The proper definition of active Avalanche defense is the "artificial triggering of small avalanches to reduce the risk of Avalanches, and to assess the accuracy of Avalanche forecasts." With this in mind, let us proceed to the actual methods currently used to actively defend Avalanches.
1.0 Manual methods
Although not employed often, the manual method of triggering small avalanches is limited to the kicking of cornices down. This is done by experienced technitions, and is usually done on only small cornices.
2.0 Explosives
The most common way that avalanches are artificially triggered is through the use of explosives. This method has meet with widespread approval in the U.S. and is by far more common than Passive Techniques in the States. It is estimated that approximately 10,000 are detonated annually in the U.S. Following is a few tidbits of explosives information, and then a brief summary of the more preffered methods of depositing the explosive charges themselves.
The premise of using explosives to trigger Avalanches may seem obvious, but a little exploration into how the explosives actually do trigger an avalanche is probably worth the time. Most importantly, the explosion causes mechanical fracturing around the explosion crater which shoot into and through the weak regions of the snowpack. The effect of this fracturing, along with the Thrust and stress waves given off by the explosion, are enough to trigger the avalanche. Although this method does sound foolproof, it should be noted that this method is not as effective in wet snow.
The choice of explosives is based on the two properties of detonation speed and density. Detonation speed relates to the rate of propegation of the explosive reaction, while the density is simply the mass to volume ratio of the explosive. Without going into boring detail, the standard charge used is 1 kg of TNT. 1 kg is used because of its ease of handling, and TNT for its high detonation speed.
2.1 hand delivery
Probably the most common method of depositing explosives on the side of the mountain, this method requires technitions to hike to the designated location and detonate the charge using a long fuse. It's popularity is based on it's simplicity and accuracy of charge detonation, but the major drawback is actually getting people there, which, at times, can be impossible due to terrain and/or weather.
2.2 aerial bombing
Although not a very popular method, aerial bombing from helicoptors is sometimes done. The advantage of this method is that terrain is no longer a factor, but weather can still be limiting.
2.3 artillery
artillery is a predominant method of avalanche control in the U.S. The advantage of this method is that explosive shells can be fired any time and in any weather. Another advantage is that artillery shells can also be fired into nearby rock surfaces, to increase the effect of the explosion. There are currently a variety of artillery pieces being used, but the bulk of the work is done by 75 and 105 mm recoiless rifles. The advantage of these guns is that they can be easily (and sturdily) mounted on a light support structure and easily used. 75 and 105 mm howitzers are also used, but to a lesser degree because of the difficulties in firing the gun and finding a location with sufficient support.
While there are many advantages for the use of artillery, there are also some disadvantages. These include: schrapnel, overshooting, duds, limited ammunition, and limited replacement parts. The biggest of these problems is the occurence of duds, which leaves live explosives on the side of the mountain.