An original and general method is proposed to predict the impact of electromagnetic effects due to the environment on the error performance of microwave communications systems. The method is based on the concept of a process-oriented model which consists of a simplified representation of the physical phenomenon responsible for transmission impairments and simulates its influence on the signals received by any given communications link. The method yields a thorough characterization of the system impairments in both the time domain, by calculating the fluctuations of the received signal during the evolution of the physical process, and the frequency domain, by inferring the communication channel transfer function from the frequency dependence of the received waveform. In addition, by introducing the simulated channel transfer function as a filter in a communications system simulation software, the method evaluates the degradation induced in the system performance. Three applications as different as scintillation due to turbulence in clouds, depolarization by rain and ice crystals, and interference in the presence of diffraction on obstacles demonstrate the ability of the method to deal with actual radio-communications system impairments.