When using temperature and precipitation to define climate, the two must be combined in a single method. As seen in the previous part of this lesson, the temperature designations were mostly latitudinally driven. Moisture varied through other considerations causing the B (dry) climates to vary throughout latitude bands. Another system was developed to account for differing climates based mostly on the water balance of a region rather, the temperature and precipitation specifically.

Thornthwaite developed another classification system dependent on the modified potential evapotranspiration (PET) of a region. His concern was with the amount of water available. The driving factor in this system is the water budget of a region. Regions were classified in categories ranging from being humid to arid. The main factor used in classifying climates is then what is most important in the classification. The Thornthwaite system uses temperature as a modifying factor in determining PET, but it is not the major factor as in the Koeppen system. Temperature in this system was a controller of PET, not a direct classification.

eq-pet

Where the index Im is based on precipitation P and Potential Evapotranspiration PE. For the US this leads to Fig. 8.6

map
Fig. 8.6 United States climate types based on the Thornthwaite system.