Ministry of Environment

Soils

1.2 Climate

D.G. Schaefer

The climate of British Columbia varies enormously from place to place. On the macro-scale, climates associated with the major soil landscape regions (Part 3) are a product of geographical location and large scale topography. On the micro-scale, conditions at the earth's surface are modified by local physiographic factors such as slope, aspect and elevation. Micro-scale climates are also affected by vegetation and the condition of the soil itself through influences on the processes of evaporation and heat exchange.

The latitude of British Columbia places the province squarely in the zone of the westerly atmospheric circulation of the northern hemisphere. Its position on the western side of North America places it in proximity to the vast Pacific Ocean which is an immense reservoir of heat and moisture. The Pacific plays the dominant role in determining the climates of the province.

Innumerable winter storms born over the North Pacific Ocean develop rapidly, move in a northeasterly direction to the Gulf of Alaska and there weaken and die. Frontal systems which break away from the storm centres impinge upon the coastline and there face another major determinant of British Columbia climates; successive large-scale mountain barriers aligned in a northwest to southeast direction. Since these are roughly perpendicular to the mean winds aloft, they determine to a major extent the overall distribution of precipitation and the degree of dominance of Pacific air masses in relation to continental air masses in the various regions of the province. Weather systems carried by the prevailing westerly winds aloft drop considerable moisture as rain or, at higher elevations, as snow, when the air is forced up the west-facing slopes of Vancouver Island, the Queen Charlotte Islands and the Coast Mountains. The wettest climates of British Columbia are found in this zone. On the eastern slopes, the air descends and is heated by compression, causing the clouds to dissipate. The driest climates of the province lie in the valley bottoms in the lee of the massive coastal barrier. As the air again ascends the Monashee and Purcell Mountains, the Cariboo and Cassiar Ranges, and finally the Rocky Mountains, additional moisture is released with the heaviest precipitation falling on the west-facing slopes of each successive mountain barrier.

The mountain ranges are effective barriers to prevailing westerly winds, with particular emphasis on low-level moisture supply; they also block the westward passage of frigid continental Arctic air masses which dominate the winter season east of the Rocky Mountains. While the Great Plains region of northeastern British Columbia lacks this protection, other parts of the province experience varying degrees of shielding from the onslaught of Arctic air, increasingly so as one moves from east to west and from north to south.

In summer, the prevailing westerlies weaken. The climate then comes under the dominance of the large, semi-permanent Pacific anti-cyclone or high pressure centre which expands northward, greatly diminishing the frequency and intensity of Pacific storms and coastal precipitation. In the interior, spring is a season of decreased precipitation. About June, precipitation again increases as an interplay of factors, which include high rates of insolation and late spring surges of cool unstable air, produces strengthened convective activity, resulting in showers and thundershowers. In addition, a number of so-called "cold low" storms normally cross the province en route to the Great Plains. By mid-summer, these normally decline as the Pacific anti-cyclone develops its dominance over western North America.

Having described some of the macro-scale climatic controls at work over British Columbia, the question remains as to the characteristics of the resulting climates of the various soil landscape regions and, more particularly, how these are modified by physiography to produce the meso- and micro-climates that directly influence soil processes. Further, there is the question of the temperature and moisture conditions that exist within the soil layer itself. These are directly related to climatic conditions experienced above ground.

Temperature cycles near the soil surface follow those in the air with only slight lag. Deeper layers experience a much smaller range or amplitude of change with a lag of one or two months at depths of a few metres. Temperature changes penetrate more deeply in rock and wet sand than in wet clay and least of all in dry sand. In any case, temperatures below 5 or 6 m are essentially constant throughout the year. Soil moisture conditions are a function of gains through precipitation or snowmelt waters and losses by runoff and evapotranspiration. A discussion of the classification of soil temperatures and moisture regimes is included in Part 2.3 below.

