(quasi) stationary front (no front is truly sta- what is an upper Front? frontal passage as well, but is much better
Reading Weather: tionary, they all at least wobble back and Cold Front Often the fronts we experience in BC and protected from the elements.
Cold and Warm Fronts forth imperceptibly). As the fronts move
about the earth, the cold front is moving
Alberta come from the eastern Pacific. Over
the water those fronts are well defined at
There are internet tools that help you to
determine where a front is moving in the
Uwe Gramann, M.Sc (Dipl.-Met.), Meteorologist faster than the warm front and will eventu- the surface. Somebody kayaking near Tofino
Warm Front next 12 hours. This will be discussed in the
ally catch up with it. When that happensthe will probably observe a lot of ‘textbook next issue. If you are eager to read more on
Living in the mid latitudes of North Ameri- forming a frontal area between them. As
fronts form what is called an occlusion. fronts’. As the fronts move east the moun- the above topics by then, check out NOAA’s
ca, two of the most important features that you can imagine temperature and moisture
Environment Canada produces weather tains will shred the lower sections of fronts Jetstream Online School at www.srh.noaa.
influence our daily weather are warm fronts will change as you are moving from one air Stationary Front
maps called Surface Analysis four times a to pieces, leaving a mixture of cloud, precipi- gov/jetstream/ or the educational site of
and cold fronts. To know what to expect mass into the other and it is this contrast of
day. They contains surface fronts, sea level tation and wind, that depends more on the the University of Illinois at ww2010.atmos.
and where to find information about fronts properties that creates the nasty weather
pressure centres and isobars (lines of equal local terrain than the front itself. As a result uiuc.edu/(Gh)/guides/home.rxml.
will help a lot with your decision making. associated with fronts.
pressure). When you look at the weather maps Occluded Front the surface front is essentially ‘ripped off’
Fronts are the interface between two In some cases it is the cooler air that Intrigued? Planning a long traverse or a remote trip? Want
always check the time stamp first usually in and meteorologists change its definition to
huge masses of air. Picture an air mass as moves towards the retreating warm air. In to learn more? Uwe Gramann of of Mountain Weather
the bottom left corner (Achtung Baby: they an ‘upper front’ that is only well-established
a huge bubble of air sitting over Hawaii. such a case we would talk of a cold front. Services will get into more detail at his upcoming weather
use Greenwich Mean Time). You can find the Upper Cold Front near and above mountain tops. So while a
Sitting there it will become warm and If the warm air is the ‘pushy’ one, then we courses (Applied Mountain Meteorology Level 1 and 2)
maps at www.weatheroffice.gc.ca/analysis/ climber on Mount Waddington may also ex-
moist over time. Add another such bubble call it a warm front. And sometimes both air scheduled for Vancouver and Victoria this fall. Dates and
index_e.html but the chart I check most of- perience a nearly ‘textbook frontal passage’,
over northern Alberta that is becoming masses are equally ‘hard headed’ causing a info at www.mountainweatherservices.com/courses.html
ten is the early morning one at www.weath- the smarter climber who sought shelter in
cool and dry. Once these air masses start stale mate where neither the warm nor the Upper Warm Front
eroffice.gc.ca/data/analysis/935_100.gif. the valleys may not be able to recognize the
moving around, they may eventually collide cold air is advancing. Such a front is called a
Images from the University of Illinois WW2010 Project.
Convective: intensity varies strongly with More Stable: less intense but steady and po-
time, but is of shorter overall duration. Hail tentially very long duration. High intensity
and thunder possible. possible near the actual surface front.
Wind Freezing rain possible when cold air trapped in valleys and
Gusty and strong intensity especially over overrunning warm air is above zero (Inversion). Watch for
exposed terrain/water but also into moun- warm fronts following arctic outbreaks.
tain valleys. Resulting snow profile is ‘upside down’ increasing avalanche
Potentially strong winds in the wake risk. • If temperature is within 0°C and 3°C, a long and steady
Cold Front rain may change to snow. Warm Front
of frontal passage, especially coastal
When cold air moves against warm air, it on a summit, but luckily there is another A warm front is entirely different from tion coupled with a gradual increase of
is driving itself like a wedge underneath very good sensor: the barometer. Moisture Slowly and steadily increasing. May be a cold front. It arrives much more slowly mountain top winds and a slow pressure
the warm air essentially lifting the air If your barometer decreases by one Hec- Potentially ‘muggy” until frontal passage steadily strong over exposed locations. and lowering cloud bases make for a good decrease.
