Chapter 8 Climatic Classification
CHAPTER 8 CLIMATIC CLASSIFICATION
Early Attempts at Global Climatic Classification
o Very early attempts by the ancient Greeks at classifying climate were logic-based, and resulted in Paramenides’ identification of three principal climate regions; the Frigid Zone, the Temperate Zone, and the Torrid Zone
o Other climatic classification schemes followed, including one by Hipparchus who updated the Paramenides classification by including information on the calculated day length for particular locations
o Logic-based climatic classification systems ruled until the development and proliferation of weather recording instrumentation
The Classical Age of Climatic Classifications
· The Modified Köppen Climatic Classification System utilizes monthly temperature and precipitation data in making calculations upon which the classification scheme is based
o Köppen identified five main climatic groups: A (tropical), B (arid), C (mesothermal or mid-latitude mild), D (microthermal or mid-latitude cold), and E (polar)
o In general, the A, C, and D climates support the growth of trees, whereas the B and E climates do not, being either too dry or too cold, respectively
o In the case of A, C, and D climates, the second-order subdivision refers to the precipitation seasonality (with “f” representing climates that are wet year-round, “s” indicating summer dry climates, “w” representing winter dry climates, and “m” representing tropical monsoon conditions)
o For B climates, the second-order subdivision is “S” if the dry climates are only semi-arid, and “W” if the dry climates are true deserts
o In the case of “E” climates, the second-order subdivision is “T” for the milder Tundra sub-type of polar climate, while “F” (frozen) is used to represent the Ice Cap subtype
o For the mesothermal and microthermal climates, third-order subdivisions identify the characteristics of summer temperatures, with “a” representing hot summers, “b” used for warm summers, “c” indicating mild summers, and the rare “d” indicating cool summers
o Arid climates have a third-order subdivision of “h” and “k” which are used to denote “hot” and “cold” arid or semi-arid regions, respectively
· The Thornthwaite Climatic Classification System is built on the physical interactions between local moisture and temperature rather than only the precipitation and temperature data. It represents a more sophisticated and precise scheme of classification based on local surface water balances
o Thornthwaite devised a number of specific indices to quantify necessary climatic components, including the moisture index (MI) and the potential evapotranspiration (PE) rate for a location
o Thornthwaite also derived a Thermal Efficiency Index (T/ET) of the ratio of temperature (T) to a calculated evapotranspiration (ET) value, and a Dryness Index (DI) and Humidity Index (HI) to identify the times of the year with water deficit or surplus
· Other Global Classification Systems
o The Holdridge Life Zones Climatic Classification System was intended to be for global application, but it became most widely used in tropical areas, where it has proved useful in ecological and alpine applications
o The Budyko Climatic Classification System uses an energy budget approach to classifying climates
Genetic Classifications
· Strategy is to classify climates solely on the basis of the major forcing mechanisms that make climate the way that it is
· The Bergeron Climatic Classification System, devised by Swedish meteorologist Tor Bergeron in 1928, is one early-modern genetic classification system. This system categorizes the climate at a location based on the manual and somewhat subjective determination of frequency with which it is dominated by certain types of weather
· Air Masses and Fronts
o The Bjerknes model classified areas of a surface cyclone and associated pressure features into specific sectors with each section exhibiting particular weather characteristics
o An air mass is a body of air that is relatively uniform in its characteristics for distances of hundreds to thousands of kilometers, with its characteristics resulting largely from the characteristics of the place where the air mass forms – the source region
o Air mass types include: the intensely-cold and dry Arctic (A) type; the slightly warmer and more humid continental polar (cP); cool, humid maritime polar (mP); warm, humid maritime tropical (mT), hot, dry continental tropical (cT), and the “E” air masses of equatorial origin – a more extreme version of the mT type
o A cold front represents a situation in which a colder air mass is pushing a warmer air mass back toward its source region
o A warm front is a region where a warmer air mass is displacing a colder one poleward
o In a stationary front, the cold and warm air mass may temporarily be at a stalemate, with neither air mass pushing the other backward
o A cold front and warm front generally emanate from a central region of low pressure in a midlatitude wave cyclone
o After the midlatitude wave cyclone travels eastward for a few days in the mid-latitude westerlies, the cold front will actually catch up with the warm front near the cyclonic center, where the two fronts are closest to each other, resulting in an occluded front
Local and Regional Classifications
· The Lamb Weather Types and Muller Weather Types are based on the mid-latitude wave cyclone model
o These are manual systems – meaning that the researcher subjectively categorizes the types based on his/her interpretation of the weather map on a given day
Quantitative Analysis to Derive Climatic Types
· Eigenvector Analysis is helpful for identifying relationships between two different sets of variables (such as spatial data points, observations of an atmospheric variable through time, or atmospheric variables) to be analyzed simultaneously
o In synoptic typing, eigenvector analysis may be used whereby the atmospheric variables may collectively represent the properties that dominate, and locations where similar characteristics of the atmosphere tend to occur simultaneously would be categorized as part of the same type for that entity of time
· Cluster Analysis
o The actual classification process in any eigenvector analysis (including PCA) comes when the matrix of loadings or scores (which usually represents the spatial and temporal variability (respectively) in the data set) is subjected to a cluster analysis
o Several types of cluster analysis can be used with each giving a slightly different classification result, but all have the common goal of selecting the groups so that the distance in n-dimensional space between the points within the cluster (within-group variability) is minimized
· Hybrid Techniques
o Typically require the investigator to identify “prototype” atmospheric circulation patterns manually – a “textbook” case of each particular genetic category, with the computerized cluster analysis used to categorize all other days (or months, if monthly mean data are used) automatically, quickly, and efficiently, into the group with the prototype day (or month) that the weather map for the day (or month) in question most resembles