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This paper illustrates the development and analysis of several transmission system models which include varying weather conditions for composite system reliability assessment. The failure rate of an outdoor component can be much higher in stormy weather than that in normal weather periods. The probability of overlapping failures in stormy weather, therefore, can be much greater than that in normal weather periods. This phenomenon is often called `failure bunching¿¿ due to the fact that components are fully or partially exposed to a common weather condition. Several methods, designated as the constant weather model, the full Markov process approach, the 4-state approximate method and the line or area addition approach are presented. The impact of these weather models in composite generation and transmission system adequacy evaluation is examined and illustrated in this paper using the IEEE reliability test system.