Weather and Climate Patterns in Canada's Prairie Grasslands

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Chapter 5 Weather and Climate Patterns in Canada’s Prairie Grasslands

Sean M. McGinn Agriculture and Agri-Food Canada Lethbridge, Alberta, Canada T1J 4B1

Abstract. The underlying uniqueness of Canada’s prairie grasslands is its aridity. The dominant air mass of this region is continental in nature and is associated with a relatively low amount of precipitation and high potential evaporation during the summer. Drought in this region is frequent, lasting one to two months on average, and presents a significant barrier to ecological capacity and agriculture. Severe droughts are associated with a stalled anticyclonic system that directs advected moist air around the region. Canada’s prairie grasslands receive on average of between 300 and 550 mm of precipitation, most of which is received as rain in spring and summer. Over the last century, changes to the climate have resulted in a general warming trend that is projected to increase further as a result of an enhanced greenhouse gas effect. Native grassland plant diversity and insect populations may shift as a result of expected climate changes.

Résumé. Les prairies canadiennes doivent leur caractère unique à leur aridité. La masse d’air dominante dans cette région est continentale, et se caractérise par un degré relativement faible de précipitations et une évaporativité élevée en été. Les sécheresses sont fréquentes dans cette région; elles durent en moyenne de un à deux mois et réduisent sensiblement la capacité limite du paysage et le rendement agricole. Les sécheresses graves sont causées par un système anticyclonique stable qui empêche les courants d’air humide de pénétrer dans la région. Les prairies canadiennes reçoivent en moyenne de 300 à 550 mm de précipitations, la plupart sous forme de pluie au printemps et en été. Au cours du dernier siècle, le changement climatique a provoqué un réchauffement général qui devrait s’intensifier sous l’effet de l’augmentation des concentrations de gaz à effet de serre. La diversité des espèces végétales indigènes et les populations d’insectes pourraient évoluer sous l’effet des changements climatiques attendus.

Introduction The prairie grasslands of Canada possess unique characteristics because of their topography, soil, fire frequency, climate, grazing, and imposed pressure from human activity (Ripley 1992). Although the current distribution of grasslands is attributed to both natural and anthropical forces, climate remains the dominant factor (Barnes 1959; Padbury et al. 2002). It is not surprising, therefore, that production of prairie flora is strongly related to weather elements such as precipitation (Smoliak 1986; Thorpe et al. 2008) and is well adapted to fluctuations in weather (Coupland 1958). For Canada’s prairie grasslands, an extension of the Northern Great Plains, the climate component is characterized as predominantly continental. Although the makeup of prairie grasslands has changed because of the introduction of agriculture (i.e., cropped grasslands), grasslands remain a distinctive landscape. Prairie grasslands extend eastward from the Canadian Rockies to the Assiniboine–Red River Valley in Manitoba and are bordered to the north by the boreal forest (see Chapter 3). Here, the seasonal extremes fluctuate between long, cold winters and short, warm summers. The

McGinn, S. M. 2010. Weather and Climate Patterns in Canada’s Prairie Grasslands. In Arthropods of Canadian Grasslands (Volume 1): Ecology and Interactions in Grassland Habitats. Edited by J. D. Shorthouse and K. D. Floate. Biological Survey of Canada. pp. 105-119. © 2010 Biological Survey of Canada. ISBN 978-0-9689321-4-8 doi:10.3752/9780968932148.ch5 106 S. M. McGinn

