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Size of tropical cyclone (TC) is often asymmetric in nature. Yet, there is a lack of systematic, clean, and intuitive definition/expression to specify the asymmetry of TC size. Here, we introduce a novel index, TC size asymmetry index (SAI), which specifies both the degree and pattern of the asymmetry synthetically. In particular, the symbolic form of SAI is vividly designated for identifying the latter. The SAI proposes 1 quasi-symmetric pattern and 28 asymmetric patterns in total. The 41-year (1979–2019) global climatology of SAI shows that the distribution of the degree of TC size asymmetry is trimodal. Elementarily, the degree and pattern of asymmetry are found to be TC intensity, TC movement, time, and space dependent. The introduction of SAI provides an insight into the subject and lays an important foundation for future applications and research. Furthermore, besides meteorology, it could inspire other fields to index the geometric asymmetries of other kinds. [ABSTRACT FROM AUTHOR]