The following sections deal separately with the five major regions of British Columbia. Table 1.2.1 provides selected climatic data for sites typifying those regions. Table 1.2.2 presents some contrasting values for stations at low and high elevations in certain regions.

Table 1.2.1
Selected Climatic Data for Major Soil Landscape Regions: Annual Means

Region
Stations
Precipi- tation (mm)
Evapotran- spiration (mm)
Snowfall (cm)
Temp. (°C)
Annual Range of Temp. (ºC)
Bright Sunshine (Hrs.)
Coast Mountains and Islands

Vancouver Internat'l Airport

Pr. Rupert

 1068
 

2415

 490
 

613

 52
 

113

 9.8
 

7.8

 15
 

12

 1930
 

1035

 Interior Plateau

 Summerland

Pr. George Airport

 296

620

 295

439

 74

233

 8.9

3.3

 25

27

 1990

1865

Columbia Mountains and Southern Rockies  

 Cranbrook
Airport

Revelstoke

 438
 

1096

 337
 

485

 178
 

412

 5.1
 

7.2

 27
 

25

 1800 E
 

1600 E

 Northern and Central Plateaus and Mountains
 Dease Lake

Germansen Landing

 394

525

 347

332

 187

257

 -1.2

0.4

 32

31

 1750 E

1755

 Great Plains
 Fort St. John
Airport

Fort Nelson Airport

 450

 

446

 383

 

380

 206

 

192

 1.3

 

-1.3

 33

 

40

 2130

 

1900 E

* Mean actual evapotranspiration from Phillips (1976), computed using the Thornthwaite water balance method, assuming a soil storage capacity of 100 millimetres of water.

E-Indicates values of sunshine in hours estimated from climatic maps.

The Coast Mountains and Islands

The marine west coast climates of British Columbia have a relative lack of sunshine, moderate temperatures year round and heavy precipitation, with a maximum in the fall on the North Coast and in the winter on the South Coast. Extensive areas, particularly westerly exposures and higher elevations, receive in excess of 2500 mm of precipitation, making them by far the wettest parts of Canada. East-facing slopes and lowlands receive substantially less. Snowfall accounts for a small fraction of the annual precipitation near sea level. The mean annual range of temperature (i.e. the mean temperature of the warmest month minus that of the coldest month) in the outer coastal zone is the smallest in Canada at 10°C.

Table 1.2.2
Comparative Climatic Data for Selected Pairs of Stations at
Low and High Elevation: Annual Means

Region
Stations
Elevation (m)
Precipitation (mm)
Snowfall (cm)
Temperature (°C)
Coast
Mountains
and
Islands
Hollyburn

Hollyburn
Ridge

46

951

1917

2939

66

811

10.1

5.1

Columbia Mountains and Southern Rockies
Trail

Old Glory

Mountain

Golden
Glacier

433

2348

788

1248

664

735

472

1493

167

558

204

970

8.7

-1.9

4.8

2.1

Northern
and
Central
Plateaus
and
Mountains
Stations
McLeod
Lake

Pine
Pass

704


933

629


2057

334


1322

1.8


0.6

A distinct climatic zone must be recognized over the southeastern lowlands of Vancouver Island, the islands of the Strait of Georgia and the Fraser River estuary. These areas which lie in the rainshadow of the Vancouver Island Ranges and Olympic Mountains have climates approaching the Mediterranean type in that summers are normally dry and warm with a high number of hours with bright sunshine, defined by a condition in which no cloud screens the instrument from the direct rays of the sun. Mean annual temperatures are the highest in Canada at just over 10ºC. The annual range of temperature of 15ºC indicates a slightly greater continental influence than on the outer coast. Precipitation amounts at sea level are as low as 650 mm, making this the driest region of the British Columbia coast.

Very different climates are encountered close to those described simply by moving to higher elevations. A transition takes place between moderate, rainy climates and colder, snowy climates. Mean temperatures decline 5ºC for every 1000 m increase in elevation. Summers become cool and short. Heavy snowpacks form in the winter and linger into mid-summer. In the extreme case extensive glaciers cover the higher peaks of the Coast Mountains.

