| ||This document is part of the Agroclimatic Atlas of Alberta.
Climate and weather are key factors in agricultural production. For example, they influence what crops can be grown in a region, the yearly variation in crop and pasture yields, and the amount of water available for livestock.
There is a difference between weather and climate. Weather is what is happening in the atmosphere now and over a short period of time. It is described as wet or dry, hot or cold, cloudy or clear, and windy, stormy or calm. Climate is the long-term average weather conditions describing what generally happens. In other words, climate is what you expect, and weather is what you get.
Four factors determine climate for a given location:
How far north or south you are from the equator has the biggest effect on climate. Both the intensity of the sun's radiation and wind circulation patterns change with latitude.
- position on the globe,
- ratio of water to land,
- wind systems, and
The sun's rays are responsible for heating the earth's surface. Energy from the sun travels about 150 million kilometres (km) through space to our planet. About half of the energy passes through the earth's atmosphere and reaches the earth's surface. The other half is reflected back into space by clouds, scattered by dust particles and water vapour, and absorbed by the gases in the atmosphere and re-radiated.
The amount of solar radiation received at the surface varies with two factors that depend on latitude (Figure 2).
The first factor is the angle of the sun's rays. When the sun appears to be directly overhead (as it is at noon at the equator during the spring and fall equinoxes), the sun's rays are at their most intense. Towards the poles, the sun is lower in the sky, so its rays are more slanted and the same amount of radiation has to pass through more atmosphere and is spread over a wider area on the earth's surface. So the sun’s radiation is most intense at the equator and least at the poles.
The second factor is the hours of daylight. At the equator, the seasonal difference in the hours of daylight is small. As you move toward the poles, the hours of daylight become longer in summer and shorter in winter.
Figure 2. Effect of latitude on seasonal solar radiation and day length
As a result of these two factors, the polar regions of the earth receive less energy from the sun than does the equatorial region. Canada's position in the northern portion of the earth's northern hemisphere means that it receives less radiation compared to countries nearer the equator. The northward decrease in solar radiation is noticeable in Alberta (Figure 2 and Annual Total Global Solar Radiation: 1971 to 2000 map). Cropping is successful in the northern agricultural area of Alberta because the longer summer day length helps compensate for the less intense solar radiation.
Warmed by the sun's radiation, the earth's surface gives off heat. The air above the earth is warmed by this heat, rather than being directly heated by the sun's rays. The amount of heat given off by the earth depends in part on whether the surface is made of water or land. Water bodies warm up and cool down relatively slowly, and maintain moderate temperatures. Land surfaces warm up and cool down more quickly, and reach more extreme temperatures. Large areas of land and water have a strong influence on the temperature and humidity of the air masses above them.
When solar radiation heats up water bodies, some of the water evaporates. The heat stored in the water vapour is released when the vapour condenses to form water droplets. This released energy drives storm systems like thunderstorms, hurricanes and tornadoes. Water vapour is an important vehicle for the transport of stored energy (called latent heat) and helps regulate the earth's temperature.
Air masses are huge bodies of air, usually several hundreds of kilometres across. Temperature and humidity change gradually within an air mass. The moisture content of an air mass is determined by whether it was formed over an ocean or a continent, while its temperature is primarily influenced by latitude. The movement and interaction of air masses create weather.
The general wind circulation in the atmosphere is driven by the exchange of heat between the cold polar regions and the hot equatorial region. This heat exchange prevents the equatorial region from becoming progressively hotter and the polar regions from becoming progressively colder. As the tropical and polar air masses try to move north and south, the rotation of the earth deflects their movement. In Canada, this deflection results in a wide belt, covering the latitude of the Canadian Prairies, known as the zone of the westerlies, so-called because that is the direction from which the wind most commonly blows.
The interaction of the westerly winds with land and water affects climate patterns in Canada. For example, the westerlies blow sea air onshore – mild air in winter and cool air in summer – to give coastal British Columbia its moderate climate. In the Maritimes, the prevailing westerlies blow offshore, giving the east coast a climate more similar to that of Ontario and Quebec. Westerly winds can vary greatly in duration and intensity.
Oceans act as huge storage areas for heat. They absorb heat in spring and summer, and release heat in fall and winter. The oceans also transfer heat and cold around the world in great ocean currents. For instance, the Japanese Current brings warm water to British Columbia's coast, while the Labrador Current on Canada's east coast moves cold Arctic water southward. These ocean currents give adjacent land areas higher or lower temperatures than they would otherwise have and help account for the differences in climate between coastal British Columbia and Newfoundland.
Topography, the physical features of the landscape, is another major factor affecting climate. In western Canada, the Rocky Mountains force air masses from the west to rise and drop their moisture as precipitation, creating wet conditions on the west side of the mountains and drier conditions on the east side. Mountain ranges can also deflect the motion of air masses so that they move parallel to the mountains. Changes in elevation on a smaller scale affect regional or local climates. For example, valleys have very different climates from those of the surrounding level areas.