  The phenomenon of Beijing Blue days first occurs on June 11, 2015, and become
a hot society topic in a short time. Thus, Beijing's blue day is not only what
ordinary people desire but national needs. In this work, using the data of
daily average meteorological observations during 1980 and 2014, we select three
standards for defining Beijing blue day (BBI), which contain no rain, clear
sky, and great dry visibility, and the accuracy is 73.4%.Our study finds that
Beijing shares the most significant acceleration in the BTH region, with a
growth rate of 2.66 d/year. The greatest seasonal average and the maximum
acceleration rate appear in winter for total BBI and continuous BBI days, the
least is in summer, while Beijing blue appears most frequently in January. BBI
is more related to relative humidity and rainless day on an annual scale, but
different in different seasons.
