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 as 28°C. 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.

Factors Influencing Temperature

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 divide.

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.