Cold Fronts
Learning Goal 5g. List the weather conditions associated with
a cold front and their relevance to snow sports.
Background information on cold fronts is covered in detail in the
Flying Learning Goal 3h. To
remind you, a cold front indicates
the boundary between
relatively warmer and colder air masses, where the colder air is
advancing.
Much of the hazardous sensible weather we experience (snow, winds,
fog), as well as changes in this sensible weather, are
associated with cold fronts. How is the weather associated with cold
fronts relevant to snowsports? How do we
identify this weather and predict its occurrence?
Temperature trends
Not only do temperatures decrease (become colder) the higher up a
mountain you go, but temperatures also become colder with time
behind a cold front, where
there is a colder air mass. I have seen temperatures as low as
-25°C on Whistler
Mountain. At these temperatures, especially with high winds that cause
wind chill (see below), frostbite
and hypothermia
are genuine concerns and you need to
be wearing the appropriate gear, covering as much skin as possible
(Fig. 5g.1).
Fig. 5g.1 These skiers are well prepared for cold
temperatures and strong
winds in the wake of a cold front on Mount Hood, OR, USA. (Credit: West)
The wind chill is not the actual
temperature, but rather the
apparent temperature, or how cold it feels
to a human due to the combined affects of temperature and wind. The
harder the wind blows, the faster heat is removed from the human body.
At a wind chill below 0°C you risk discomfort, and over a long period
of
time, if not properly dressed, hypothermia. Below -10°C you may suffer
frostbite and hypothermia after a short amount of time (again, if not
properly dressed). For wind
chill values below -55°C, skin can freeze in just two minutes! The wind
chill formula was developed as a joint effort by the US and Canada, in
part by using volunteers in a refrigerated wind tunnel. You can check
out the wind chill chart below.
Wind chill is particularly a concern on ridgetops (where it's windier),
during arctic air outbreaks at the coast (Learning
Goal 6l), and in regions where very cold arctic air is more
common, like the Canadian
Rockies.
Fig. 5g.2. Wind chill. Start with the actual
air temperature on the bottom axis, move straight up until you get to
the wind speed. The curved line that passes through that point is the
wind chill. If your point is between two curved lines, then interpolate
(estimate the in-between value based on how close it is to the curved
lines). Example, if the actual air temperature is -20 degC, and the
wind speed is 40 km/hour, then the wind chill is about -36 degC.
(Credit: Stull)
Winds
Winds are typically stronger in the vicinity of fronts. Cold fronts
generally bring stronger and gustier winds than warm
fronts. The location of the strongest winds relative to the cold front
varies storm to storm. Learning Goal 5j
will teach you about predicting high winds. Also, as is discussed in
Learning Goal 5f, wind direction typically shifts with the passage of a
front.
Precipitation
Don't forget that fronts are three-dimensional. The denser, colder
air behind the front undercuts the warmer
air ahead of it as it moves forward, causing that warmer air to rise
up, cool, condense, often forming tall cumulonimbus clouds (Learning Goal 1a), sometimes filling the depth
of the entire troposphere.
As a result, cold fronts generally bring a narrow (relative to the
length of the front) band of organized
precipitation along the frontal zone. The band can be 10-100 km wide
(Fig. 5g.3). This typically brings
with it a period of steady moderate or heavy snowfall. Strong cold
fronts in the spring may be accompanied by thunderstorms (even
thundersnow!).
Fig. 5g.3 Typical clouds and precipitation
associated with a cold front.
Strong vertical motion at the front causes
clouds and precipitation. (Credit: Stull/West)
In winter and at higher elevations in the mountains, cold fronts
generally bring precipitation in the form of snow, and sometimes
graupel and ice pellets (see Learning Goal 7k).
Rain or mixed rain and snow can occur if temperatures are warm enough.
In warmer storms, or warmer (lower) elevations, cold frontal
precipitation may start as rain, and transition to snow as the
temperature drops behind the front. While cold fronts themselves may be
accompanied by hazardous weather for snow sports enthusiasts, the snow
that accumulates on the ground during their passing makes for great
skiing. Often the best skiing comes in the hours following a cold
front passage, as the skies tend to clear somewhat. We'll talk about
determining the rain/snow line in Learning Goal 7a.
Visibility
This may sound obvious, but it really helps if
you can see where you are going while skiing or snowboarding. This is
far more important when you are skiing in the backcountry, i.e. not in
a
resort. Resorts have markers, signs, and ropes at the ski area
boundaries, and even then it is possible to go a different way than you
intended. However, you will likely always remain in a safe and
patrolled area, even if you lose your friends. This is not the case
when skiing in the backcountry, where you and your crew are the
wayfinders and there are no boundary ropes, no "cliff" warning signs,
and definitely no ski patrol.
