idea

There are three types of avalanches:

Slab - well bonded snow on top of a weak layer. 95% of skier triggered avalanches, 98% of deaths
Loose snow - loose snow releases at a point and snowballed.
Glide snow - loss of friction to the ground, not triggered by humans.

Slabs are usually formed by

recent new snow
wind, which can carry snow from one side of the mountain to the other and form a slab.
cold temperature (preventing the new slab to bind with previous snow).

There are twice as many avalanches on north face than south, due to delayed settlement. East facing are also bad. However in wet condition, things reverse, since the hot snow get wetter and will result in wet avalanches. During prolonged cloudy situations, there's little difference.

3/4 of avalanches happen on slopes between 35 and 45 degrees (+2d on maritime climate). Steepness is a much better predictor than terrain shape^[1]. Past 45 risk decreases.

Evaluation is danger + consequence.

Danger is evaluated using --/-/0/+ on:

Signs of instability
Presence of tracks (given that most avalanches are triggered by the first skier)
New snow, and existing snow foundation
Slope steepness.
Direction of snow.
Elevation mentioned in report
Windyp
presence of anchors (trees, rocks) - note that too few anchors are "false" anchors ehich become hazards if an avalanche is triggered

Consequences are calculated with: --/-/0/+ on:

Slope size (more snow above = more burried)
How much snow and how heavy will move
Terrain traps that could make it more serious
Safe grouping spots

Always avoid high consequence

During ascent:

Traverse new snow as high as possible
Stay on ridges and flats
Spread
Avoid >30 degree terrain
avoid bottom of risky terrain that can be triggered from below
use a test slope - a small, consequence free slope to test the snow pack

During descent:

30m min distance
Alone when slope > 30d
Weaker skier in middle of the group
if there's a risk, go completely across the risk towards a safe island, and don't stop, keep momentum to escape the potential slab (aka ski cut).

10 commandments

Go one by one
Have an escape route planned. If avalanche triggers, keep a 45d angle to escape, keep your altitude as snow below you can't burry you.
Don't go first (try to go on tested terrain)
Don't trust a cornice
Obsess over consequences
Start small (use a practice slope to test the snow)
Communicate
Have a belay rope
Be equipped
Terrain terrain terrain

wind slabs

Wind slabs are the worst. Wind loads snow much faster, and errodes it, making for a cohesive slab that can easily trigger. It overloads the pack.

They are recognisable because of their chalky white aspect, pillow on the snow, harder consistence, sound hollow, they feel slabby. If you encounter one, stop and turn around. Go to a safe spot and investigate

slabs

Slabs are made of

a slab
a weak layer
a bed on top of which the slab will slide (sometimes the bed forms during the avalanche)

Weak layers are usually made of

faceted snow (angular and large grained made by big gradient)
surface hoar (frost)
low density
ice crust.

Knowing that these exist makes you suspicious when they get burried under new snow.

Snow does not like rapid change, such as temperature, new snow, rain,...

Rapid change is usually the recipe for a slab. Increase in temperature or rain adds stress on weak layers because of percolation, and create wet slabs.

Storm slabs form due to new snow, and usually settle within a day

signs

avalanches
wind drifted snow
new snow
collapses or cracks
rapid change of temperatures
pinwheels (happens when dry snow becomes wet and indicates risk of wet Avalanches)
Testing

Pole test

Bury your pole to feel for weak layers. Potentially use the handle side for hard snow. If hard then soft, it's the sign of a slab. Stop and investigate.

track test

Go above the track, then try to trigger a mini slab to see how the snow handles

Compression tests

Perform on surface, then deep (DCT). dug a pit. Isolate column of 30cm by 30cm. Cut the back. Perform tests.

Place the shovel gently on top of the block. If fractures occur, result is very easy (CTV)
Position hand over shovel (don't rest on it). Perform 10 taps with the fingers, moving hand from the wrist. If at any time factures occur, note depth, mark "Easy" (CTE).
Level column so it's flat.
Repeat ten times, but this time moving the arm from the elbow. If fracture occurs, mark as moderate. (CTM)
Level if it's not horizontal.
Repeat moving arm from the shoulder. Mark as hard. (CTH)

Shovel sheer

Indicates where in a snowpack a sheer fracture could happen

Cut two columns (25cm wide / 35cm upslope)
Start by one 50cm column
Remove snow from one side, cut a wedge between both
Put shovel vertical on the back wall at the top. Make sure not to apply pressure, cut a wedge to insert it without pressure
Pull on shovel with both hands
If a clean sheer fracture occurs, mark where. If the snow just comes, repeat test deeper.
Mark where fracture occured in backwall
Repeat on second column, but edge of shovel placed 30cm above where the first column fractured.

If fracture happens at the bottom, repeat deeper. If fracture happens at different level on both test, or effort is different, repeat elsewhere.

Note effort required to fracture: easy, medium, hard.

Hand sheer

Same as shovel sheer, but with hand and easy to do. Isolate a 30x30 colum, 40cm deep.

Start with digging 40cm in front of snow,
then cut sides using poles doing a sawing motion
Then put arm behind the block, and pull

Note fracture level and effort required.

Rutschblock (Rouge block)

Perform on at least 25deg and preferably 30deg and steeper. Isolate a block 2m * 1.5m upslope. If using a cord or saw, build a trapeze to make sure. If digging sides it doesnt matter. Reach below any weak layer (e.g 1.5m deep). Place 2 probes on top 2 corners and use a cord to cut, or use ski poles / skis, or dig.

Result vary on loading step. Score depends when the layer slides.

Slides during isolation
Tester gently steps on block with skis, within 35cm from upper wall.
Bent legs without jumping
Jump on same spot
Jump again on the same compacted spot
Step another 35cm to approx mid block with skis, jump 3 times.
Didn't slide. Release type:

Whole block (WB)
Most block (MB)
Edge block (EB)

references

[1]: Bruce Temper / avalanche essentials