Canada is a very large country with many different climates. It extends 4500 km, from 42° N lat (at Pelee Island, Lake Erie) to 83° N (the tip of Ellesmere Island). Thus, there are great differences in length of day from south to north: in December southern Canada receives 8 hours of daylight; Canada's northern tip, none. Latitude is also responsible for the generally westerly air flow in Canada.
Southern Canada is often the battle zone between cold air from arctic regions and warm, moist air from the south; hence, airflow is channelled east-west. Latitude is not the only geographic factor significant to climate; position on the continent, especially distance from the OCEANS, also plays a part. Therefore, places with roughly the same latitude, eg, Victoria and Winnipeg, have very different climates since Victoria's climate is modified by the warm Pacific Ocean and Winnipeg's is not.
Temperature is the degree of warmness or coldness. Air temperatures have been measured in some places in southern Canada for over 100 years, but in the Arctic for a much shorter time. Climate statistics are usually expressed as 30-year averages published every 10 years, the most recently published being those for 1961-90. The next climate statistics published will be for 1971-2000. In winter, when northern Canada receives very little solar radiation, temperature differences from north to south are great.
The average maximum January temperature of Alert, at the northern tip of Ellesmere Island, is -28.1°C, that of Windsor, Ont, -0.7°C, a difference of 27.4°C. In summer the long days in northern Canada result in smaller north-south differences, with maximum temperatures in July of 6.8°C for Alert and 27.8°C for Windsor.
Monthly air temperatures give an indication of the hotness or coldness of a place at a particular time of year, and monthly temperature maps can be drawn for Canada. However, it is also desirable to have a single map which expresses the total amount of heat received by a given place.
To express this total, climatologists have formulated an index of potential evapotranspiration (PE), derived from monthly temperatures above 0°C. Potential evapotranspiration is the amount of water that potentially would evaporate and transpire from a vegetated surface; therefore, PE is an index of heat, but expressed in terms of depth of water at the Earth's surface.
The high arctic islands, with a PE of under 200 mm, receive very little heat; the warmest areas in southern Ontario, Québec and BC receive 3 times as much. Canada is obviously not a warm country since tropical areas have an annual PE of over 1500 mm.
The coldness of a place is also of interest, especially in these days of expensive energy, when coldness can be equated with the cost of heating homes. A parameter which summarizes coldness is the heating degree day (HDD), a unit which sums the temperatures below 18°C, the point at which household furnaces must be turned on. Southern Ontario has 4000 HDD annually, while the high arctic islands have 12 000 HDD.
The most important climatic parameter after temperature is precipitation, ie, moisture that falls to the Earth's surface as RAIN, snow, HAIL, etc. Because of Canada's size, there are great differences in the amounts of precipitation received. Since warm air can hold a great deal more moisture than cold, southern parts of Canada have more precipitation than northern parts. Because WINDS usually blow from west to east, West Coast areas receive the most precipitation. There are other complicating factors: the north-south trend of the western mountains greatly complicates the precipitation regime there, and the Great Lakes in central Canada affect precipitation in that region.
Usually, measurements of average annual precipitation are made where people live; therefore, the precipitation map is most accurate for southern Canada. The isohyets (lines of equal precipitation) are very generalized in mountainous regions and refer to valley conditions; precipitation totals at higher altitudes are not well known. Annual precipitation ranges from 100 mm in the High Arctic to over 1500 mm on the windward side of BC's mountains. Areas of anomalously high precipitation caused by increased winter snowfall appear east of the Great Lakes.
Defining climatic regions for any country is difficult; usually, climatologists discuss the various climatic parameters (eg, temperature, precipitation) and conclude by using vegetation as an index. Within a geographical area, climates gradually change from one type to another. There are 5 main climatic regions in the southern populated area of Canada: East Coast, Great Lakes, Prairies, Cordilleran and West Coast.
