This research was an investigation of changes in driving behavior that occurs in response to rainfall intensity, especially focusing on risky behaviors. This was done using driving records of 620 taxis in Seoul (South Korea). We utilized driving volatility as a quantitative measure of driving behavior. This parameter indicates the variability of vehicle movement as indicated by vehicular acceleration and jerk. The result verified that, as the rainfall intensity increases, driving patterns deviate more from those without rainfall. From these changes, a measure of aggressiveness was derived considering these behavioral differences under different rainfall conditions. In particular, volatile and risky driving decisions with respect to jerk occur more frequently as rainfall intensity increases. This implies that changes in acceleration (e.g., acceleration after deceleration, deceleration after acceleration) are prevalent in rainy days. Furthermore, using crashes and law violation information about taxis, this research verified that higher volatility is related to a higher likelihood of crashes and law violations. The contributions of this study are that it quantifies the aggressiveness of drivers as a reflection of changing driving behavior under different rainfall conditions, and verifies the volatility index by relating to crashes and traffic law violations of individual drivers.