An avalanche is a mass of snow sliding, tumbling, or flowing down an inclined surface. Slab avalanches can be particularly powerful and destructive due to the speed and force of the mass of snow and the rush of air that sometimes precedes the avalanche.  For a slab avalanches to occur, these four conditions are necessary:

• A sufficiently steep slope (most avalanches occur on slopes between 30 and 45 degrees)
• A weak layer in the snowpack
• A slab of snow above that weak layer
• A trigger initiates the fracture and collapse of the weak layer.  Triggers can be the accumulation of snow, rain, wind loading, or human/animal disturbance

An avalanche occurs when the stress (from gravity) trying to pull the snow downhill exceeds the strength (from bonds between snow grains) of the snow cover. Four conditions must be present for an avalanche to occur:

• a steep slope,
• snow cover,
• a weak layer in the snow cover, and
• a trigger.

Avalanche danger increases with major snowstorms and periods of thaw. About 2,000 avalanches are reported to the U.S. Forest Service Avalanche Center in an average winter. More than 80 percent of these occur during or just after large snowstorms. The most avalanche-prone months are, in order, February, March, and January. Avalanches caused by thaw occur most often in April.

Vulnerable Areas

About 90 percent of all avalanches start on slopes of 30-45 degrees; about 98 percent of all avalanches occur on slopes of 25-50 degrees. Avalanches release most often on slopes above timberline that face away from prevailing winds. This is because leeward slopes collect snow blowing from the windward sides of ridges. Avalanches can occur, however, on small slopes well below timberline, such as gullies, road cuts and small openings in the trees. Very dense trees can anchor the snow to steep slopes and prevent avalanches from starting, however, avalanches can release and travel through a moderately dense forest. Most avalanches occur in the backcountry, outside of developed ski areas.

How to Minimize Risk

You can reliably avoid avalanches by recognizing and avoiding avalanche terrain. Travel on the valley floor away from large avalanche runouts, along ridgetops above avalanche paths, in dense timber, or on slopes of 25 degrees or less that do not have steeper slopes above them. Avoid cornices, or hanging masses of hardened snow.

You cannot entirely eliminate risk if you travel in avalanche terrain, but you can minimize risk by using good technique: climb, descend, or cross avalanche areas one at a time; cross a slope at the very top or bottom if possible; climb or descend the edge of a slope rather than the center; carry and know how to use avalanche rescue gear; and turn back or alter your route if you detect signs of unstable snow.

Signs of Danger

When the snow cover is very unstable, nature often broadcasts clear danger signals. Fresh avalanches are the best clue. Snow that cracks, collapses, or makes hollow sounds is also unstable. Weak layers that are found by digging snow pits are signs of unstable snow. Snow that has become wet from thaw or rain can be dangerous.

Even if you find no signs of unstable snow, you should always travel observing the techniques listed above to minimize risk.

An avalanche is a large amount of snow moving quickly down a mountain, typically on slopes of 30 to 45 degrees. When an avalanche stops, the snow becomes solid like concrete and people are unable to dig out. People caught in avalanches can die from suffocation, trauma or hypothermia.

Avalanches can:

How to Protect Yourself from an Avalanche

The most important actions you can take to survive an avalanche are done before it happens.

Know Your Avalanche Risk
Learn about your local avalanche risk. Know the signs of increased danger, including recent avalanches and shooting cracks across slopes. Avoid areas of increased risk, such as slopes steeper than 30 degrees or areas under steep slopes. Get training on how to recognize hazardous conditions and avalanche-prone locations. Sign up for alerts from a U.S. Forest Service Avalanche Center near you. Your community may also have a local warning system.

Preparing for Avalanche
Get proper equipment to protect yourself from head injuries and create air pockets. Receive first aid training so you can recognize and treat suffocation, hypothermia, traumatic injury and shock. Wear a helmet to help reduce head injuries and create air pockets. Wear an avalanche beacon to help rescuers locate you. Use an avalanche airbag that may help you from being completely buried. Carry a collapsible avalanche probe and a small shovel to help rescue others.

