Cold and Warm Fronts

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					                                                                                                  (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



                                                                                                   Precipitation                                     Precipitation




                                                                                                                                                                                                                                                                                                                                       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
                                                                                                   locations.                                        Wind
   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
                                                                                                   Clouds
   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

				
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