away from a weak layer underneath and slides as a block, often reaching 80 mph (130 km/h) within seconds. The weak layer can be buried surface hoar, faceted snow or old ice crust.

Loose-snow avalanches

Start at a point and fan out as a growing teardrop of snow. They are usually smaller, but can still sweep a skier over a cliff. Wet loose avalanches occur when sun or rain melts the snow.

What causes them

• Heavy new snow or rapid snowfall
• Wind loading: wind builds slabs on the downwind side of ridges
• Rapid warming or rain on snow
• A weak layer deep in the snowpack
• Slope: most slab avalanches release on slopes of about 30° to 45°

The danger scale

North American forecasts rate danger from 1 Low to 5 Extreme. Level 3 Considerable is where most fatalities occur, because conditions look inviting. At levels 4 and 5, avoid avalanche terrain, including the runout zone below slopes.

Staying safe

• Read the avalanche bulletin first (avalanche.org in the US, avalanche.ca in Canada).
• Carry a transceiver, probe and shovel and practise with them; consider an airbag pack and a helmet.
• Cross suspect slopes one at a time, and avoid terrain traps such as gullies.
• Take an avalanche course.

If caught

Try to get away to the side, grab a tree, and fight to stay on the surface ("swim"). As it slows, make an air pocket around your face. Companions must start a search immediately: the chance of survival falls quickly after about 15 minutes buried, and in the US roughly 25 to 30 people die in avalanches each year.

Before you leave

• Check the forecast and road conditions (511 in the US and Canada, or your province's or state's site).
• Clear all the snow and ice from the roof, hood, lights and windows.
• Fill the fuel tank, and tell someone your route and when you will arrive.
• Charge your phone and carry a charger.

Winter car kit

• Blankets, warm clothes, boots, hat and gloves
• Flashlight with spare batteries, and a first-aid kit
• Snow shovel, ice scraper, and sand, salt or cat litter for traction
• Jumper cables, a tow strap, flares or reflective triangles
• Water and non-perishable snacks

How to drive

• Slow down. On ice, stopping distances can be up to ten times longer than on dry roads.
• Accelerate, steer and brake gently and smoothly.
• Leave much more room: stay back from the vehicle ahead and from snowplows, and do not pass a plow.
• Use low beam headlights, and stay in tracks that are already cleared.

If you skid

Stay calm. Take your foot off the accelerator, don't slam the brake, and steer gently in the direction you want the front of the car to go. With anti-lock brakes, press the brake firmly and steadily if you need to stop; do not pump it.

Tyres and drive type

Tyres matter more than drive type. Winter tyres (with the mountain-and-snowflake symbol) give much better grip than all-season tyres. All-wheel drive helps you start moving but does not help you stop or turn on ice.

Black ice and whiteouts

Black ice is a thin, nearly invisible glaze, most likely at around freezing, on bridges, overpasses and shaded corners, which freeze first. In a whiteout or snow squall, slow down, turn on your lights, and if you cannot see, pull well off the road, switch on your hazards and wait.

If you get stuck

• Stay with your vehicle: it is shelter, and it is easier to find than you are.
• Call for help and tell them your location.
• Keep the exhaust pipe clear of snow and crack a window if you run the engine, because carbon monoxide can build up. Run the engine only about ten minutes an hour for heat.
• Move your arms and legs to keep warm, and tie something bright to the antenna.

Snow water equivalent

The water in snow is called its snow water equivalent (SWE). Typical snow is about 10 percent water, so 10 inches of snow melts to about 1 inch of water, but the ratio ranges from about 5:1 for wet, heavy snow to 20:1 or more for cold powder. In the western US the amount of water in the snowpack on April 1 is a key benchmark for the summer water supply.

Too little snow: snow drought

A poor snow year means low rivers, shrunken reservoirs, less groundwater recharge and more wildfire. In 2015 California's snowpack was about 5 percent of normal on April 1, a record low, and the state ordered water restrictions. Wells go dry and farmers pump more groundwater, which makes the water table fall further.

Too much snow

A huge snowpack can melt in days and cause floods, as on the Red River of the North in 1997 and 2009, or deep, wet snow can trigger landslides and avalanches and collapse roofs. High water tables then flood basements and fields for weeks.

Slow melt recharges, fast melt floods

When snow melts gradually, the water has time to soak in and recharge groundwater. When warm rain falls on snow ("rain on snow") or the ground is frozen solid, the water cannot soak in and runs straight off, causing floods and little recharge.

A warming climate

Warmer winters turn more snowfall into rain, shrink the snowpack and make it melt earlier, shifting river peaks earlier in the year. That means less water in late summer when it is needed most, a worry across the western US and for the rivers fed by Himalayan and Alpine snow.

Who relies on snow?

Mountain snow and glaciers feed rivers on which well over a billion people depend, including the Indus, Ganges, Colorado and Rhine. See how rivers carry the water in our Planet River site. For snowpack measurements see the US Natural Resources Conservation Service SNOTEL network and NOAA's National Operational Hydrologic Remote Sensing Center.

How a snowflake forms

A snow crystal grows when water vapour freezes onto a tiny particle of dust or pollen in a cloud below freezing. Its shape depends on temperature and humidity: thin plates and stars form near −15 °C, and needles and columns at other temperatures. Many flakes stick together to make the fluffy clumps we see falling.

Snow, sleet and freezing rain

• Snow: air stays below freezing all the way down
• Sleet: snow melts in a warm layer, then refreezes as ice pellets near the ground
• Freezing rain: rain falls on a freezing surface and glazes it with ice, often the most damaging winter hazard for trees and power lines

What is a blizzard?

In the US a blizzard is a storm with winds of at least 35 mph (56 km/h) and visibility under a quarter mile for at least three hours. A blizzard does not need new snow: wind can lift snow already on the ground. In Canada the wind threshold is 40 km/h with visibility under 400 m for four hours.

Kinds of winter storm

• Nor'easter: a coastal storm over the US east coast
• Alberta clipper: a fast storm from Canada with light snow and bitter cold
• Lake-effect snow (see above)
• Snow squall: a brief, intense burst with whiteout conditions
• Ice storm: heavy freezing rain

Snowfall records

• Greatest snow depth in a season: about 1,140 inches (29 m) at Mount Baker, Washington, in 1998–99
• Most snow in 24 hours (official): about 76 inches at Silver Lake, Colorado, in April 1921
• Snowiest large city: Aomori, Japan, with about 8 metres in a typical winter

Winter safety

• Dress in layers and keep skin covered; frostbite can set in within minutes in strong wind and cold.
• Shovelling snow strains the heart: take breaks and push, don't lift, if you have heart problems.
• Never run a generator or grill indoors: carbon monoxide kills in power cuts.
• Check on elderly neighbours and keep pets indoors.