Abstract: This paper is focusing on discussing EERTD,which stands for equal effects reasoning in three dimensions.EERTD is able to describe the developing procedures of weather systems fully and exactly in three aspects of time,space and properties,with the help of time-strength-coefficient, space-shifting coefficient and concept-transfering-coefficient.EERTD can help to overcome the brittll- ness of expert systems,and enhance the capabilities of expert systems adapting to initial evidences and intermediate evidences.