Avalanche

A powder snow avalanche in the Himalayas near Mount Everest.
Heavy equipment in action after an avalanche has interrupted service on the Saint-Gervais–Vallorcine railway in Haute-Savoie, France (2006).
The terminus of an avalanche in Alaska's Kenai Fjords.
Loose snow avalanches (far left) and slab avalanches (near center) near Mount Shuksan in the North Cascades mountains. Fracture propagation is relatively limited.
15 cm deep, soft slab avalanche triggered by a snowboarder near Heliotrope Ridge, Mount Baker in March 2010. Multiple crown fracture lines are visible in the top-middle of the image. Note the granular characteristic of the debris in the foreground that results from the slab breaking up during descent.
Avalanche on Simplon Pass (2019)
In steep avalanche-prone terrain, traveling on ridges is generally safer than traversing the slopes.
A cornice of snow about to fall. Cracks in the snow are visible in area (1). Area (3) fell soon after this picture was taken, leaving area (2) as the new edge.
Avalanche path with 800 m vertical fall in the Glacier Peak Wilderness, Washington state. Avalanche paths in alpine terrain may be poorly defined because of limited vegetation. Below tree line, avalanche paths are often delineated by vegetative trim lines created by past avalanches. The start zone is visible near the top of the image, the track is in the middle of the image and clearly denoted by vegetative trimlines, and the runout zone is shown at the bottom of the image. One possible timeline is as follows: an avalanche forms in the start zone near the ridge, and then descends the track, until coming to rest in the runout zone.
After surface hoarfrost becomes buried by later snowfall, the buried hoar layer can be a weak layer upon which upper layers can slide.
After digging a snow pit, it is possible to evaluate the snowpack for unstable layers. In this picture, snow from a weak layer has been easily scraped away by hand, leaving a horizontal line in the wall of the pit.
United States Forest Service avalanche danger advisories.
Snow fences in Switzerland during summer.
Avalanche blasting in French ski resort Tignes (3,600 m)
Avalanche warning sign near Banff, Alberta
Radar station for avalanche monitoring in Zermatt.
frameless

Rapid flow of snow down a slope, such as a hill or mountain.

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Mudflow

Form of mass wasting involving "very rapid to extremely rapid surging flow" of debris that has become partially or fully liquified by the addition of significant amounts of water to the source material.

Mailboxes caught in a mudflow following the May 1980 Mount St. Helens volcanic eruption.
The Mameyes mudflow disaster, in barrio Tibes, Ponce, Puerto Rico, was caused by heavy rainfall from Tropical Storm Isabel in 1985. The mudflow destroyed more than 100 homes and claimed an estimated 300 lives.

Some broad mudflows are rather viscous and therefore slow; others begin very quickly and continue like an avalanche.

Avalanche control

An avalanche snow bridge near a ski-resort in Vorarlberg.
Avalanche blasting in the French ski resort of Tignes (3,600 m)
Gazex installation
U.S. Forest Service team using a 106mm recoilless rifle for avalanche control at Mammoth Mountain in the Inyo National Forest.
Forest and structures to protect against avalanches.
Trains passing in an avalanche gallery on the Wengernalpbahn in Switzerland.
Snow bridges in Switzerland.

Avalanche control or avalanche defense activities reduce the hazard avalanches pose to human life, activity, and property.

Natural hazard

Natural phenomenon that might have a negative effect on humans and other animals, or the environment.

San Francisco was devastated by an earthquake in 1906
Puʻu ʻŌʻō
Young steer after a blizzard, March 1966
Hurricane Katrina

There are 18 natural hazards included in the National Risk Index of FEMA: avalanche, coastal flooding, cold wave, drought, earthquake, hail, heat wave, hurricane (tropical cyclone), ice storm, landslide, lightning, riverine flooding, strong wind, tornado, tsunami, volcanic activity, wildfire, winter weather.

Landslide

Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows.

A landslide near Cusco, Peru, in 2018
The Mameyes Landslide, in the Mameyes neighborhood of barrio Portugués Urbano in Ponce, Puerto Rico, was caused by extensive accumulation of rains and, according to some sources, lightning. It buried more than 100 homes.
The landslide at Surte in Sweden, 1950. It was a quick clay slide that killed one person.
The Costa della Gaveta earthflow in Potenza, Italy. Even though it moves at a rate of just a few millimeters per year and is hardly visible, this landslide causes progressive damage to the national road, the national highway, a flyover, and several houses that are built on it.
A rock slide in Guerrero, Mexico
Hotel Panorama at Lake Garda. Part of a hill of Devonian shale was removed to make the road, forming a dip-slope. The upper block detached along a bedding plane and is sliding down the hill, forming a jumbled pile of rock at the toe of the slide.
Deep-seated landslide on a mountain in Sehara, Kihō, Japan caused by torrential rain of Tropical Storm Talas
Landslide of soil and regolith in Pakistan
A Wireline extensometer monitoring slope displacement and transmitting data remotely via radio or Wi-Fi. In situ or strategically deployed extensometers may be used to provide early warning of a potential landslide.
Rhine cutting through Flims Rockslide debris, Switzerland
Global landslide risks
Ferguson Slide on California State Route 140 in June 2006
Trackside rock slide detector on the UPRR Sierra grade near Colfax, CA
Before and after radar images of a landslide on Venus. In the center of the image on the right, the new landslide, a bright, flow-like area, can be seen extending to the left of a bright fracture. 1990 image.
Landslide in progress on Mars, 2008-02-19

An avalanche, similar in mechanism to a landslide, involves a large amount of ice, snow and rock falling quickly down the side of a mountain.

