Avalanche Types
Understanding the four main types of avalanches and how they form is fundamental to backcountry safety. Each type has different characteristics, triggers, and avoidance strategies.
Slab Avalanches
Key Characteristics
- •Most dangerous and deadly type
- •Cohesive layer of snow breaks as a unit
- •Clear fracture line at the crown
- •Can release suddenly and propagate quickly
- •Often triggered by a person or external force
How It Forms
A cohesive slab of snow forms over a weak layer. When stress exceeds strength, the slab fractures and slides as a unit. The weak layer acts as a sliding surface.
Typical Conditions
- ▸Wind-loaded slopes
- ▸Recent storm snow
- ▸Temperature gradients creating weak layers
- ▸Buried surface hoar or faceted snow
- ▸Rain crust with new snow on top
Where They Occur
Slopes 30-45°, all aspects depending on problem type, commonly on lee slopes (downwind)
⚠️ Danger Signs
- !Recent avalanche activity
- !Whumpfing sounds (collapse)
- !Shooting cracks
- !Recent wind or snow loading
- !Unstable snow test results
✓ Avoidance Strategies
- ✓Avoid slopes steeper than 30° when slab problems exist
- ✓Choose windward slopes over lee slopes
- ✓Avoid convex rollovers (stress points)
- ✓Space party widely when crossing suspect terrain
- ✓One person at a time in avalanche terrain
Loose Snow Avalanches
Key Characteristics
- •Start at a point and fan out downhill
- •No distinct fracture line
- •Snow lacks cohesion
- •Generally smaller and less dangerous than slabs
- •Can be dry or wet
How It Forms
Individual snow grains lose cohesion and begin sliding, entraining more snow as they descend. Forms a fan or inverted "V" shape.
Typical Conditions
- ▸Fresh, non-cohesive snow
- ▸Very cold, faceted snow (dry loose)
- ▸Warming or rain (wet loose)
- ▸Steep, smooth surfaces
- ▸Solar radiation on sun-exposed slopes
Where They Occur
Slopes steeper than 35°, often on very steep terrain (40°+), any aspect depending on trigger
⚠️ Danger Signs
- !Rollerballs forming naturally
- !Pinwheels of snow rolling down
- !Warming temperatures or sun exposure
- !Rain or heavy wet snow
- !Fresh powder on smooth surfaces
✓ Avoidance Strategies
- ✓Avoid steep terrain during warming
- ✓Get off sun-exposed slopes before afternoon
- ✓Watch for terrain traps below
- ✓Be cautious after fresh snow on smooth surfaces
- ✓Consider runout zones and consequences
Wet Avalanches
Key Characteristics
- •Snow saturated with water
- •Heavy and slow-moving
- •Very dense debris
- •Can be loose or slab type
- •Predictable timing (diurnal warming)
How It Forms
Water infiltrates the snowpack, reducing strength and cohesion. Weight increases, bonds weaken, and snow loses ability to support itself.
Typical Conditions
- ▸Spring conditions with warming
- ▸Rain on snow events
- ▸Prolonged sunshine
- ▸Temperatures above freezing
- ▸Wet, heavy new snow
Where They Occur
Any slope angle steep enough to slide, often start spontaneously, sun-exposed and south-facing aspects most common
⚠️ Danger Signs
- !Pinwheels and rollerballs
- !Free water in snow
- !Wet loose slides starting
- !Temperatures above freezing overnight
- !Rain or heavy wet snow
- !Deep, heavy snow penetration when walking
✓ Avoidance Strategies
- ✓Early morning starts (frozen snow)
- ✓Descend before warming occurs
- ✓Watch for signs of water saturation
- ✓Avoid afternoon exposure on sunny aspects
- ✓Get off steep terrain when warming begins
- ✓Know the freeze-thaw cycle
Glide Avalanches
Key Characteristics
- •Entire snowpack slides on ground
- •Very slow to sudden release
- •Glide cracks visible days before
- •Full-depth avalanche
- •Unpredictable timing
How It Forms
Water lubricates the ground surface beneath the snowpack. Entire snow column slowly creeps downhill until it loses adhesion and releases as a full-depth avalanche.
Typical Conditions
- ▸Smooth ground surface (grass, rock slabs)
- ▸Water at snow-ground interface
- ▸Warm ground temperatures
- ▸Deep snowpack
- ▸Spring or warm winter periods
Where They Occur
Smooth, grassy slopes and rock slabs, any aspect, often below treeline, slopes 30-45°
⚠️ Danger Signs
- !Glide cracks (obvious openings in snow)
- !Creeping snow (slow movement visible)
- !Recent glide avalanche activity
- !Smooth ground surface visible
- !Warm temperatures with deep snow
✓ Avoidance Strategies
- ✓AVOID ALL SLOPES WITH GLIDE CRACKS
- ✓Give glide cracks a wide berth (100+ feet)
- ✓Cannot predict when they will release
- ✓Stay off terrain below glide cracks
- ✓No safe way to test or manage glide risk
Key Takeaways
Slab Avalanches = Most Deadly
Responsible for 90% of avalanche fatalities. Always take slab problems seriously and heed forecaster warnings.
Loose Snow = Common but Manageable
Generally smaller and more predictable. Watch for warming trends and avoid steep terrain when signs appear.
Wet Avalanches = Timing is Everything
Predictable based on temperature cycles. Early starts and knowing when to turn around are key.
Glide Avalanches = Unpredictable
Cannot be predicted or managed. Avoid all slopes with glide cracks—no exceptions.