Types of Spring and Summer Avalanches
Originally prepared for the CSAC Avalanche Center.
Here are descriptions of four types of avalanches which are most
likely in spring and/or summer and information on what causes them
and how to avoid them. Note that the defining characteristic which
these types of slides all have in common is warming. In spring and
summer the sun is higher and more direct, average ambient temperatures
are higher, and the sun is up for a longer time than in winter. Refreezing
of snow overnight due to radiational cooling may or may not help to
counteract these factors. Exposure to these spring and summer avalanches
can be greatly reduced by watching the weather and planning accordingly
and by timing your activities early in the day.
New Snow Avalanches
New snow may create winter-like conditions at any time of year in
the mountains, especially at higher elevations. During a storm the
avalanche concerns may be much the same as in winter. However, the
first couple of sunny days will generally cause widespread avalanching.
This can be immediately after the storm or it may be later after several
cooler and overcast days. If it has recently snowed but there has
not yet been a few warm and sunny days be cautious. Consider avoiding
avalanche terrain or postponing your plans. Should you choose to be
in avalanche terrain be sure to get an early enough start to be out
of such terrain before the direct sun hits it.
Wet Loose Snow Avalanches
Loose snow avalanches will begin at a point and fan out as they run
downslope. Ultimately they may entrain large amounts of snow and be
very powerful. They may also carry a person into, through or over
a terrain trap. These slides will begin to occur as the sun weakens
the surface layers. This will usually be sometime after the late morning
and may be in the afternoon. However, if overnight refreezing has
not occurred this type of slide may occur at any hour. The most common
reason for a lack of refreezing is cloud cover. Even a thin covering
of clouds is enough to block the radiation which cools and refreezes
the snow. As long as overnight refreezing occurs the key to avoiding
these slides is time of day. By the time you are penetrating the snow
to ankle depth (traveling on foot) you should be thinking about getting
off of steep slopes. Skiers will want to climb early in order to get
a few runs in on the much sought out "corn
snow" before having to quit for the day. Climbers will want
to get an alpine start in order to complete their descent early enough
in the day. In either case the early start may necessitate an ice
axe and crampons to ascend the still refrozen snow.
Deep Wet Slab Avalanches (Climax slides)
If the weather is too warm for too long the weakening of the snow
bonds will occur to greater depths. In addition meltwater may lubricate
buried layers and interfaces which can serve as failure plains. Rain
and/or warm winds can also contribute to this process. Crown
lines of resulting slides may be quite deep and even go to the ground.
These slides are generally unsurvivable. Typically the weather leading
up to such an avalanche
cycle will be very warm for an extended time (at least a couple
days) without any refreezing of the snowpack overnight. Be wary if
such a warm spell occurs. These conditions usually lead to numerous
large slides, not isolated events. Word generally gets around that
such a cycle is underway - check with the local avalanche forecast
center, ski patrol, forest ranger or warden, and local climbers and
These occur on glaciers. Icefall events are, to a large extent, a
result of glacier motion. They may become more likely well into or
just following an extended warm spell during which the glacier motion
increases. Their dependence on the time of day is not particularly
great. Unlike snow avalanches these events are rarely human triggered.
The key to avoiding them is route selection and timing - don't be
in the wrong place when they happen. Icefall areas are typically obvious
so try to choose a route which avoids them. Be cautious if the visibility
is too poor to choose or follow a route easily. When it is necessary
to cross an icefall area do so quickly.