This paper introduces user-friendly atmospheric data, so-called objective analysis data, for ornithological studies. The data have been interpolated onto grid points distributed at a regular interval in space and time, and are suitable for analyzing using computer data analysis software. Data assimilation techniques, which are basically the application of optimization and control theories, are utilized for producing objective analysis data in order to reduce errors as much as possible and obtain the most reliable dataset. Some examples of objective analysis data are shown and their features are described. Some cautionary notes are also given in order to avoid misinterpretation of the data.