Temperature-Related Statistics

characterize Natural Regions and Subregions (Tables 3-1 and 3-2, respectively) are presented at the end of this Part and are discussed below.

Figure 3-8 presents an overview of monthly temperatures for all Natural Subregions (these charts are presented in larger format in Part 4).

Monthly color patterns in Figure 3-8 illustrate Natural Subregion trends by latitude and elevation. Figure 2-1 (in Part 2) shows mean

annual temperature variations between and within Natural Subregions across Alberta.

3.2.1.1 Mean Annual Temperature and Monthly Variations

Mean annual temperature (MAT) decreases with both elevation and latitude (Figure 3-2).

The Alpine Natural Subregion at the highest elevations, and the Boreal Subarctic, Kazan Uplands and Northern Mixedwood Natural Subregions at the most northerly latitudes, have the lowest MAT values. In contrast, the

southerly Grasslands Natural Region receives higher insolation and the Mixedgrass and Dry Mixedgrass Natural Subregions have the highest MAT values.

3.2.1.2 Mean Temperature of Warmest and Coldest Months

Mean temperatures of the warmest and coldest months by Natural Subregion are shown in Figure 3-3. Mean temperature of the warmest month (MTWM) is either July or August.

Temperatures are coolest at high elevations (the Alpine and Subalpine Natural Subregions), and warmest in the Grassland Natural Region.

Summers can be quite warm even at northerly latitudes; however, growing degree day totals and monthly temperature charts (refer to individual Natural Subregion descriptions in

Boreal Cordilleran Grassland Ecoclimatic Province

Figure 3-1. General climatic relationship between Natural Regions and Ecoclimatic Provinces.

Part 4) indicate that warm periods are brief in the Canadian Shield, Boreal Forest, Foothills and Rocky Mountain Natural Regions, and longer in the Grassland and Parkland Natural Regions.

Mean temperature of the coldest month

(MTCM), which is either December or January, varies widely between Natural Regions and Subregions. It decreases with increasing latitude and reflects both the shorter days and the influence of cold, dry continental polar and

Arctic air masses for extended periods in the Boreal Forest and Canadian Shield Natural Regions.

Pacific air masses and Chinook events influence southern Alberta climates. Those Natural Subregions adjacent to and within the lower valleys of the Front Ranges (the Foothills Fescue, Foothills Parkland and Montane) enjoy the warmest average winter temperatures in the province.

-4.0 -2.0 0.0 2.0 4.0 6.0

Kazan Uplands Boreal Subarctic Northern Mixedw ood Peace-Athabasca Delta Athabasca Plain Upper Boreal Highlands Low er Boreal Highlands Central Mixedw ood Dry Mixedw ood Peace River Parkland Central Parkland Foothills Parkland Foothills Fescue Northern Fescue Mixedgrass Dry Mixedgrass Low er Foothills Upper Foothills Montane Subalpine Alpine

Average temperature ^oC

Figure 3-2. Mean annual temperature of Natural Subregions, grouped by Natural Region.

3.2.1.3 Mean Daily Maximum and Minimum Values

Mean daily maximum values are the average daily maxima for June, July and August;

mean daily minimum values are the average daily minima for December, January and February. Figure 3-4 shows the same latitudinal and elevational trends for Natural Subregions as those for MTCM and MTWM discussed in the previous section.

The effect of elevation on mean daily maxima are more noticeable; the Alpine, Subalpine, Upper Foothills, Upper Boreal Highlands and Boreal Subarctic Natural Subregions all have mean daily maxima less than 20°C, and all are at higher elevations than the surrounding terrain. Mean daily maximum values vary the most in the Alpine and Subalpine Natural Subregions, with a range of 18° and 15° in July compared with a 2° to 8° range for other Natural Subregions (Appendix 2).

These two Natural Subregions occur across a broad range of elevations (about 600 m for the Subalpine and up to 1000 m for the Alpine compared to 300 m or less for other Natural Subregions. The corresponding temperature ranges are higher, with highly variable slopes and aspects also contributing (e.g., solar energy is lower on steep northerly slopes than on steep southerly slopes).

3.2.1.4 Growing Degree-Days Growing degree-days (GDD) are used to match plant requirements for heat to the amount of heat available, and permit

comparisons of areas in terms of plant growth potential. The reference temperature for calculating GDD is the minimum threshold temperature at which plant growth starts. For most plants, this is considered to be 5^oC.

Growing degree-days are calculated by taking the average daily temperature and subtracting the reference temperature. For example, a day with an average temperature of 21°C

accumulates 16 GDD using a reference

-30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 Low er Boreal Highlands Central Mixedw ood

Average temperature ^oC

Mean temperature, coldest month

Mean temperature, w armest month

Figure 3-3. Mean temperatures of the coldest and warmest months, grouped by Natural Region.

temperature of 5°C². This measure is abbreviated as GDD5, and is the most commonly reported GDD statistic.