Consider Overlapping Hazards: Coronavirus Disease 2019 (COVID-19) Remember, there is no evidence that cold weather and snow can kill the coronavirus disease. Be sure to have several clean masks to use in case your mask becomes wet or damp from snow. Cloth masks should not be worn when they become damp or wet. Be sure to wash your mask regularly.

Masks may make it difficult to breathe, especially for those engaging in high intensity activities. Remove your mask if you are having difficulty breathing. If you are unable to wear a mask, maintain a distance of at least six feet between yourself and others.

Signs of Avalanche
Learn the signs of an avalanche, and how to use safety and rescue equipment.

Follow avalanche warnings on roads. Roads may be closed, or vehicles may be advised not to stop on the roadside.

Stay Safe During an Avalanche

• Use and carry safety equipment and rescue gear.
• If your partner or others are buried, call 9-1-1 and then begin to search if it is safe to do so.
• If you have the proper training, treat others for suffocation, hypothermia, traumatic injury or shock.

Returning After an Avalanche

Know the signs and ways to treat hypothermia.

• Hypothermia is an unusually low body temperature. A body temperature below 95 degrees is an emergency.
  • Signs: Shivering, exhaustion, confusion, fumbling hands, memory loss, slurred speech and drowsiness.
  • Actions: Go to a warm room or shelter. Warm the center of the body first—chest, neck, head and groin. Keep the person dry and wrapped up in warm blankets, including the head and neck.
• Engage virtually with your community through video and phone calls. Know that it’s normal to feel anxious or stressed. Take care of your body and talk to someone if you are feeling upset.

An avalanche is a rapid flow of snow down a hill or mountainside. Although avalanches can occur on any steep slope given the right conditions, certain times of the year and types of locations are naturally more dangerous. While avalanches are sudden, there are typically a number of warning signs you can look for or feel before one occurs. In 90 percent of avalanche incidents, the snow slides are triggered by the victim or someone in the victim's party. Avalanches kill more than 150 people worldwide each year. The National Weather Service provides current weather conditions and forecast information to regional avalanche forecast centers that in-turn issue avalanche forecasts. Avalanche warnings and special advisories are included on NWS websites and broadcast over NOAA Weather Radio. Refer to your local avalanche center for current snowpack conditions!

How to Stay Safe

Get Training.

• Know the three factors required for an avalanche:
  • Slope - Avalanche generally occur on slopes steeper than 30 degrees
  • Snowpack - Recent avalanches, shooting cracks, and “whumpfing” are signs of unstable snow.
  • Trigger - Sometimes it doesn’t take much to tip the balance; people, new snow, and wind are common triggers.
• Determine if you are on or below slopes that can avalanche
• Find out if the snow is stable
• Get the Advisory: Refer to your local avalanche center for current snowpack conditions!
• Get the gear...and learn how to use it! Have these three avalanche safety essentials in your pack. :
  • Transceiver: so you can be found if covered by the snow
  • Shovel: so you can dig out your partner
  • Probe: so you can locate someone who has been covered by the snow

Avalanche survival rates plummet after about 15 minutes for victims who do not die from trauma. Saving your partner is up to you! Practicing realistic scenarios beforehand is essential.

Avalanche Warning Signs

The following are a few of the warning signs of unstable snow and possible avalanches:

• You see an avalanche happen or see evidence of previous slides.
• Cracks form in the snow around your feet or skis.
• The ground feels hollow underfoot.
• You hear a "whumping" sound as you walk, which indicates that the snow is settling and a slab might release.
• Heavy snowfall or rain in the past 24 hours
• Significant warming or rapidly increasing temperatures
• You see surface patterns on the snow made by the force of strong winds. This could indicate that snow has been transported and deposited in dangerous drifts that could release.