The genesis of tropical cyclones depends on atmospheric circulations with a wide range of scales both in time and in space. While synoptic-scale disturbances exist all the time in the tropics, only a small percentage of them developed into tropical cyclones. Grey (1974) identified environmental conditions suitable or necessary for the development of tropical cyclone (TC). Due to recent improvements of numerical models, data assimilation techniques and the application of remotely-sensed data, global analysis can provides us much more reliable representation of atmosphere conditions than before. In this study, the global daily analysis from the U. S. Navy Operation Global Atmosphere Prediction System (NOGAPS) is examined to understand why some tropical disturbances formed tropical cyclones while others did not. Among the atmospheric parameters examined the mid-level moisture content of the disturbance, vertical wind shear and large-scale forcing stand out as important factors. The differences between TC formation conditions in the Atlantic and the Western Pacific will be reported.