Two other measures of GDD that use different reference temperatures include “negative”

degree-days where degree-days below 0°C (DD<0) are accumulated, and growing degree days greater than 0°C (GDD>0). The date on which accumulated GDD5 degrees equal 100 (GDD100) is considered potentially important to plant development (Alberta Environment 2005).

The Rocky Mountain Natural Region has the lowest GDD5 values. (Photo: K. Crockett)

GDD5 and DD<0 are shown in Figure 3-5.

Because they are calculated from basic temperature statistics, they reflect geographic trends in seasonal conditions similar to the previously reported temperature values.

Overall, the Rocky Mountain Natural Region has the lowest GDD5 values, followed by the Foothills, Canadian Shield, Boreal Forest and Parkland Natural Regions. The Grassland Natural Region has the highest average GDD5 values.

There is considerable variation within some Natural Regions; for example, in the Boreal Forest Natural Region, average GDD5 ranges from a low of about 870 in the higher

elevation, high-latitude Boreal Subarctic Natural Subregion to a high of about 1300 in the lower elevation and lower latitude Dry Mixedwood Natural Subregion. Figure 2-3 (in Part 2) shows the distribution of GDD5 conditions across Alberta.

2Source: Alberta Agriculture,

www.1.agric.gov.ab.ca/$department/deptdocs.nsf/all/sag6301

The differences in winter severity between Natural Regions and Subregions is readily apparent from the DD<0 summary on the left side of Figure 3-5. The Canadian Shield and Boreal Forest Natural Regions are more strongly influenced by continental polar and continental arctic³ air masses than other Regions, producing a larger accumulation of negative degree-days in northerly latitudes.

GDD100 dates are widely variable between and sometimes within Natural Regions. They are closely related to GDD5 trends, from which they are derived, and less closely related to frost-free period. The earliest dates at which 100 GDD5 days are accumulated are in early to mid-May on average in the

Grassland and Parkland Natural Regions. By the third week of May, the Boreal Forest Natural Region has usually accumulated 100 degree-days, except for the higher elevation Upper Boreal Highlands and Boreal Subarctic Natural Subregions.

Croplands are the most common in the Grassland and Parkland Natural Regions, in part due to their more favorable GDD5 values.

(Photo: L. Allen)

Decreasing temperatures at higher elevations produce a significant lag in GDD100 dates for the Rocky Mountain Natural Region. It is not until late June or early July on average that 100 growing degree-days are accumulated in the Alpine and Subalpine Natural Subregions.

3Refer to Part 1 (Climates) for more detail.

Figure 3-4. Mean daily minimum values (Dec., Jan., Feb.) and maximum values (Jun., Jul., Aug.).

Figure 3-5. DD<0^oC (solid bar) and GDD>5^oC (striped bar).

-30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 Kazan Uplands

Boreal Subarctic Northern Mixedw ood Peace-Athabasca Delta Athabasca Plain Upper Boreal Highlands Low er Boreal Highlands Central Mixedw ood Dry Mixedw ood Peace River Parkland Central Parkland Foothills Parkland Foothills Fescue Northern Fescue Mixedgrass Dry Mixedgrass Low er Foothills Upper Foothills Montane Subalpine Alpine

Average temperature ^oC

Mean daily minimum Mean daily maximum

-3000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000

Kazan Uplands B o real Subarctic No rthern M ixedwo o d P eace-A thabasca Delta A thabasca P lain Upper B o real Highlands Lo wer B o real Highlands Central M ixedwo o d Dry M ixedwo o d P eace River P arkland Central P arkland Fo o thills P arkland Fo o thills Fescue No rthern Fescue M ixedgrass Dry M ixedgrass Lo wer Fo o thills Upper Fo o thills M o ntane Subalpine A lpine

Accum ulated degree-days

3.2.1.5 Frost-Free Period

Frost-free period is another indicator of temperature regimes that are favourable or unfavourable to plant growth. This variable is often used for assessing crop growth

suitability, but also gives some indication of the growth period available to native species.

Frost-free trends are not as closely linked to latitude as mean annual temperature. The Peace–Athabasca Delta and Athabasca Plain Natural Subregions in the far north have almost as long a frost-free period as more southerly areas such as the Northern Fescue and Central Parkland Natural Subregions. Figure 3-6 shows the predicted frost-free period (striped bars) and the range between the maximum and minimum predicted period (solid bars).

The factor that appears to contribute most to short, erratic, frost-free periods is terrain variability and elevation; generally, the higher the elevation and the rougher the terrain, the shorter and more unpredictable the period.

This is probably due to aspect variations (north vs. south slopes) and cold air drainage from higher to lower terrain. The latter can produce

“reverse” tree lines in valley bottom locales within the Upper Foothills and Subalpine Natural Subregions where cold air pools at night and retards tree growth.

