Local Influences on Weather
While most sources refer to large scale
climatological influences on weather, like the infamous El Nino, local weather
patterns are also affected on a small scale. For instance, the difference
between sun and shade (or between north and south facing slopes) can be as much
North facing slopes also tend to be wetter than south facing slopes. The end
result will see many north facing slopes supporting cool, wet spruce forests
while the arid south facing slopes may host a preponderance of aspen and poplar.
A quick walk around Emerald Lake in Yoho National Park quickly illustrates this
difference as you begin in an open, sun exposed forest not significantly
different from those found on the eastern slopes. However as you round the far
end of the lake and begin to return to the lodge, the forest takes on a wet
character and typical western slope plants like western red cedar, western
hemlock and Devil’s club become common.
The Continental Divide is another excellent
example of a local influence on weather. Moving from one side of the divide to
the other has a significant affect on the weather experienced. Since the divide
runs perpendicular to the prevailing western winds, as well as to the less
common eastern weather systems, our weather must climb and crest the divide. The
eastern slopes are characterized by much drier climates than areas only a few
kilometres on the western side of the divide. Since the majority of weather
patterns arrive from the west, the eastern slopes represent a rain shadow
protected from the brunt of western storms by the steep ascent the storms take
prior to cresting the divide. Rain shadows may also occur on a smaller scale,
where individual mountains and small ranges may provide a similar protection for
their downwind faces.
Just as the weather is influenced by global,
regional and local conditions, the temperature also varies with an equally
diverse list of conditions. Temperature is the end result of many factors. It is
not merely a measure of the direct solar radiation hitting a particular
location, but the synergistic result of many interdependent conditions.
Latitude and altitude represent two of the
most well understood influencers of temperature. Latitude, quite simply affects
the angle with which the sun’s rays hit the Earth. As one moves farther from the
equator, the sun’s rays impact the Earth at increasingly shallow angles. With
the northern latitude of the Rockies, in excess of 50°
North, the maximum angle of the sun is 65°.
Our sunlight never comes from directly overhead
Also, as you climb in elevation, the air
pressure decreases. Thinner atmospheres are less capable of holding heat,
resulting in a gradual drop in temperature. In general, temperatures drop 1.5 to
3°C for every 300 metres climbed. The difference may also be accentuated by the
general increase in winds with elevation. As you climb, you quickly move out of
the protection of the valley and into the unobstructed winds that whip over the
mountain summits. It is a rare day to crest a high ridge and not feel the sting
of a strong wind. As an average, a climb of 30 m (100 ft) delays spring by one
day. Locally, temperature differences with elevation may seem marginal, but in
most situations, these changes will be evident. .
Also, since solar radiation also increases
with altitude, those surfaces exposed to the sun rapidly heat up, and those not
exposed stay cool. This results in a drastic difference between sun and shade.
In some areas the difference may be as great as 22-28°C (40-50°F). This is one
of the major factors allowing some of our remnant glaciers to survive today. By
sticking to the north facing slopes, they receive significantly less sunlight
and melt at a slower rate than the long departed south facing glaciers.
This same mechanism rapidly heats mountain
locations during the day and cools them in the evening. As a result, daily
temperature variations in alpine areas are much higher than those of lower
elevations. In addition, the boiling point for water drops with altitude until
at 3050 metres (10,000 feet) it boils at around 90°C.
Regional Influences on Weather
The Pacific Influence
The prevailing winds in the Canadian Rockies
originate off the coast of British Columbia and move inland towards Alberta. As
these west winds approach the western slopes, they rise to crest the summit of
the continental divide, and in doing so release large amounts of moisture. This
moisture helps to support dense rainforests within the British Columbia areas of
the Rockies. By the time these winds crest the divide, they are generally
drained of much of their moisture, turning the eastern slopes into a more arid
rain shadow. The difference between western and eastern slope moisture content
can be seen in an almost immediate change in vegetation as one crosses the
The Prairie Influence
While the majority of our weather comes from
the west, occasionally the winds with switch to the east. In this case, the
weather system will still hold much moisture as it approaches the eastern
slopes. As it climbs up the foothills and mountains the eastern side of the
mountains will be bathed in rainfall, and the weather may clear as one travels
west into British Columbia.