Statistically expected early first-frost days with return periods from 2 to 30 y (EFFDk, where k represents the return period) were calculated by fitting frost records obtained at weather stations to the Weibull distribution function. To create 1 km2 mesh maps of EFFDk, a method to estimate EFFDk was developed by relating k to the normals of daily minimum air temperature (Tm).An equation to calculate the probability of occurrence of the first frost before day x ((P (X ≤ x)) from Tm was derived. Risk of early first-frost damage to soybeans was defined as (P (X ≤ maturity day). Day of maturity was estimated with existing soybean developmental models, and risk maps of first-frost damage to soybeans were created.
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Sameshima et al. (2007) studied this question.