Snowpack

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

Digging a snowpit on Taku Glacier, in Alaska to measure snowpack depth and density
Graph of changing Wyoming Snowpack

Assessing the formation and stability of snowpacks is important in the study and prediction of avalanches.

Rockslide

Type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses en masse and not in individual blocks.

Rockslide at Oddicombe Beach in Devon, UK

Fast-flowing rock slides or debris slides behave similarly to snow avalanches, and are often referred to as rock avalanches or debris avalanches.

Snow

Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.

Norwegian train plowing through drifted snow
Extratropical cyclonic snowstorm, February 24, 2007—(Click for animation.)
Frontal snowsquall moving toward Boston, Massachusetts
Cold northwesterly wind over Lake Superior and Lake Michigan creating lake-effect snowfall
Freshly fallen snowflakes
An early classification of snowflakes by Israel Perkins Warren
An animation of seasonal snow changes, based on satellite imagery
New York City during a 2016 blizzard, which
 produced local wind gusts up to 42 mph and dropped 27.5 in of snow, breaking the city's one-day snowfall record.
Snow-covered trees in Kuusamo, Finland
Fresh snow beginning to metamorphose: The surface shows wind packing and sastrugi. In the foreground are hoar frost crystals, formed by refrozen water vapor emerging to the cold surface.
Firn—metamorphosed multi-year snow
Snow drifts forming around downwind obstructions
A powder snow avalanche
Snowmelt-induced flooding of the Red River of the North in 1997
Snow pit on the surface of a glacier, profiling snow properties where the snow becomes increasingly dense with depth as it metamorphoses towards ice
Snowfall and snowmelt are parts of the Earth's water cycle.
Traffic stranded in a 2011 Chicago snowstorm.
Winter conditions on Ontario Highway 401 in Toronto due to a snowsquall.
Deicing an aircraft during a snow event
Satellite view of the Indus River, showing snow in the Himalayas, which feeds it, and agricultural areas in Pakistan that draw on it for irrigation.
Extreme snow accumulation on building roofs
Icings resulting from meltwater at the bottom of the snow pack on the roof, flowing and refreezing at the eave as icicles and from leaking into the wall via an ice dam.
Alpine skiing.
Algae, Chlamydomonas nivalis, that thrive in snow form red areas in the suncups on this snow surface
Arctic fox, a predator of smaller animals that live beneath the snow
Trucks plowing snow on a highway in Missouri
Airport snow-clearing operations include plowing and brushing
Swiss low-profile, train-mounted snowplow
Bivouac of Napoleon's Grande Armée, during the winter retreat from Moscow
Finnish ski troops during the invasion of Finland by the Soviet Union
Army vehicles coping with snow during the Battle of the Bulge of World War II.
Norwegian military preparations during the 2009 Cold Response exercise
Navy SEALs training for winter warfare at Mammoth Mountain, California.

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

Ski resort

Resort developed for skiing, snowboarding, and other winter sports.

Ski resorts in the world by country
Map of world ski resorts (interactive map)
Mzaar Kfardebian Ski Resort in Lebanon
This image of Zauchensee, Austria, shows the pistes, served by a gondola lift, detachable chairlift and a funicular. There is a snow fence to prevent snowdrift; and avalanche towers and avalanche barriers to mitigate the risk of avalanches
Jasná ski resort in Slovakia
Cerro Catedral Ski Resort, Argentina
Ski resorts can also be situated on a volcano like this one on Etna in Sicily
Gambarie, a ski resort above the Strait of Messina
Ruka, Finland
Valle Nevado, Chile
Małe Ciche, Poland
Blue Cow, Perisher ski resort, Australia
Sierra Nevada Ski Station, Spain
Shahdag Mountain Resort, Azerbaijan
Tsakhkadzor, Armenia
Killington Ski Resort, Vermont, United States
Winter Park Resort, Colorado, United States

With enough excess water, the likelihood of landslides and avalanches may be drastically higher.

Powder snow avalanche

Example of powder snow avalanche

A powder snow avalanche is a type of avalanche where snow grains are largely or completely suspended and moved by air in a state of fluid turbulence.

Angle of repose

Steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping.

Angle of repose
Talus cones on north shore of Isfjord, Svalbard, Norway, showing angle of repose for coarse sediment
This pile of corn has a low angle of repose
Sand pit trap of the antlion

It is also commonly used by mountaineers as a factor in analysing avalanche danger in mountainous areas.