Avalanche and Snow Guide

Slab Avalanche:  This is the White Death, the Snowy Torrent, the Big Guy in the White Suit. Dry slab avalanches account for nearly all the avalanche deaths in North America. A “slab” is a cohesive plate of snow that slides as a unit on the snow underneath. The bonds holding a slab in place typically fractures at 350 kilometers per hour (220 miles per hour) and it appears to shatter like a pane of glass. It’s typically about the size of half a football field, usually about 30-80 centimeters (1-3 feet) deep and it typically reaches speeds of 30 km/hr (20 mph) within the first 3 seconds and quickly accelerates to around 130 km/hr (80 mph) after the first, say, 6 seconds.

Loose Snow Avalanche: Loose snow sliding down a mountainside is called a loose snow avalanche. Small Loose snow avalanches are called “sluffs”. Loose snow avalanches usually start from a point and fan outward as they descend, and because of this they are also called “point releases.” Very few people are killed by sluffs because they tend to be small and they tend to fracture beneath you as you cross a slope instead of above you as slab avalanches often do.

Cornice Fall Avalanches: Cornices are the fatal attraction of the mountains, their beauty matched only by their danger. Cornices are elegant, cantilevered snow structures formed by wind drifting snow onto the downwind side of an obstacle such as a ridgeline. the weight of a falling cornice often triggers an avalanche on the slope below, or the cornice breaks into hundreds of pieces and forms its own avalanche—or both. Be aware that cornice fragments often “fan out” as they travel downhill, traveling more than 30 degrees off of the fall line. Cornices tend to become unstable during storms, especially with wind, or during times of rapid warming or prolonged melting.

Wet Avalanches: Wet avalanches usually occur when warm air temperatures, sun or rain cause water to percolate through the snowpack and decrease the strength of the snow, or in some cases, change the mechanical properties of the snow. Once initiated, wet snow tends to travel much more slowly than dry snow avalanches–like a thousand concrete trucks dumping their load at once instead of the hovercraft-like movement of a dry avalanche. A typical wet avalanche travels around 15 to 30 km/hr (10 or 20 mph) while a typical dry snow avalanche travels 100 to130 km/hr (60 or 80 mph). Wet slides are also harder for a person to trigger than a dry slide. Because of these two facts, wet avalanches don’t account for nearly as many avalanche fatalities as dry snow avalanches. But they’re certainly not insignificant. They still account for a sizeable percentage of avalanche fatalities in maritime climates, especially to climbers. Wet slides can also do quite a bit of damage to property or to forests and often cause significant hazards on highways.

Glide Avalanches: Glide occurs when the entire snowpack slowly slides as a unit on the ground, similar to a glacier. Don’t mistake glide for the catastrophic release of a slab avalanche that breaks to the ground. Glide is a slow process, that usually occurs over several days. Glide occurs because melt water lubricates the ground and allows the overlying snowpack to slowly “glide” downhill. Usually, they don’t every produce an avalanche but occasionally they release catastrophically as a glide avalanche. So the presence of glide cracks in the snow do not necessarily mean danger. It’s often difficult for a person to trigger a glide avalanche but at the same time it’s not smart to be mucking around on top of them and especially not smart to camp under them.