The large amount of precipitation on the outer coast ensures that soils there are constantly moist. The small annual range of temperature implies slowly changing conditions within these moist soils. absent in any but extreme surface layers. Heavy snowpacks insulate soils from frost at higher elevations. In the lowlands surrounding the Strait of Georgia, warming and drying of the soil normally results in a mid- to late-summer moisture deficit estimated to range from less than 100 to about 200 mm depending on the site. Again, winter temperatures are moderate with frost penetration slight and infrequent.

The Interior Plateau

The Interior Plateau lies in the rainshadow of the towering Coast Mountains and Cascades. Precipitation in the semiarid valleys of the south averages as little as 250 to 350 mm annually. In the more open valleys to the north, amounts reach 400 to 600 mm. Over the rolling uplands, precipitation is considerably greater, reaching 750 mm or more. Precipitation is well distributed throughout the year with both a winter and a summer peak.

The continental nature of the thermal climate of the region is revealed by an annual range of temperature of 25ºC, double that on the coast. In southern valleys, this is a reflection of hot summers and moderate winters. In the uplands and in the north, it is more a reflection of cold winter conditions. Mean annual temperatures range from 10°C in southern valleys to about 3ºC in the north and at higher elevations elsewhere. More than 1800 hr of bright sunshine are received annually over most of the Interior Plateau.

Aridity and summer heat are significant factors affecting the soil in southern parts of the region. Strong insolation makes this particularly true of south-facing slopes where very high soil temperatures can result. Further north and over higher ground, more moisture is available and summer heating is less pronounced. Summer moisture deficits are estimated to range from 400 mm in southern valleys to about 200 mm near Williams Lake and to

only 100 mm near Prince George. The season of moisture deficit includes most of April to October in southern valleys, May to September near Williams Lake and June to August near Prince George. Moisture deficits are reduced and the season of deficit is shortened at higher elevations throughout the Interior Plateau. In winter, frost penetrates the soil to varying depths depending on location, depth of snow cover and type of material. In colder areas, frost penetration to a depth of 50 cm is common.

The Columbia Mountains and Southern Rockies

Southeastern British Columbia is an area of great vertical relief with strong climatic gradients. Mountain slopes over which westerly winds must rise receive annual precipitation totals of 1500 to 2000 mm, second only to amounts on coastal slopes. About half of this precipitation falls as snow, leading to the maintenance of glaciers on higher peaks. The narrow valleys of the area are semiarid, receiving 500 to 750 mm, only slightly more than the parched valleys of the southern Interior Plateau.

Mean annual temperatures in the valleys of southeastern British Columbia are close to 5ºC in contrast to values of 10°C for valleys further to the west. This is because the main valley bottoms are at progressively higher elevations as one moves from the Okanagan Valley to the Rocky Mountain Trench (this is illustrated incidentally in Figure 1.4.1) and because Arctic air readily invades valleys close to the Great Plains. Cold winters and cool summers lead to a mean annual range of 25ºC, similar to that over the Interior Plateau. With around 1800 hr of bright sunshine per year, southeastern valleys are less sunny than those to the west which receive in excess of 1900 hr. Fewer hours of bright sunshine are received on higher terrain due to increased amounts of cloud. Sunlight is also significantly reduced on steep, north-facing slopes.

The great range of climatic conditions from the valleys to the mountain slopes of southeastern British Columbia dictate a similarly wide range of temperature and moisture conditions in the soils found there. Moisture deficits of 200 mm or more occur in valley bottoms in the southwestern part of the region. The season of deficit extends from June to September. Further east and north, valley bottom deficits of 100 to 150 mm are common. Over higher terrain, deficits are limited to a few tens of millimetres in July and August.