continental polar air mass that typically resides over this region produces relatively low amounts of precipitation. Periodic interruptions of moist maritime polar air in winter and summer by colder and drier arctic air masses generate the variety of weather experienced across the prairie grasslands. The seasonal extremes in air temperature, and the low amount of precipitation, are common elements of grassland ecosystems throughout the world. The extreme nature of climate on the prairies was first highlighted in relation to agricultural capacity during the 1856 expedition of Captain John Palliser (Spry 1959). Having spent a short time in the region, Palliser reported that southern Alberta and Saskatchewan, and a small area in southwestern Manitoba, would not support viable agriculture. The normal climate of this arid region, which became known as Palliser’s Triangle, was later proven to be capable of supporting agriculture if specialized practices were used (Grace 1987a). However, even with the advancements in agriculture today, periods of extreme weather, especially that of drought, continue to shape the agricultural and ecological capacities of the region. The climate of the prairie region prior to direct weather observations has been reconstructed by using many types of proxy data (Nkemdirim 1991). Such reconstruction of the climatic record has given insight into the potential for future climate. For example, drought intensity as experienced in the 1930s may have been more frequent in the past (Laird et al. 1996) and suggests that we are currently experiencing a climate without these extreme fluctuations.

Precipitation across the Prairie Grasslands Precipitation amounts and timing are the principal climate features of the Canadian prairie grasslands that effectively determine the vitality of this ecosystem. In the absence of regional changes in topography and coastlines, the main force controlling regional precipitation patterns is the circulation of the atmosphere (Borchert 1950). In particular, the movement of cyclonic (low-pressure) systems across this region is a critical process that delivers advected moisture. The Canadian prairies receive an annual average of 454 mm of precipitation (McGinn and Shepherd 2003), which is much less than the Canada-wide average of 535 mm (Phillips 1990). However, the distribution of precipitation across these grasslands is highly variable, ranging from an annual low of 300–350 to a high of 500– 550 mm. In general, the driest region coincides with Palliser’s Triangle (Fig. 1). The distribution of precipitation is seasonal. Approximately 70–80% of the annual precipitation across the prairies is received as rain (Wheaton 1998). The rain received in June and July accounts for 20–35% of the yearly total precipitation and 42–54% of the growing- season precipitation (Chakravarti 1972). For the southern grasslands (between 49 and 52–54° N latitude), the peak month for rainfall is June, coinciding with the early summer position of the jet stream (a narrow band of strong winds in the high troposphere) and more frequent tracking of cyclonic systems. At higher latitudes, the peak month for precipitation is July, again coinciding with the predominant position of the jet stream at that time of the year. Within the Canadian prairies, the province of Saskatchewan receives the least annual precipitation (395 mm), followed by Alberta (482 mm) and Manitoba (486 mm) (Shepherd and McGinn 2003). In winter, the southern corridor of Alberta and central Saskatchewan receives 40 mm or less precipitation, whereas the southern border of Alberta and Saskatchewan (around Cypress Hills) receives more precipitation (Fig. 2). More than 60 mm of precipitation is received in northern Alberta and along the foothills, as well as in Manitoba. During the summer, areas of southeastern Alberta and southwestern Saskatchewan receive precipitation of between 120 and 145 mm. The maximum average precipitation in the summer occurs in central-northern Alberta (200–300 mm) and in eastern Manitoba (220 mm). Weather and Climate Patterns in Canada’s Prairie Grasslands 107

Fig. 1. Average annual precipitation (mm) across the prairie provinces (from Wheaton 1998).

Prairie Drought

Three basic weather patterns dominate the Canadian prairie grasslands in the summer: a mid-troposphere (approximately 10 km in height) ridge that occurs 72% of the time, a mid-troposphere trough with a 14% occurrence, and a zonal flow occurring 14% of the time (Dey 1982). A prolonged decrease in precipitation on these grasslands coincides with the mid-troposphere ridge pattern that remains stationary. An anticyclonic system (high- pressure area) associated with the ridge drives the jet stream northward, allowing moist maritime polar air to flow into the prairie region. This advection of atmospheric moisture into the prairie region accounts for about 65–76% of the rainfall received in the summer (June, July, and August; Raddatz 2000). The causes of drought continue to be evaluated. Bonsal and Wheaton (2005) reported that, unlike the typical 1961 and 1988 droughts of the Canadian prairies, where the formation of ridges and troughs had a key role, the 2001–2002 drought on the Canadian prairies had no relationships to large-scale circulation patterns. Instead, the drought was caused by a unique circulation pattern related to a northward extension of a persistent drought of the continental United States. Although the meteorological causes of drought are well recognized after they occur, they are often difficult to predict. In addition, the impact of drought is not always straightforward. A clear definition is needed when describing drought because the degree of impact is related to the intensity and duration of a drought event. Drought on the prairies is not characterized solely by a prolonged period of reduced precipitation (meteorological drought), because the soil water status that depends on the infiltration rate of precipitation, existing stored water, and surface evaporation must also be considered. 108 S. M. McGinn

