The grouping of climates into broad types according to their shared characteristics. There are three principal approaches to the task. (a) Generic classification is based on levels of temperature and aridity as these relate to vegetation boundaries. Aridity is usually expressed as ‘effective precipitation’, which is calculated as the ratio of rainfall to temperature. Climatic types are defined by the response of flora to them. The Köppen system, with its modifications, uses this approach. (b) Classifications based on the moisture budget and ‘potential evapotranspiration’ (i.e. the maximum moisture that will be transferred from the ground to the atmosphere, provided that sufficient moisture is available) which do not rely on vegetation boundaries. The Thornthwaite system uses this approach. (c) Genetic (i.e. pertaining to its origin) classification, based on factors related to the atmospheric circulation of major winds and air masses, and on other factors that cause climate, is used in the systems of H. Flohn (1950) and A. N. Strahler.