9.3: Climate Classification
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The purpose of classification is to organize a set of data or information about something to effectively communicate it in an informative way. Classification helps synthesize information into smaller units that are more easily understood. When considering the Earth's climate, there is such an enormous amount of information that one has to break it down into areas of commonality to easily understand it. Climatologists have therefore created several ways to organize the wealth of information about Earth's climate to bring order and understanding to it.
Climate classification systems
There are three fundamental types of classifications used in climatology. First there are empirical systems of classification that are based on observable features. The Koeppen system discussed below is an empirical system based on observations of temperature and precipitation. These are two of the easiest climate characteristics that can be measured, and probably the ones with the longest historical record. It's fairly easy to collect air temperature readings with a thermometer and precipitation with some sort of collecting device that can measure the amount of precipitation. Climates are grouped based on annual averages and seasonal extremes.
Genetic classification systems are those based on the cause of the climate. A genetic system relies on information about climate elements like solar radiation, air masses, pressure systems, etc. The important point here is that we assume we know what causes climate. Though atmospheric science is progressing everyday, we still have a long way to go before we have a complete understanding of the workings of our climate. These are inherently the most difficult classifications to create and use because of the multitude of variables needed.
Applied classification systems are those created for, or as an outgrowth of, a particular climate-associated problem. The Thornthwaite classification system is one based on potential evapotranspiration and thus groups climates based on water requirements. Research conducted by C.W. Thornthwaite and his associates attempted to formulate a water budget technique that assessed water demand under different environmental conditions. His classification system grew out of the issue of trying to predict the supply and demand for water in different climate regions.
Köppen climate classification system
The Köppen climate classification system one of the most widely used systems for classifying climate because it is easy to understand and data requirements are minimal. It is an empirical system largely based on annual and monthly means of temperature and precipitation.
The Köppen system uses a letter coding scheme to classify climate. There are three levels of letter coding except for the A-type climates. The five main groups of climates are designated by capital letters, all but the dry climates being thermally defined. These are:
- Tropical climates (sometimes identified as "equatorial" climates)
- Dry climates (sometimes identified as "arid" climates)
- Warm temperate climates
- Subarctic climates (sometimes identified as "snow" or "boreal" climates)
- Polar climates
The second letter relates to the seasonality of precipitation and the third to an additional temperature qualifier. For the B-type (dry) climates the first two letters are combined, BW for desert and BS for steppe. The third letter is used to subdivide these on the basis of temperature.
As you read through this text you'll notice that the names given to some of the climates reflects the vegetation found there, like tropical rainforest or savanna. The Köppen system has undergone several changes since it was first created and the names have been changed to ones that reflect climate rather than vegetation.