A

B

Fig. 2. Average historic precipitation (mm) for the periods A, December to February and B, July to August (from Shepherd and McGinn 2003). Weather and Climate Patterns in Canada’s Prairie Grasslands 109

An example of the dominance of drought in the southern prairies can be seen from the frequency and average duration of drought at Lethbridge, Alberta (Table 1). Two drought indices are examined, the Rainfall Anomaly Index (RAI; Oladipo 1985), which reflects abnormal rainfall amounts, and the Palmer Drought Index (PDI; Palmer 1965), which incorporates soil water storage. The RAI series for Lethbridge reveals 233 drought events of slight intensity (values < −0.5), each lasting an average of 2.2 months (49% of the total duration). Moderate drought intensity (−2) occurred 163 times and lasted on average 1.3 months (20% of total duration). There were no extreme RAI events recorded. According to the PDI series, fewer drought events occurred than were found with the RAI, but the duration of each category increased (39, 22, and 6 events lasting on average 11.4, 7.2, and 2.8 months, respectively, for slight to extreme drought intensities). On average, the PDI series indicated that slight, moderate, and extreme droughts existed 42, 15, and 2% of the total time for Lethbridge between 1902 and 1990. In the past century, the 1930s and 1980s have seen an abnormal number of successive drought episodes. Although the “dirty thirties,” especially 1936 and 1937, are perceived as dry years, in terms of meteorology, the 1986–1988 drought was more severe. The summers during 1986–1988 were much warmer than those experienced in the latter drought years of the 1930s, which predictably resulted in greater evaporation and soil water deficits in 1986–1988 than in the 1930s (Lang and Jones 1988). The recurring nature of drought on the prairies has a significant impact on all grassland ecosystems. For example, warm, dry conditions associated with summer drought can favour higher densities of insect populations such as grasshoppers (Capinera and Horton 1989).

Temperature across the Prairies The spatial distribution of temperature on the North American grasslands, in the absence of any dramatic change in topography and mitigating impact of oceans, is generally dominated by a latitude effect imposed by insolation (Borchert 1950). Overlaid on this latitude effect

Table 1. Number of droughts, average duration in months, and standard error (parentheses) between 1902 and 1990 at Lethbridge, Alberta, for the Rainfall Anomaly Index (RAI) and the Palmer Drought Index (PDI).

RAI PDI Intensity of Average Average Drought Number of Number of Duration in Duration in Occurrences Occurrences Months Months

Slight (−0.5) 233 2.2 (0.1) 39 11.4 (2.0)

Moderate (−2.0) 163 1.3 (0.1) 22 7.2 (1.2)

Extreme (−4.0) 0 0 6 2.8 (0.9) 110 S. M. McGinn

are temporal fluctuations resulting in part from the modifying effect of frequent anticyclonic (high-pressure) conditions (Ripley 1992). Anticyclones produce subsidence (descending air) that generally coincides with clear skies. In addition, subsiding air produces warming (by compression), which is most noticeable in the winter. In the summer, clear skies lead to warm daylight temperature (increased insolation) and cool night temperature (increased long-wave radiation loss). The annual mean maximum and minimum air temperature on the Canadian prairie region is 8.1 and −4.1 °C, respectively (McGinn and Shepherd 2003). However, the variability between seasons is tremendous, as illustrated by the extreme air temperature. On average, the summer maximum and minimum temperatures are 22 and 28 °C higher, respectively, than those in winter across the prairie region. The extreme temperature experienced between summer and winter ranged from a high of 45.0 °C one year (Midale and Yellow Grass, southeastern Saskatchewan) to a low of −61.1 °C in another (Fort Vermilion, northern Aspen Parkland Ecoregion in Alberta).