The Northern and Central Plateaus and Mountains

The northern interior has long, cold winters and short, cool summers with moderate precipitation which is well distributed throughout the year. Mean annual temperatures below 0ºC in the northern valleys with even lower values on the slopes indicate the severity of the climate. Precipitation in the valleys averages 400 to 500 mm per year. Up to 900 mm fall on much of the mountainous terrain, with more than half in the form of snow. Precipitation increases from west to east, culminating in a distinctly wet area in the Rocky Mountains northeast of Prince George. There the annual precipitation averages from 1500 to 2000 mm, including over 1000 cm of snowfall. It is estimated that an average of 1700 to 1800 hr of bright sunshine occurs over much of the northern interior.

Due to the short summer and the harsh winter, the soils in the region are frozen to considerable depth from late October to mid-April or May. Scattered permafrost exists in some of the most northerly areas. Although evapotranspiration rates are relatively low, the modest amounts of precipitation lead to summer moisture deficits of 100 mm in soils at low elevation sites removed from the Rocky Mountains.

The Great Plains

The Great Plains region lies to the east of the Rocky Mountains and, therefore, has the most continental climate of any part of the province. What has already been said of the long, cold winters and short summers of the northern interior also applies here. Since the summers are somewhat warmer east of the Rockies, the mean annual range of temperature exceeds 30°C over most of the area and reaches 40°C in the extreme northeast, which is only slightly less than the greatest range observed in Canada. Annual temperatures average just above 0ºC in the Peace River basin and a few degrees lower in the Fort Nelson basin and over higher terrain. Precipitation over the Great Plains is moderate, averaging only 400 to 500 mm per year. Maximum rates occur during the mid-summer growing season, another continental feature of the climate of the area. Solar radiation is plentiful. Fort St. John has an annual average of over 2000 hr of bright sunshine, closely rivaling Victoria as the sunniest location in British Columbia.

Despite the peaking of the distribution of precipitation in summer months, high insolation and relatively high temperatures in that season produce sufficient demand for water for evapotranspiration to lead to a significant water deficit. Exposed soil surfaces tend to be warm and dry during mid to late summer, particularly on south-facing slopes. Estimated summer moisture deficits of 150 mm occur in the lower elevation terrain of the region. However, the summer is short. As in other parts of northern British Columbia, cold winter temperatures soon lead to frost penetration to depths of several metres, particularly in the Fort Nelson basin where scattered permafrost exists.

Further Reading

  • Atmospheric Environment Service, Canada Department of Fisheries and the Environment - numerous publications including climatic maps, normals and data summaries.

    Details regarding these various data sources are available from the Atmospheric Environment Service.

  • Bryson, R.A. and F.K. Hare, Editors, 1974. World Survey of Climatology: Volume 11, Climates of North America, Elsevier, New York. 420 pp.

    An in-depth volume including descriptive material but with considerable technical detail.

  • Chapman, J.D., 1952. The Climate of British Columbia, paper presented to the Fifth British Columbia Natural Resources Conference, University of British Columbia, February 27, 1952. 47 pp.

    A readable paper presenting basic information on climatic controls but with emphasis on a description of the climates of the regions of British Columbia.

  • Hare, F.K. and M.K. Thomas, 1974. Climate Canada, Wiley, Toronto. 256 pp.

    This text contains sections dealing with basic theory, regional climates, applications and meteorological services along with numerous tables of data. A readable and useful reference.

  • Phillips, D.W., 1976. Monthly Water Balance Tabulations for Climatological Stations in Canada, DS No. 4-76, Atmospheric Environment Service, Toronto. 6 pp plus tables.

    A brief paper describes the method. The complete set of tables (only a selection are included in the paper) provides a valuable source of information on soil moisture surplus and deficit conditions across Canada. Details can be obtained from the Atmospheric Environment Service.

  • Williams, G.P. and L.W. Gold, 1976. Ground Temperatures, Canadian Building Digest, Division of Building Research, National Research Council of Canada, Ottawa. 4 pp.

    A brief paper oriented toward building design which also provides basic information on the behaviour of the sub-surface temperature regime.