The Rocky Mountain and Foothills Natural Regions, and the Foothills Parkland, Foothills Fescue and Dry Mixedwood Natural

Subregions, all exhibit relatively high frost-free period variability. With the exception of the Dry Mixedwood Natural Subregion, all occur at average elevations over 1000 m and with average slopes of greater than 2°.

The Dry Mixedwood Natural Subregion is variable, probably because it extends from just north of the Bow River Corridor in the south to just south of the Caribou Mountains in the north. This is a south-to-north distance of nearly 800 km, and includes a number of weather stations in variable topographic locations.

Frost-free period and average dates of last spring frost and first fall frost reported in Table 3-2 are highly unreliable because of both

year-to-year variations in weather patterns and topographic variability, especially in the Natural Regions and Subregions mentioned above. For more precise information, readers should consult climate station data in the locale of interest.

3.2.1.6 Continentality

“Continentality” is a relative index of the degree to which an area is affected by

continental rather than Cordilleran influences.

It is calculated simply by subtracting the mean temperature of the coldest month from the mean temperature of the warmest month.

The area with the highest continentality (Figure 3-6) is the Kazan Uplands Natural Subregion in extreme northeastern Alberta.

Here the long, cold winters resulting from short days and Arctic frontal influences during the winter months alternate with short, warm summers. This is a typical continental regime.

At the other extreme is the Alpine Natural Subregion, which is entirely within the Cordillera. In this Subregion, the cold

conditions prevail throughout the year and the temperature range is correspondingly lower.

Figure 2-4 (in Part 2) indicates continentality trends across Alberta. This figure shows that several Natural Subregions are influenced by their proximity to the mountains. The Lower and Upper Foothills, Foothills Parkland, Foothills Fescue, Montane, Alpine and Subalpine Natural Subregions are all within or close to the Rocky Mountain Front Ranges and have relatively low continentality values. The comparatively higher continentality of the Lower and Upper Boreal Highlands Natural Subregions compared to the Lower and Upper Foothills Natural Subregions supports re-assigning the 1994 northerly Foothills outliers to the Lower and Upper Boreal Highlands.

Figure 3-6. Frost-free period, mean length (days) and range (days).

0 20 40 60 80 100 120

Kazan Uplands Boreal Subarctic Northern Mixedw ood Peace-Athabasca Delta Athabasca Plain Upper Boreal Highlands Low er Boreal Highlands Central Mixedw ood Dry Mixedw ood Peace River Parkland Central Parkland Foothills Parkland Foothills Fescue Northern Fescue Mixedgrass Dry Mixedgrass Low er Foothills Upper Foothills Montane Subalpine Alpine

Average length of frost free period, days (upper striped bar) Range betw een highest and low est values, days (low er solid bar)

0 10 20 30 40 50 Kazan Uplands

Boreal Subarctic Northern Mixedwood Peace-Athabas ca Delta Athabas ca Plain Upper Boreal Highlands Lower Boreal Highlands Central Mixedwood Dry Mixedwood Peace River Parkland Central Parkland Foothills Parkland Foothills Fes cue Northern Fes cue Mixedgras s Dry Mixedgras s Lower Foothills Upper Foothills Montane Subalpine Alpine

Contine nta lity

Figure 3-7. Relative continentality of Natural Subregions.

Figure 3-8. Comparison of mean monthly temperature trends for all Natural Subregions. (Red= >15°C, Yellow = 10 to 15°C,Green = 0 to 10°C, Light Blue = -10 to 0°C, Dark Blue = -20 to -10°C, Purple = -30 to -20°C. Time axis is January to December. Refer to Part 4 for more detailed bar charts.)

Rocky Mountain Natural Region Foothills Natural Region Grassland Natural Region Parkland Natural Region Canadian Shield Natural RegionBoreal Forest Natural Region

Decreasing elevation

Lower Foothills -30

-20

20 Month Higher elevations near Front RangesIncreasing latitude

Central Parkland -30

Peace River Parkland -30

-20

-10

010

20 JFMAMJJASOND Month Kazan Upland -30-20-1001020 Month Far northeast

Dry Mixedgrass -30-20-1001020 Month

Foothills Fescue -30

Northern Fescue -30-20-1001020 Month Increasing latitudeHigher elevations near Front Ranges Far north, elevated plateausNorthern Alberta, elevated plateausand plateau slopesWidespread across central to Northern Alberta

Lower Boreal Highlands -30

-20

-10

010

20 Month

Upper Boreal Highlands -30

-20

-10

010

20 Month

Athabasca Plains -30-20-1001020 Month

Northern Mixedwood -30

-20

-10

010

20 Month

Peace-Athabasca Delta -30-20-1001020 Month Far north and northeastern Alberta, low-elevation plains

Central Mixedwood -30

-20

-10

010

20 Month

Dry Mixedwood -30-20-1001020 Month

Boreal Subarctic -30

-20

-10

010

20 Month

3.2.2 Precipitation-Related

Dans le document Natural Regions and Subregions of Alberta (Page 32-41)