The mean maximum air temperature (Tmx) for the prairie grasslands in Alberta, Saskatchewan, and Manitoba throughout the year is 8.4, 8.3, and 7.6 °C, respectively. In winter, Tmx decreases from about −2.5 °C in southwestern Alberta to at least −10.5 °C, as depicted by the isotherm (Fig. 3A) extending from northern Alberta through central

Saskatchewan and into southern Manitoba. This Tmx gradient, as in the case for the mean minimum air temperature (Tmn) in winter, reflects the influence of the cold continental air mass contrasting with more mild Pacific air, as well as the effect of the chinook winds (a dry and warm air mass) along the east slope of the mountains. In summer, the southern grassland region experienced Tmx values of about 25.5 °C, which decreased north (latitude effect) and west (e.g., elevation effect) (Fig. 3B).

Annual historic values of Tmn averaged between −4.0 (Alberta) and −4.2 °C (Saskatchewan and Manitoba) (Shepherd and McGinn 2003). However, the prairies experience considerable seasonal variability in air temperature. During winter across the prairies, a well-defined gradient existed in mnT , extending from a high of around −12.5 °C in southern Alberta and decreasing to the east and north to at least −20.5 °C (Fig. 4A). This pattern reflects both a latitudinal effect and the effect of a cold continental air mass in winter. The pattern of Tmn over the prairies during summer was less distinct, although an elevation effect of cooler air to the west was evident (Fig. 4B). In summer, the highest

Tmn (>10.5 °C) existed just north of the Canada−US border in southeastern Saskatchewan and southern Manitoba. The higher winter minimum temperature in southern Alberta coincides with a region known for its frequent chinook winds that have a moderating effect on the average winter temperature.

Sunshine Hours and Solar Radiation Incident solar radiation is the principal driving force for many ecological and hydrological processes. Because of the clear skies associated with continental air masses, the prairies are recognized as the sunniest region in Canada. On average, the southern prairie has greater than 2,400 hours of sunshine (unobstructed by clouds and pollution) annually (Phillips 1990) compared with the rest of Canada of between 1,200 and 2,000 h. Typically, about 312 to 322 days of the year have some hours of sunshine. The sunniest month is July. The mean solar radiation each day varies from about 23 to 25 MJ/day in July to about 4 MJ/day in December (Canadian Climate Normals 1951–1980, Environment Canada 1982). Weather and Climate Patterns in Canada’s Prairie Grasslands 111

A

B

Fig. 3. Average historic maximum temperature (°C) for the periods A, December to February and B, July to August (from Shepherd and McGinn 2003). 112 S. M. McGinn

A

B

Fig. 4. Average historic minimum temperature (°C) for the periods A, December to February and B, July to August (from Shepherd and McGinn 2003). Weather and Climate Patterns in Canada’s Prairie Grasslands 113

Upper-Level Wind Patterns The dominance of strong upper-level westerly winds east of the Rockies over the entire North American prairie grasslands maintains the continental-like and arid conditions of the air mass over this region (Borchert 1950). In fact, the extension of drought toward the east in many years is linked to the extent of upper-level westerly winds to the east. The wedge shape of the dominant upper-level westerly wind ranges from central Alberta and Saskatchewan, southeast to the lower Great Lake region, and southwest to Texas. The regions of North America to the southeast of the grasslands are influenced by southerly winds that bring tropical Atlantic air. To the northeast of the prairie region, winters are characterized by more abundant snowfall resulting from arctic air across the region.

