Snow

Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals.

A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, then falls through the Earth's atmosphere as snow.

As snow accumulates into a snowpack, it may blow into drifts.

Snowpack forms from layers of snow that accumulate in geographic regions and high altitudes where the climate includes cold weather for extended periods during the year.

Snow affects such human activities as transportation: creating the need for keeping roadways, wings, and windows clear; agriculture: providing water to crops and safeguarding livestock; sports such as skiing, snowboarding, and snowmachine travel; and warfare.

Skiing can be a means of transport, a recreational activity or a competitive winter sport in which the participant uses skis to glide on snow.

Mid-latitude cyclones are low pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards. Snow flurry, snow storm and blizzard describe snow events of progressively greater duration and intensity.

In the United States, the National Weather Service defines a blizzard as a severe snow storm characterized by strong winds causing blowing snow that results in low visibilities.

A cold front, the leading edge of a cooler mass of air, can produce frontal snowsqualls—an intense frontal convective line (similar to a rainband), when temperature is near freezing at the surface.

A snowsquall (or snow squall) is a sudden moderately heavy snow fall with blowing snow and strong, gusty surface winds.

In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow.

Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is an unusual kind of thunderstorm with snow falling as the primary precipitation instead of rain.

The areas affected by lake-effect snow are called snowbelts.

Snowbelt is a term describing a number of regions near the Great Lakes in North America where heavy snowfall in the form of lake-effect snow is particularly common.

It pertains to frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.

Snow and ice sublime, although more slowly, at temperatures below the freezing/melting point temperature line at 0 °C for most pressures; see line below triple point.

Mid-latitude cyclones are low pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards. Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air. Snow flurry, snow storm and blizzard describe snow events of progressively greater duration and intensity.

A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form (i.e. freezing rain).

Micrography of thousands of snowflakes from 1885 onward, starting with Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within a classifiable set of patterns.

He first became interested in snow crystals as a teenager on his family farm.

A cold front, the leading edge of a cooler mass of air, can produce frontal snowsqualls—an intense frontal convective line (similar to a rainband), when temperature is near freezing at the surface.

Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain or snow.

Where the climate is cold enough for year-to-year accumulation, a glacier may form.

A glacier ( or ) is a persistent body of dense ice that is constantly moving under its own weight; it forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries.

Snow that persists into summer evolves into névé, granular snow, which has been partially melted, refrozen and compacted.

Névé is a young, granular type of snow which has been partially melted, refrozen and compacted, yet precedes the form of ice.

A sub-specialty is avalanches, which are of concern to engineers and outdoors sports people, alike.

An avalanche (also called a snowslide) is a cohesive slab of snow lying upon a weaker layer of snow in the snowpack that fractures and slides down a steep slope when triggered.

The International Classification for Seasonal Snow on the Ground defines "height of new snow" as the depth of freshly fallen snow, in centimeters as measured with a ruler, that accumulated on a snowboard during an observation period of 24 hours, or other observation interval.

A snowboard (US) or Weaverboard (Canada) is a meteorological tool used to aid in the obtaining of accurate measurement of snow accumulation.

Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime.

Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through atmospheric supercooled cloud droplets.

At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge.

Ultrasonic snow depth sensor for measuring depth of snow

After deposition, snow progresses on one of two paths that determine its fate, either ablation (mostly by melting) or transitioning from firn (multi-year snow) into glacier ice.

Firn (from Swiss German firn "last year's", cognate with before) is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé.

This can be improved with the use of snow tires, which have a tread designed to compact snow in a manner that enhances traction.

Snow tires—also called winter tires—are tires designed for use on snow and ice.

In the Southern Hemisphere, snow is confined primarily to mountainous areas, apart from Antarctica.

Glaciologists in Antarctica are concerned with the study of the history and dynamics of floating ice, seasonal snow, glaciers, and ice sheets.

Common examples include cross-country skiing, Alpine skiing, snowboarding, snowshoeing, and snowmobiling.

A snowshoe is footwear for walking over snow.

Common examples include cross-country skiing, Alpine skiing, snowboarding, snowshoeing, and snowmobiling.

Alpine skiing, or downhill skiing, is the pastime of sliding down snow-covered slopes on skis with fixed-heel bindings, unlike other types of skiing (cross-country, Telemark, or ski jumping), which use skis with free-heel bindings.

Snow flurry, snow storm and blizzard describe snow events of progressively greater duration and intensity.

Snow shower

Orographic or relief snowfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains.

As the altitude increases, the main form of precipitation becomes snow and the winds increase.

Snow science often leads to predictive models that include snow deposition, snow melt, and snow hydrology—elements of the Earth's water cycle—which help describe global climate change.

Precipitation: Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. Approximately 505000 km3 of water falls as precipitation each year, 398000 km3 of it over the oceans. The rain on land contains 107000 km3 of water per year and a snowing only 1000 km3. 78% of global precipitation occurs over the ocean.