ahead of itself. This happens pretty topascal per one hour (without changing brings moisture down considerable. Inland valleys are usually more protected visual indicator as to when to expect the Weather associated with a warm front
quickly and is generally confined to altitude), consider it your first of three red from warm frontal winds. front. is usually not as violent and the clouds
several dozens of kilometers ahead of flags. Start looking around for other signs Temperature / Freezing level As the warm air approaches towards the more stable (stratus-type clouds). Precip-
Will generally fall behind the frontal Moisture
the front. of deteriorating weather. The second red colder and heavier air mass, it begins to itation out of these clouds is generally
passage. Frequent exception: Increases ahead of front as precipitation
The resulting upward motion is pro- flag is a decrease of 3 Hectopascal in three slide upwards onto the cold air. This process not as intense but longer lasting, so now
mountain valleys. falls from overrunning front into cooler air
ducing mostly convective clouds, the hours. Now it is time to consider descend- is slow and gradual. As the warm air is the problem becomes the duration. Ac-
typical ‘mushroom’ or cumulus-type ing or at least plan alternative routes that Temperature / Freezing level pushed higher, clouds start to form that are cumulated precipitation during a warm
clouds. In a cold front there are often are less exposed and over lower altitude. If Pressure horizontally very expansive and can reach frontal passage can be huge and several
Gradual decrease as front is still beyond Will generally increase behind frontal
too many clouds to distinctively see you are on coastal waters or in high eleva- passage. In some winter and spring cases as far as 1000km ahead of the front. feet of snow overnight can wreak havoc
the mushroom shape, but the cloud top tions, chances are that winds have already 100km away, then rapid decrease. After That far ahead of the front, the clouds with your trip timing.
frontal passage well-defined pressure rise, quite drastically so. Frequent exception:
often gives it away. You may see blue picked up by now and cloud bases have mountain valleys. have reached 10 kilometers in height or Especially over exposed water and
skies 100 km ahead of the front and as lowered. If your barometer drops 4 hPa (or sometimes very rapid (windy!) more and are fairly thin. These are the high mountain terrain, winds can
the front moves closer, weather quickly more) over 4 hours (or less) and other indi- Pressure clouds that you will first see when a well- become an issue, too. While a cold front
deteriorates. Convective clouds within cators are signifying deteriorating weather: Gradual and slow decrease all the way developed warm front approaches. The is usually over within a couple of hours
Cumulus type growing and lowering their
a cold front are locally quite confined; Act! Cold fronts can be ferocious especially to the frontal passage. Then only slight problem with most warm fronts in western or so, the precipitation of a warm front
bases within 2 hours of frontal passage (not
this means that precipitation and wind in spring through fall. recovery right after passage and then Canada is that this indicator of approach- can last 12 hours and if the front should
a lot of warning).
do not last very long, but they vary a lot Do not ever get caught in a cold front continued steady/light drop. ing high clouds (Cirrus) is very often a false stall, even longer. As a result, if you get
and can be very intense. One famous during those times over exposed terrain alarm. Since the front is still so far out it caught by a cold front you may be able
mushroom cloud is the Cumulonimbus or water – the risk is not worth it. Another barometer. You may have to count on a few Clouds may change direction and miss you. So, high to wait it out, if you find good shelter,
(CB), that is commonly known as the intensifying factor is daytime heating. When more hours of wind, especially over the Stratus type. Starting with very high and thin clouds are just one of many indica- but a warm front may be persistent
“thunderstorm cloud”. a cold front arrives during the warmest time coastal waters of BC and when the front was thin clouds changing to mid height and tors that you should watch for. enough to force you to move due to
If you stand on a mountain top, you of the day (afternoon) you have to count on fast moving. Freezing levels will have low- thickening over several hours. Further Similarly, the barometer will decrease darkness or even hypothermia.
can see a cold front from far away as a particularly intense weather. ered in some cases significantly, which will lowering of clouds together with onset of only quite subtly making it often an incon- Freezing levels during a warm frontal
line of mushroom clouds that sometimes Once the cold front passes, you will usually make the next night potentially chilly. precipitation within a few more hours of clusive measure. The best indicators for a passage will climb and if it is near freez-
appears as a wall. Unfortunately you do see a fairly quick improvement of cloud the frontal passage. warm front are lowering and thickening ing, snow may change to rain and make
not always have the luxury of standing heights and precipitation as well as a rising cloud bases as well as onset of precipita- a mess of slopes and paths.
Avalanche Echoes • September 20 09 • 6 7 • Avalanche Echoes • September 20 09