Surface Wind and Evaporation The average annual surface wind speed across the prairies varies between 14 and 22 km/h, as compiled from 13 major weather stations (measured at 10 m). Spring and fall are usually periods of higher wind speeds close to the ground. Wind gusts of 117 (Edmonton, Alberta) and 171 km/h (Lethbridge, Alberta) have been recorded at these sites. At the mid-latitudes of Canada’s prairie grasslands, the wind direction is highly dependant on the west-to-east movement of low-pressure systems. The wind speed near the surface is important in dictating the movement of winged insects (Pedgley 1982). The layer of air adjacent to the ground within the convective boundary layer (CBL; the height below which air is affected by surface processes to about 1,000 m), where the wind speed is less than the air speed of winged insects, is referred to as the insect’s boundary layer (to about 10 m). However, transport of insects (winged and wingless) also occurs aloft within the CBL. Geerts and Miao (2005) used Doppler radar to report on the transport of insects in the CBL. Wind speed also has an important role in modifying the environment close to the ground. High wind speeds can have a desiccating effect through mechanical mixing of the air adjacent to a surface. However, this is true only when surface water is readily available (e.g., open water surfaces, wetlands, wet soils, and non-water-stressed plants). Increasing wind speed has little effect on surface evaporation when the surface is dry soil or water- stressed plants with high stomatal resistance. Where surface water is limited, the wind speed increases convective heat exchange (warming the air above the surface) to reduce the heating of the surface. Such an exchange is shown in Table 2 for a rangeland site on two adjacent days in 1989 with high and low wind speeds, respectively. In Table 2, although the wind speed on July 2 was more than twice that of the following day, the proportion of radiant energy used to evaporate water did not change (16%). In addition, the windier day saw an increase in convective heat exchange (H) over that of latent heat exchange (LE). This suggests that the higher wind speeds increased H while the LE of the dry surface was not affected.

The Chinook Wind in Southern Alberta A recognized weather feature in southern Alberta is dry and warm air accompanying high westerly wind speeds from the mountains, a feature that is referred to as a chinook wind. Also known as the föhn winds in Europe, these strong winds sweep downward off the eastern slope of the mountains and can desiccate the foothill and prairie landscape in 114 S. M. McGinn

Table 2. Average daytime (0800–1900 h) energy ratios over rangeland near Lethbridge, Alberta, in 1989. H is the convective heat exchange (energy transferred from the surface to air), LE is the latent heat exchange (energy used to evaporate water), and Rn is the net radiation absorbed at the surface (SMM, unpublished).

Parameter July 2 July 3

Wind speed m/s 8.4 3.0

H / LE 4.4 4.1

LE / Rn 0.16 0.16

winter. The importance of the chinook in this region was noted by early settlers such as McCaul (1888) who wrote:

As I sit on my verandah I can see the huge cumuli … depositing their moisture in heavy rains on their snowy summits; and I know that in a couple of hours the heat, that is being evolved by that process, will come down with the western breeze to dry up the mud-puddles …

