Atwater wrote. “It is continually being
pushed, pulled, pressed, bent, warmed,
chilled, ventilated, churned.” The top-
most layer might be evaporating into
the night air; at the same time, radiant
heat from the ground, or from nearby
trees, could be melting the lowest layer.
When the temperature differences be-
tween the layers are small, snow tends
to sinter, or coalesce: the crystals knock
off one another’s arms, becoming rounded
grains that fuse into a strong, dense snow-
pack. When the differences are larger—
say, between the pack and the ground—
snow vaporizes upward and refreezes,
creating hollow, cup-shaped crystals. The
result is brittle, spiky snow, called depth
hoar. (In ice cream, a similar process cre-
ates freezer burn.)
Neither settled snow nor weak hoar
is dangerous in itself. The problem arises
when a dense layer lies atop a weak layer
to which it is poorly bonded. Depth hoar
is “the eeriest stuff on any mountain,”
Atwater wrote; it grows unseen, rotting
the snow until it is weak and potted. It
is strong in compression but weak in
shear. Like a row of champagne glasses
slowly loaded with bricks, it can hold a
surprising amount of weight until, with
the slightest shove, the structure falls
apart, creating a slab avalanche.
The word “avalanche” is too graceful
for the phenomenon it describes. On
slopes shallow enough to accumulate
snow but steep enough for it to be un-
stable—the sweet spot is said to be thirty-
nine degrees—the layers will separate,
and the slab will crack and slide. Churn-
ing violently, the snow reaches eighty
miles per hour within a few seconds. A
skier who avoids colliding with trees and
rocks is likely to be pulled under, then
pinned in place by thousands of pounds
of snow that harden like concrete. Very
few people can dig themselves out; most
can’t even move their fingers. Within
minutes, an ice mask forms around your
face. You asphyxiate on your own ex-
haled carbon dioxide.
In his book “Staying Alive in Ava-
lanche Terrain,” from 2008, Bruce Trem-
per, the former director of Utah’s Ava-
lanche Center, offers a taxonomy of
avalanches. In slab avalanches—the most
dangerous kind—an entire layer releases
at once. In storm slabs or wind slabs, the
releasing layer falls from above; in wet
slabs, a layer lower down is weakened by
water; in a persistent slab, it was weak to
begin with. A soft slab, composed of pow-
dery snow, tends to break where you stand;
a hard slab breaks above you, which is
more perilous. Non-slab avalanches are
said to be “loose.” In a dry loose ava-
lanche, powder releases in disconnected
sloughs. Wet loose avalanches—por-
tended by “pinwheels,” small snowballs
that leave streaks as they roll—are slower
but stickier, and more likely to bury you if
you get caught. Mixed avalanches, which
start dry and get wet lower on the slope,
have become increasingly common. So
have glide avalanches, caused by melt-
water seeping in below the snowpack.
Students of tsunamis or volcanoes
must wait for nature to deliver their di-
sasters, but an avalanche can be pro-
voked. In the nineteen-fifties, Atwater
used a technique now called “ski-cut-
ting.” Two patrollers descended danger-
ous slopes; while one looked on, ready to
stage a rescue, the other skied to a safe
point on the far side, picking up enough
speed to try and ride through any ava-
lanches he might start. In theory, the
slopes that slid were safer because of it;
the ones that didn’t were deemed stable
enough for everyone else.
It wasn’t practical to ski-cut every hill.
Knowing that the Swiss used bombs to
combat avalanches, Atwater tapped the
Forest Service’s wartime supply of tetry-
tol, the high-powered explosive; he askedhis supervisor whether he could have
some artillery, for distant targets. National
Guardsmen arrived with a First World
War-era French 75. (“What would ava-
lanche research be without war surplus?”
he later wrote.) For mid-range targets,
too close for artillery but too distant for
hiking or skiing, Atwater tried rifle gre-
nades, bazookas, bombs dropped from he-
licopters, and an air-to-air rocket known
as the Mighty Mouse. These methods
were too costly, or unsuited to the snow;
in the end, a modified ball machine, of
the sort used for batting practice, was
the most reliable delivery mechanism.
Richards’s team still uses Atwater’s “Av-
alauncher” to shoot about thirty rounds
each morning.
Atwater worked with Ed LaCha-
pelle, who had done a stint at the Swiss
Avalanche Institute, to create a “snow
study plot”—a clearing where they could
measure snowfall and take samples of
the snowpack at regular intervals. They
tracked the snow’s rate of accumulation
and weight in water, discovering that
weight mattered far more than depth:
when placed atop a layer of hoar, a foot
of fluffy powder was less dangerous than
three inches of dense slush. Wind, they
learned, could deposit many feet in just
a few hours; pillows of windblown snow
looked tranquil but were deadly. Study-
ing how snow settled, Atwater wrote,
“We saw things going on within that“Open up for the cleaning crew.”