Same Charge, Different Slope: Why Your Shot Doesn’t Always Work

How explosives really trigger avalanches

Petr Explosives Group · Practical Explosives Training School · October 2, 2026

Every avalanche blaster has seen it. A hand charge clears one start zone, and the same charge on the next slope does nothing. The explosive didn’t change. What changed was where the charge sat, whether the target was uphill or downhill of it, how steep the slope was, and what the snow was like.

Those differences are large. A 1 kg charge raised on a pole can reach about 70 m. Thrown on the snow, it reaches about 49 m. Sunk into the snow, about 8 m. In wet snow, the effect stops within about 10 m, whatever the charge size.

Petr Explosives Group has built an Avalanche Mitigation Blast Calculator and a training course that put numbers on these differences, in US and metric units. Even experienced avalanche blasters can use it to plan where to place a charge, how high and how much.

What the pressure does: SLF Release Criteria for Avalanche Control with Explosives

An avalanche-control charge does not push the slab off the slope. It works in three stages:

1. Air blast loads the surface. The blast wave sweeps over the snow and presses on it for a few milliseconds.
2. A stress wave travels down through the slab. That adds a short extra load to the weak layer, on top of the load the slab’s own weight already puts there.
3. The weak layer fails, and the crack runs. If the extra load starts a fracture over a large enough patch, the crack spreads through the weak layer on its own and the slab releases.

So the “critical pressure” is the surface pressure needed to start a fracture. Whether the crack then spreads depends on the snowpack, which is why there is a range and not one number.

Where each number comes from

• Gubler (1977) fired charges over instrumented snow at Davos and measured both the air pressure on the surface and how fast the snow moved. Fractures started when the snow moved at roughly 0.001 to 0.02 m/s, which corresponded to surface pressures of 250 to 2,000 Pa.
• The link between snow movement and stress is given in the 2012 guideline as stress = snow density × particle velocity × wave speed, with a wave speed of about 500 m/s. For a slab of 280 kg/m³ moving at 0.01 m/s, that gives 280 × 0.01 × 500 = 1,400 Pa (1.4 kPa), which sits inside Gubler’s range. The same guideline describes this as comparable to the stress a single skier puts on the weak layer under a 0.5–1 m slab.
• Simioni and Schweizer (2018) measured about 1 kPa at roughly 60 m from 2.4–4.8 kg elevated charges, and 320–480 Pa at 100 m, which they judged too low to fail a weak layer. Avalanches that release farther out than about 60 m do so because a crack started closer in and ran outward.

The 1 kPa “intermediate” value is a working figure based on that measurement. The SLF did not publish it as a threshold.

How the calculator applies them

1. It predicts the air-blast pressure at the point of interest (Pso).
2. It multiplies by the placement ratio: 1.0 for an elevated charge, 0.6 on the snow surface, 0.05 buried. The result is the pressure reaching the snow.
3. It compares that pressure with the three thresholds and reports “Met” or “Not met” for each.
4. It finds the distance at which the pressure falls to each threshold. Those are the three effective ranges.
5. It applies the snow-type limit: 10 m (33 ft) for wet snow and very stable hard slab.

Example: 1 kg cast booster, 1.5 m above the snow, Loveland Pass

Read this as: within about 43 m, the blast should be able to start a fracture even in stable snow. Out to about 70 m it should work in average conditions. Between 70 and 188 m it would only affect a very unstable snowpack, and at those distances release depends on crack propagation, not direct pressure.

What the criteria cannot tell you

• They are about starting a fracture, not about release. A fracture can start and then stop if the weak layer does not let the crack run.
• A negative result is not proof of stability. A charge that produces no avalanche only tests the snow within its effective range.
• The calculator’s pressures are high-side estimates. They assume hard ground. Gubler’s own field fit for the example above gives about 1.2 kPa, where the calculator gives 3.39 kPa. Plan on the smaller of the calculated and guideline ranges.
• Snow type can override everything. In wet snow, the blast dies out within about 10 m, whatever the pressure calculation says.

Note on sources

The Gubler and SLF figures in this document were taken from automated summaries of the papers, not from a full reading of the originals. Check them against the originals before using them in course material.

1. Gubler, H. (1977). Artificial release of avalanches by explosives. Journal of Glaciology 19(81), 419–429.
2. Gubler, H., Wyssen, S. and Kogelnig, A. (2012). Guidelines for Artificial Release of Avalanches. Wyssen Avalanche Control AG.
3. Simioni, S. and Schweizer, J. (2018). Avalanche control: comparing the effect of solid explosives and gas exploders. Proceedings, International Snow Science Workshop, Innsbruck.