During a chinook, increases in air temperature of 20 °C within a few hours are normal in mid-winter, with winds gusting between 60 and 100 km/h (Grace 1987b). An example of an extreme chinook episode is shown in Fig. 5 for Lethbridge, Alberta, where significant warming occurred (+30 °C), depleting the snowpack (15 to 3 cm), and thawing the near-surface soil. Golding (1978) reported that during a chinook event, the snowpack can lose (evaporation and sublimation) an average of 1.2–2 mm/day; the maximum recorded rate was 10 mm/day. Once the winter snowpack is lost, the near- surface soil environment can undergo more frequent freeze-thaw and freeze-drying cycling, thus affecting the soil physical properties and soil erodibility (Bullock et al. 1999). These cycles also impact habitats for insects. For example, the removal of snow exposes soil surfaces and reduces surface temperature, thus having an impact on the eggs of grasshoppers located at a depth of about 5 cm (Mukerji and Braun 1988). As an indication of the frequency pattern of chinook winds, wind speeds in excess of 50 km/h in the Lethbridge area occur 6% of the year (Grace 1987b), dropping to 1% near Calgary (approximately 200 km north) and 0.6% close to Edmonton (approximately 500 km north) and Medicine Hat (approximately 150 km east). Longley (1967) reported winter days (December to February) with chinook winds (defined by a maximum air temperature of 14.4 °C) that ranged from 36 days for Lethbridge, decreasing to the north (27 days for Calgary) and east (24 days for Medicine Hat).

Climate Change: Past and Future Understanding climate change is critical for developing adaptive strategies to meet potential changes to prairie ecosystems, such as insect dispersal into new regions. Understanding climate trends also allows for a more complete explanation of ecological shifts of the recent past. Weather and Climate Patterns in Canada’s Prairie Grasslands 115

Recent Historic Trends The most striking aspect of climate change in the prairie region over recent decades has been the strengthening trend toward increased temperatures. This is manifested in several ways. Between 1900 and 1998, the number of days with extreme low air temperature declined in winter, spring, and summer in southern Canada (Bonsal et al. 2001), whereas the days with extreme high temperature increased in winter and spring. Between 1950 and

1989, Tmx and Tmn increased by 1.7 and 1.1 °C, respectively (Skinner and Gullet 1993).

During this period, mean winter Tmx and Tmn increased by 2.4 and 2.1 °C, respectively, with warming in the spring even greater at 3.8 and 2.8 °C, respectively. Associated with this warming is evidence of a decrease in extreme Tmn (temperatures lower than two standard deviations from the mean minimum temperature) in January and February between 1914 and 1994 (Lawson 2003). Most evident is a decrease in January extremes for Saskatchewan. Further confirmation of a warming trend on the prairies has been the significant shift toward a longer growing season. The last spring frost is occurring earlier and the first fall frost is occurring later, thereby increasing the frost-free period and the number of growing degree-days (Bootsma 1994). Coincident with earlier spring warming has been a decrease in the amount of snow and number of snowfall events in winter (converted to rain) in southern Saskatchewan, as well as an earlier spring runoff (Cutforth et al. 1999). Long-term monitoring of ground surface temperatures across Alberta has also suggested a warming trend since 1950, a trend similar to changes in surface air temperature (Majorowicz and Skinner 1997). The increase in ground surface temperature was

Fig. 5. Air and soil temperature, wind speed, and depth of snow on the ground associated with a chinook episode on January 6, 1996, at Lethbridge, Alberta. 116 S. M. McGinn

attributed to both an enhanced greenhouse gas effect and land-cover changes (Skinner and Majorowicz 1999). Changes in precipitation have also been evident on the prairies. Between 1920 and 1995, the number of precipitation events has increased significantly to 16 within a year, mainly due to low-intensity events. During this period, the average amount of precipitation has increased 0.62 mm each year (Akinremi et al. 1999). Between 1956 and 1995, rainfall has also increased by 16%, most of which is presumably due to the conversion of snowfall to rain in spring, coinciding with a warmer and earlier spring (Akinremi et al. 2001). The overall picture of prairie aridity encompasses both increased warming and precipitation. In general, warming increases aridity by enhancing surface evaporation of wet surfaces, whereas increased precipitation directly decreases aridity. The interaction of these factors and whether the prairies are becoming drier is not currently well understood.

