What Is Climate?
The climate of a place may be defined as a "composite" of the long-term prevailing weather that occurs at that location. It is the
normal weather pattern for that place and we know that it will recur, with some small variations perhaps, from year to year. Our
sense of a place's climate leads us to expect that temperatures at a particular time of year will fall within a
certain range, that They are unlikely to rise above or fall below certain extreme values, that enough rain will fall at certain times of year to sustain
crops, and do on. Even the exceptions to these patterns often occur with some regularity. Thus, although rainfall may normally
be reliable in a particular region, we may also know that it is unlikely that a ten-year period will go by without at least one
serious drought.
It is this regularity that makes climate so important, because it creates the long-term conditions that allow ecosystems to evolve
and survive. For human societies, this reliability is equally important for such basic activities as agriculture and settlement. When
climates change, all species, including humans, must either adapt or migrate if they are to avoid major stress.
Ultimately, climate is a consequence of the way the atmosphere redistributes the sun's energy. Because the intensity of solar
radiation changes with latitude, the tropics are heated much more intensely than the poles. It is this imbalance that drives the
complex pattern of atmospheric and oceanic motions that redistribute heat and moisture from one part of the globe to another
and cause our weather.
Local regional climates are highly influenced by latitude, altitude, topography and the proximity of large water bodies such as
the oceans. Globally, climate is affected by complex interactions involving the sun, the land, the sea, the air, the earth's ice cover,
its plants and all other life forms.
Measuring Climate
Climate is usually described in terms of the familiar elements of the weather. Temperatures and precipitation are the essential
indicators, but other are sunshine, wind, cloud cover, atmospheric pressure, humidity and evaporation can be added to provide
a fuller and more complete picture. When these elements are measured systematically at a site over a period of several years,
we eventually accumulate an archive of observations from which we can construct an accurate summary of that place's climate.
Using a variety of statistical techniques, we can compute averages for different climate elements as well as measures of
variability and frequency of occurrence.
The standard practice is to base the calculations on the last three complete decades. The resulting summaries are referred to as the climate "normals" and
they are updated at the beginning of every new decade. The climate record since 1941, for example, is covered by
three sets of normals: 1941-1970, 1951-1980 and 1961-1990.
Like any other statistical measure, climate summaries are only as good as the data on which they are based. For the past 150
years or so, regular programs of observation using reliable instruments under controlled conditions have produced a large body
of relatively high quality data for many locations around the world. At present there are some 2200 climate stations across Canada which provide twice daily readings of temperature and
precipitation. More detailed information for weather forecasting purposes comes from another 500 principal and automatic
stations.
Created by: Erin Stewart and Christina Duskey