Global climatic factors, especially temperature and precipitation, most strongly influence soil formation. Temperature acts on the reactions involved in soil processes in a variety of ways. It controls the speed of chemical reactions as described by the well-known Van't Hoff's temperature rule. However, all chemical reactions cease in the absence of soil moisture due to freezing or drying up. Temperature also determines the type and biomass of vegetation closely relating to soil formation. Soil moisture supplied by precipitation is crucial to the forming and functioning of the soil. It contributes to the dissolution, neoformation and translocation of materials and facilitates the growth of vegetation that also acts on soil formation (Buol et al., 1997).

Although there are many treatises dealing with this subject, we will select a conceptual and comprehensive one published by Ugolini and Spaltenstein (1992) and will begin this chapter by introducing it. We will then describe the influence of climatic factors on soil formation, mainly based on our studies of volcanic ash soils which have several distinct advantages for pedogenesis studies. The age, rock types, and chemical and mineralogical properties of volcanic ash as a parent material can be easily determined. Volcanic ash is commonly unconsolidated and comminuted, and is dominated by volcanic glass, which shows the least resistance to chemical weathering (Shoji, 1986). Therefore, volcanic ash rapidly forms a variety of soils in all climate zones, and the soil processes involved are highly accentuated. Intermittent volcanic ash deposition and repeated pedogenesis commonly create sedentary multistoreyed soils.