The vast, mostly uninhabited area of northern Canada contains several different climatic regions but for the present purposes will be subdivided into Arctic and Subarctic. The stations described below are examples of the above regions, with the exception of the Cordilleran.
The Cordilleran, or Rocky Mountain, region is a composite of many climatic types. The southern BC valleys have climates that are the driest in Canada, while some of the higher elevations, especially in the YT area, contain permanent ice caps. No one station is typical of its province.
Climate types result from the relationship between monthly potential evapotranspiration and precipitation. By comparing the PE or need for water with the supply for each month, graphs of the water balance can be obtained. In the settled parts of Canada, water need is usually zero during the winter months, rising to an average of 130 mm in July.
Only Pacific Coast regions have temperatures high enough to cause a need for water in every month of the year. Monthly precipitation varies widely throughout Canada, and nowhere does it coincide with monthly PE. Most localities receive too much moisture in winter, when the PE is zero, and too little in summer, when maximum need occurs.
East coast climates are represented by Halifax, NS. Precipitation is fairly uniform throughout the year and only in July does water need exceed supply. The vegetation does not suffer immediately since a moisture reserve is stored in the soil (see GROUNDWATER). For most soils the reserve is approximately 100 mm.
In Halifax, this reserve is not exhausted before precipitation rises above need and the stored moisture is again built up to its maximum. Thus, for Halifax the actual amount of water loss is almost equal to the potential water loss. After soil moisture has reached its maximum, surface or subsurface runoff occurs; in Halifax, total annual runoff is 773 mm.
Southern Ontario climates are typified by Windsor. Precipitation is rather uniform throughout the year (although less so than at Halifax), but Windsor's summer PE is greater than that of the East Coast, reflecting the higher summer temperatures in the interior of the country.
In May PE rises above precipitation; by mid-July, in an average year, the stored soil moisture is exhausted. From then until precipitation is again sufficient to build up the stored soil moisture, vegetation must depend solely on current precipitation. In Windsor this is insufficient and a water deficiency of 86 mm occurs in an average year.
Prairie climates are exemplified by Edmonton. Here, the annual precipitation of 447 mm is inadequate to meet the PE of 555 mm and deficits, averaging 120 mm, are common in the summer months. With low winter precipitation, soil moisture is not always restored to capacity in an average year and water surplus averages only 7 mm (see DROUGHT).
Subarctic and Arctic
At Inuvik, north of the Arctic Circle, the monthly PE and precipitation resemble those of stations in the Prairies, but the growing season is shorter. Thus, there is a small amount of runoff as well as a deficiency of 100 mm. Alert, in the High Arctic, has a more severe arctic climate, with a growing season of one month and low precipitation. It too experiences water deficiency but has an annual runoff of 76 mm.
West coast climates are characterized by a winter maximum and summer minimum precipitation regime. Victoria is a typical station. Mean monthly temperatures are usually above 0°C and water is needed in every month. In an average winter, 130 mm of rain falls every month and the PE is only 14 mm; therefore, large amounts of runoff occur in winter. In summer, precipitation is least when need is greatest and large water deficiencies occur.
The moisture map shows the distribution of moisture deficiency and runoff in Canada. The lines showing average annual water deficiency indicate the areas where precipitation is inadequate for vegetation needs and moisture is a problem. Even in southern Ontario, deficiencies occur regularly in summer. West Coast and East Coast areas rarely suffer moisture deficiencies.
Much of the precipitation that falls in Canada evaporates again into the atmosphere (evapotranspiration); the water that does not evaporate runs off into the rivers and lakes and eventually to the ocean. The lines showing average annual runoff are almost the reverse of those showing average water deficiency. Canada's interior has little or no runoff. Eastern and western areas have large amounts; these are the areas which have major hydroelectric developments.
The above description of Canada's climatic types refers to average conditions; however, climate is very variable. For example, half the time the precipitation is less than the amounts shown and half the time it is greater. Consequently, climate involves more than average weather, and another series of maps showing the "expectation of weather" could be drawn.
Author MARIE SANDERSON
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