Environment Canada - Atlantic Climate Centre

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 invlving 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.

Because climate is variable, a single locality's climate record may show different climates for different periods of time. This is particularly likely where the climate record is long and substantial changes, such as urban growth, have greatly altered local conditions. How then do we decide which of these climates is the normal climate for that place? 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.

The use of standard climate normals makes it much easier to compare climate data from different locations. As long as the base values, length of record and method of computation are compatible, reliable comparisons can be made between various parts of a country or between different parts of the world. To facilitate such comparisons, the World Meteorological Organization (WMO), an agency of the United Nations, has issued guidelines and regulations for recording and processing climate data. These have greatly benefited international climatological studies and have been essential to current international efforts to study global climatic change.

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. Nevertheless, changes in instruments, sites and procedures, alterations in local conditions and incomplete observations can all introduce errors which may give a false reading of climatic change. Careful processing of the data, however, can minimize the effects of such distortions.

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.

(from Gullett & Skinner, SOE Report No. 92-2, Environment Canada, July 1992)