...Avalanche Triggers...

Snow is said to be a "visco-elastic" substance, basically meaning that it can behave as a liquid or a solid. It's choice of behavior is determined by the rate of strain--i.e. the snowpack can withstand a much larger strain without avalanching if the load is applied slowly. Think of an elastic band or chewing gum on a cold day (other visco-elastic substances): they will stretch if pulled slowly, but will break if rushed.

Like the stretched elastic band, the snow slab will not break and will stay where it is as long as the strength of the forces holding it in place is greater than the stress of the forces pulling it downhill. Sometimes, this balance is very tenuous, indeed.

With this in mind, the idea of a "trigger" becomes crystal clear: a snowpack that is narrowly balanced between stress and strength can go one of two ways--either it will adjust to it's load over a period of time, or something abrupt will happen to tip the scale, causing an avalanche.

This "something" is called a trigger and can be natural or artificial. Professional avalanche workers cataloguing avalanche activity attempt to record the trigger whenever possible. The following are some examples of triggers and their recording codes:

Trigger: Artificial-Other...

ARTIFICIAL TRIGGERS:

Skier	AS
Snowboarder	AB
Explosive:   Hand-Charge   Artillery   Avalauncher	AE AA AL
Cornice fall	AC
Other: (examples of past artificial triggers)   coyote   elk   snowmobiles   climbers   snowcats	AO

NATURAL TRIGGERS:

Snowfall	N (Natural releases are generally not differentiated for record- keeping purposes)
Wind-loading
Rain
Rapid temperature increase
Tidal pull (controversial)