Predictions of Future Climate Changes in the composition of the atmosphere brought on by the enhanced emission of greenhouse gases into the atmosphere is predicted to cause global warming of 1.8 to 4.0 °C by 2100 (Intergovernmental Panel on Climate Change 2007). The projected warming will not be spatially or temporally uniform, and the projection varies depending on the forecast model used. For a doubling of carbon dioxide concentration between 2040 and 2060, the Canadian Coupled Global Climate Model (CGCM1-A; Hengeveld 2000) predicts that Canada’s prairie region will warm by about 3.1 °C (McGinn and Shepherd 2003). Precipitation is projected to increase by 4% annually, with Alberta receiving the greatest increase relative to the eastern prairie (Shepherd and McGinn 2003). Rainfall during the July-to-August period in southern Manitoba and southeastern Saskatchewan is projected to decline by 30 mm. For the cultivated grasslands, agronomic models driven with CGCM1-A data indicate earlier seeding dates of 18–26 days. This projection suggests similar shifts toward earlier spring activity for the entire prairie ecosystem, which will presumably impact insect emergence and population dynamics. Thorpe et al. (2008) investigated the climate projections of five global climate models on the productivity of Canadian native grasslands and concluded that despite significant increases in temperature, projected productivity (to 2050) did not change much from current productivity. Elevated carbon dioxide concentrations are expected to moderate any impact of an increase in aridity on grassland ecosystems but will make grasslands more susceptible to nutrient limitations, such as in nitrogen and phosphorus (Sossana and Lüscher 2007). A shift in species diversity may be expected as a result. For example, elevated carbon dioxide concentrations benefit the production of 3C over C4 plant species, but this effect diminishes in more arid grassland environments (Campbell and Stafford Smith 2000).

Confounding the issue further, C4 species benefit more than 3C species in response to increased temperatures (Sossana and Lüscher 2007). The repercussions of changes to prairie ecosystems, in terms of species and nutrients, likely will affect the populations of prairie arthropods.

Microclimate versus Weather and Climate Variability The environment measured near a surface is often different from that reported by nearby weather stations. Weather station criteria for sitting instruments are established to represent the broad landscape of the region and not local features. The difference is especially important in the case of insects where selective habitats have evolved that may differ Weather and Climate Patterns in Canada’s Prairie Grasslands 117

from the broad landscape of the region. Hence, the use of data from weather stations may not be appropriate for characterizing microhabitats of insects that can self-regulate their thermal environment. Archibold et al. (1996) illustrated the differences between selective habitats in the grassland–parkland region, where mid-summer near-surface soil temperature was 6–8 °C higher on the grassland compared with an adjacent aspen grove. Even at the same location, the vertical profile in temperature can be dramatic. For example, at Onefour, in southeastern Alberta, on July 4, 2001, the maximum air temperature was 36.2 °C while the maximum grass height air temperature was 54.1 °C. Such extreme near-surface air temperatures are related to the low evaporative rate and low convective heat exchange between the surface and air. The use of climate data (30-year weather averages) to generally characterize conditions at a point in time would also be inappropriate because the temporal variability in most parameters is high. For example, at Onefour, the 1971–2000 average precipitation (www. climate.weatheroffice.ec.gc.ca/climate_normals/index_e.html) for May is 52.8 mm, whereas the lowest value on record was 0.8 mm (1928) and the highest was 138.8 mm (1965).

Conclusions The principal climatic features of Canada’s prairie grasslands are its low annual precipitation and extremes in temperature. However, these features can be highly variable between locations and seasons. The persistence of peak precipitation early in the summer season, especially in the south, generally ensures some productivity during the remaining more arid summer. Periodically, the prolonged exclusion of cyclonic systems in the region results in low precipitation and meteorological drought. Depending on the timing for plant productivity and antecedent soil water reserves, the effect of drought can vary between locations and years. In general, weather and climate data, and the larger scale patterns depicted by these data, are not well-suited to quantify aridity or thermal regimes of microhabitats needed for ecological studies. However, these data may be useful in describing the relative differences between locations and between years. A good example of this use is the documentation of climate shift denoted by the warming trend across Canada’s prairie grasslands.

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