Thornthwaite's scheme of climatic classification is a comprehensive water-balance-based system that first estimates potential evapotranspiration (PE) from mean monthly temperature using an empirical formula, and then derives a moisture index by comparing actual precipitation with this calculated potential evapotranspiration over the course of the year; because the calculation of PE itself depends on temperature, and the classification further incorporates precipitation values directly, the overall scheme is fundamentally based on the interplay of temperature, precipitation, and potential evapotranspiration together, rather than on any single pairwise comparison alone.