Right-side Up and Upside Down Snow
Learning goal 7c: Describe right-side-up and upside-down
snowfall and their significance to skiing and avalanches
In Learning Goal 7b we discuss the factors
influencing the
density of newly-fallen snow. Snow density matters because it affects
avalanche danger and ski quality. Here we talk about what happens when
the density of falling snow changes over the course of a storm.
Right-side-up snow
Right-side-up snow has higher-density (heavier)
snow underlying lower-density (lighter) snow. This typically
occurs when temperature and/or wind speed decreases during the
course of a storm or storm cycle. A storm cycle is simply a
series of storms in quick succession, with short breaks (day or less)
or no breaks in between.
A change in temperature and/or wind during the course of a
storm will affect the density of the newly-fallen snow. A typical
right-side-up storm would be cold-front dominated. That is, most of the
snow occurs with and following the cold front. Since temperatures
decrease following a cold front, the newly-falling snow would decrease
in density as the storm passes, leading to a right-side-up new snow
layer.
Often it is also windiest near the cold front, becoming less windy
following the front. In this case the temperature and wind effects on
density are working in concert to deliver a right-side-up new snow
layer.
Fig. 7c.1 -
Right-side-up new snow in Niseko, Japan. Right-side-up snow tends to
feel bouncier and floatier to ski in. If it's deep enough, the
low-density snow on top often sprays and billows up around you.
(Credit: West)
Right-side-up snow is good for skiing because the vertical
density gradient has a tendency to lift your skis or snowboard. It's
more bouncy and floaty to ski in. Since it lifts your skis or board so
easily, it's relatively easy to ski in. It's also a ton of fun!
The benefits don't end there. Recall from Learning Goal 7f
that higher-density snow underneath lower-density snow is a stable
configuration of snow layers. So, right-side-up new snow layers tend to
be more stable, and thus less prone to storm-snow avalanches.
Upside-down snow
Upside-down snow in the snowpack is, as you might guess, the
opposite: higher-density snow overlying
lower-density snow. This typically occurs with a warm front (you will
learn more about these in the next module). Temperatures increase
following a warm front, leading to increasing newly-fallen snow density
with time. Upside-down snow can also occur if it gets windy towards the
end of a storm.
For new snowfall within a storm cycle, you only need to
consider density changes within the most recent ~24 hours. Snowfall
density changes prior to that will have been largely eliminated by
settling within the snowpack. Don't forget that persistent weak layers
(e.g., surface hoar) can linger for months within the snowpack.
Upside-down snow is difficult to ski. Your ski tips want to
dive down under the snow. It can be hard to keep them above the
surface. Snowboarders have the advantage here; they will be able to
more easily float and bust through this stuff. Skiers with fat skis
will fare far better than those without. Telemarkers,
however, typically suffer the most in these conditions. It's on these
days that
they can be seen faceplanting and cursing (they too may end up
"upside-down") while snowboarders blast by them with a smile on their
face.
Fig. 7c.2 - "During" and "after" shots of a telemark
skier faceplanting into a somersault in upside-down snow, in the
Wasatch Mountains of Utah, USA. (Credit: West)
Upside-down snow also makes for bad avalanche conditions. Remember
that higher-density snow overlying lower-density snow is an unstable
configuration.
Thus, when you have newly-fallen snow that's upside-down, you're more
likely to have problems with storm-snow avalanches.
It is possible for the temperature and wind speed effects to
counteract each other.
For example, you may have a storm with decreasing temperatures, so the
snow that's falling is decreasing in density. However, towards the end
of the storm the wind increases. A fast enough wind speed could
overcome the density-lowering effects of cooling temperatures.
This could yield a new snow layer that is right-side-up in the lower
part
(due to decreasing temperatures), but then becomes upside down towards
the top (due to winds). Remember, however, that you only need one
unstable layer somewhere in the snowpack for the whole snowpack to be
unstable.
The right tool for the job
Snowboarders have a definite advantage in high-density and
upside-down snow; having 2 legs controlling one board is easier than 1
leg per board (skis). Snowboards tend to both power through snow and
float more readily. I know people that ski, telemark, and snowboard,
and when the new-snow is going to be difficult to make turns in, their
snowboard is the weapon of choice. Choosing the right tool for
difficult or deep snow, whether it be fat skis or a snowboard, can make
the difference between having a frustrating, unenjoyable day and an
awesome powder day.
Story time:
Back when I lived in Utah, most storms
were right-side up. After moving here to the south coast of BC, I found
the snow quality to be far less consistent. Storms more frequently
featured higher-density, and upside-down new snow. In Utah I telemarked
95% of the time, and skied AT skis about 5% of the time. It didn't take
long after I moved here to become 95% AT skis, and 5% telemark.
Ultimately, you want to be riding down the hill on whatever you're
going to have the most fun on.
Keywords: stable, storm cycle,
storm-snow avalanche, unstable
Figure Credits
Stull: Roland Stull
West: Greg West
Howard: Rosie Howard
COMET/UCAR: The source of this material is
the
COMET® Website at http://meted.ucar.edu/ of the University Corporation
for Atmospheric Research (UCAR), sponsored in part through cooperative
agreement(s) with the National Oceanic and Atmospheric Administration
(NOAA), U.S. Department of Commerce (DOC). ©1997-2016 University
Corporation for Atmospheric Research. All Rights Reserved.
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