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The proposed climatic index, named C value, is the coefficient of the third correlative equation that characterizes the dryness (or wetness) of climate, and the third correlative equation deals with heat and water balance related to evaporation. In this article, C value, mean temperature, and summer temperature are combined to predict the distribution of vegetation zones in world. The overall impression from examining the resulting vegetation map is that the location and distribution of vegetation zones in the world are predicted fairly well. Comparison between the predicted vegetation map and the Holdridge Life Zones map are made based on Kappa statistics, which indicates very significant agreement for the Ice/polar desert and Desert. Agreement is also significant for the categories of tundra, boreal forest, temperate mixed and deciduous forest, temperate steppe, subtropical mixed and deciduous forest, subtropical xerophytic woods/shrubs, tropical rain forest, tropical seasonal forest, tropical savanna, and tropical thorn woods/shrubs, even though much larger area of tundra and tropical thorn woods/shrubs were predicted compared to those on the Holdridge life zones map. The results show that C value has a strong correlation with vegetation distribution. As a climatic index, C value can be used for bioclimatic mapping at global scale.