5.7: Climate Classification

Overview 

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:

1. Tropical climates (sometimes identified as "equatorial" climates)
2. Dry climates (sometimes identified as "arid" climates)
3. Warm temperate climates
4. Subarctic climates (sometimes identified as "snow" or "boreal" climates)
5. 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 second lowercase letter specifies the seasonal distribution of precipitation for groups A, C, and D:

• f (Wet/Moist) Rain falls relatively evenly throughout all seasons; there is no distinct dry period (e.g., Af - Tropical Rainforest).
• m (Monsoonal): Experienced in tropical regions undergoing intense seasonal wind shifts, featuring a short, severe dry season countered by torrential rains during the rest of the year.
• w (Winter Dry)The winter season is distinctly dry, with the majority of precipitation falling during the high-sun summer months.
• s (Summer Dry) The summer season is distinctly dry, a defining characteristic of Mediterranean climates where winters are wet and stormy.

For the B (Dry) group, uppercase second letters designate the level of aridity:

• W (Desert): True arid wasteland conditions receiving minimal precipitation.
• S (Steppe): Semi-arid grassland regions acting as a transition zone between true deserts and more humid climates.

The third letter is used to subdivide these on the basis of temperature. 

• a: Hot summer (warmest month averages above 22 degrees Celsius).
• b: Warm summer (warmest month is below 22 degrees Celsius, but features at least four months above 10 degrees Celsius).
• c: Cool, short summer (fewer than four months over 10 degrees Celsius).
• d: Extremely severe winter (coldest month drops below minus 38 degrees Celsius, found only in Group D).
• h: Hot sub-type (used in Group B to indicate an annual average temperature above 18 degrees Celsius).
• k: Cold sub-type (used in Group B to indicate an annual average temperature below 18 degrees Celsius).

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.

Figure \(\PageIndex{1}\): World Climate patterns according to Köppen (Courtesy NOAA (Source))

The map in Figure \(\PageIndex{1}\) shows a world map of Köppen Climatic Zones using color shading to represent climate regions. A legend along the left side lists climate classifications including Af, Am, As, Aw, BSh, BSk, BWh, BWk, Cfa, Cfb, Cfc, Csa, Csb, Csc, Cwa, Cwb, Cwc, Dfa, Dfb, Dfc, Dfd, Dsa, Dsb, Dsc, Dsd, Dwa, Dwb, Dwc, Dwd, EF, and ET.

• Group A (Tropical Climates) appears in dark and medium green shades along the equatorial regions of South America, Central Africa, Southeast Asia, and Indonesia.
• Group B (Dry Climates) appears in yellow and tan shades across the Sahara, Arabian Peninsula, Central Asia, interior Australia, western North America, and parts of South America and southern Africa.
• Group C (Warm Temperate Climates) appears in light green shades along many midlatitude coastal regions, including the southeastern United States, western Europe, eastern Asia, southern South America, southern Africa, and southeastern Australia.
• Group D (Continental or Severe Midlatitude Climates) appears in orange and brown shades across much of Canada, Alaska, northern Europe, and northern Asia.
• Group E (Polar Climates) appears in light purple and pink shades across Greenland, the Arctic margins, and Antarctica. Labels ET and EF identify tundra and ice-cap regions.
• Latitude and longitude grid lines are shown across the map. Oceans are labeled including the Pacific Ocean, Atlantic Ocean, and Indian Ocean.