Visibility on mountain slopes can be reduced due to a number of
factors
associated with cold fronts, some which we have already talked about:
Heavy snowfall
- Heavy precipitation in the form of snowfall
can reduce visibility down to tens of metres or less for sustained
periods of time. This can happen both with the cold frontal snowfall,
as well as for more brief periods of time in the showers in the
postfrontal airmass.
Fig. 5g.4 Heavy snowfall causing limited
visilibity and drooling. Falls Lake, Coquihalla, BC. (Credit: West).
Blowing snow
- Blowing snow can be snow that's falling and blowing, or snow
that's
been lifted up from the ground. It only takes a 14 km/h wind to pick up
snow off the ground and start moving it around. Blowing snow can be
continuous, or, in gusty conditions, vary a great deal from one moment
to the next. When the wind is strong, visibility may be limited to a
few metres. Perhaps more importantly, visibility is zero when your eyes
are closed! Which they may have to be if you're not properly equipped
with goggles.
Storytime
I was in a situation in the Blackcomb backcountry that seemed
nearly
unsurvivable. The wind was blowing hard, there was drifting snow making
it nearly impossible to open my eyes. I couldn't see to move forward,
frostbite seemed imminent. A friend calmed me down, and told me to put
on my goggles and balaclava.
These are essential items that should always go in your backcountry
pack. They're also recommended for resort skiing if the weather is less
than ideal. Just like that, the situation became manageable, and we
continued with our ski day, moving to a less windy location.
Clouds and fog
- In general, clouds block visibility. This could be vertical
visibility
(can only see uphill to the cloud base), or horizontal visibility
(can't see route down or the next peak over if a cloud is blocking your
view).
Fig. 5g.5 - Standing in clear air, next to a cloud
that's limiting
horizontal visibility. Mount Rainier, WA, USA. (Credit: West)
An interesting aspect of being at the same elevation at which
clouds
frequently exist, is that sometimes you find yourself inside
a cloud. Of course, when a cloud exists at ground level, we typically
refer to it as fog . In mountains
you can call it either. This
typically limits your
visibility severely, and it can even limit your ability to see the
ground
at your feet. This is known as a
whiteout since all you can
see is a kind of white in every direction (fog in the air and snow on
the ground) (Figs. 5g.6 and 5g.7).
I've had friends navigating in the backcountry, worried about
walking off a
corniced cliff, that repeatedly throw their ski pole a few feet in
front of
them just to give some definition
to the ground in front of them.
Fig. 5g.6 - Whiteout conditions, inside of a
cloud, at the top of Mount Garibaldi, BC. (Credit: West)
Fig. 5g.7 - Near-whiteout conditions on Mount
Rainier
WA, USA. (Credit: West)
In the wake of a cold front, after the main band of
clouds/precipitation, there is typically a gradual drying trend. This
leads to breaks in the (cumulus) clouds and precipitation.
Convective showers are possible because the air mass is typically unstable
due to cold air moving in aloft (Fig. 5g.8).
Fig. 5g.8 - A convective snow shower in the wake
of a cold front.
Visibility is improving, as the skies begin to clear and the sun comes
and goes, but the snow shower still creates a short-term hazard with
clouds/fog, intense but short-lived snow, and gusty winds. Hurley
Silver Mine area, Duffey Road, BC. (Credit: West)
Since the instability is often driven by cold air moving in over a
sun-heated ground, postfrontal clouds often dissipate in the evening as
the heating source goes away, and the atmosphere stabilizes.
Regardless, eventually drier air moves in, and the clouds
dissipate.
Blizzard conditions
- During the passage of a cold front, it is not uncommon to
experience everything discussed above while on
a mountainside. With that combination, you have yourself some fierce blizzard
conditions (Fig. 5g.9)! In this scenario, it may be more
sensible to stay inbounds in a resort rather than venture into the
backcountry, or head to the lodge until the worst is over. It is up to
you to determine the intensity of the storm to make your decision.
Fig. 5g.9 - Skiers trudging through blizzard
conditions in the Mount Baker backcountry, WA, USA. (Credit: West)
The weather must satisfy ALL of the following conditions to be classified as a blizzard:
Weather Element
|
Canada
|
USA
|
wind speed of at least
|
40 km/hr
|
35 mph (=56 km/hr)
|
visibility at or below
|
400 m
|
1/4 mile (= 400 m)
|
snow
|
blowing and/or falling
|
blowing and/or falling
|
duration of at least
|
4 hours
|
3 hours
|
Key words: cold front, wind chill, isotherms,
frontal zone, wind barbs, fog, whiteout, frostbite, hypothermia,
blizzard
Figure Credits: Howard: Rosie Howard, West:
Greg West, Stull: Roland Stull