Abstract: The discovery of the atmospheric long wave (Rossby wave) together with the establishment of the related theory is an important milestone in the history of meteorology. The ideas beneath atmospheric long wave dynamics are deeply and widely permeated in modern dynamical meteorology and climate dynamics. It serves as a theoretical basis for the numerical weather prediction and in a sense, turns meteorology into a science in name and in fact. This paper briefly reviews how Carl-Gustaf Arvid Rossby discovered the atmospheric long wave and derived its phase speed from his theory. The emphasis is mainly on the process and the scientific methods during the development of the atmospheric long wave theory in 1940s and 1950s, so that readers can realize the physical intuition from a higher plane and the excellent mathematical treatment of Rossby. By this means, it is hoped that both meteorologists and young students could be motivated.