An avalanche is a rapid flow of snow, ice, and debris down a sloping surface. The duration of an avalanche, from its initial fracture to the moment the snow settles, is not a fixed measurement but is highly variable depending on the characteristics of the terrain and the snow itself. Understanding the timeline requires breaking down the event into the brief period of active movement and the far longer period of its immediate aftermath.
The Active Slide: Typical Duration of Movement
The active phase of an avalanche, the time the snow is actually in motion, is surprisingly short, often lasting less than a minute. For smaller, localized slides, the duration can be as brief as a few seconds, with the snow rapidly losing momentum once it hits a flatter area. Larger avalanches, particularly those with a significant vertical drop and long runout, may remain in motion for one to three minutes.
Dry slab avalanches, the most common and dangerous type, accelerate extremely quickly. These slides can reach speeds of 60 to 80 miles per hour within approximately five seconds of fracturing. This rapid acceleration means that escape is nearly impossible once the slide begins, as the duration of the slide is a function of the speed attained and the distance traveled before friction halts the mass.
Factors Determining Avalanche Speed and Runout
The variability in avalanche duration is governed by the interaction of terrain and snow conditions. The slope angle is a major influence; starting zones typically occur on slopes between 30 and 45 degrees, allowing for rapid acceleration. As the avalanche moves into the track, the slope angle often decreases to 20 to 30 degrees, and the snow continues to travel until it reaches the runout zone, defined by terrain less than 20 degrees.
The type of snow involved dictates the flow dynamics and the slide’s velocity. Dry powder avalanches, characterized by a cloud of suspended snow particles, move at high speeds, sometimes exceeding 100 miles per hour, which allows them to travel extensive distances. Wet snow avalanches, in contrast, are much denser due to water saturation and travel significantly slower, averaging around 20 miles per hour.
The overall volume of the snow mass also directly influences both speed and runout distance. Larger avalanches generate greater momentum, allowing them to overcome friction for a longer period and travel further across gentler terrain. The vertical drop of the path and the average gradient determine the overall time the snow remains in transit. The path’s configuration, such as whether it is channeled or open, also plays a role in how far and how long the snow flows before coming to rest.
Post-Movement Hazards and Survival Window
Although the active slide is brief, the duration of the resulting hazard extends far beyond the moment the snow stops. Once the moving snow mass settles, it undergoes rapid consolidation, quickly setting into a dense, concrete-like state within minutes. This rapid setting makes it incredibly difficult for a buried victim to self-extricate.
The most critical time measurement is the survival window for a person caught and buried in the debris. Statistical analysis shows a sharp decline in survival probability after the first few minutes of burial. Recent studies indicate that the phase where survival probability is greater than 90 percent lasts only about 10 to 18 minutes after the snow stops moving.
After this short period, the survival rate drops precipitously, largely due to asphyxiation as the victim exhales carbon dioxide into the limited air space. The chances of survival are three times higher when a victim is rescued by companions than by organized rescue, highlighting the importance of immediate action.