Scatter plots of the observed minus fitted f_oF₂, Δf_oF₂, versus <ap>_24 h at Petropavlovsk in (a–d) four seasons; the red dots and error bars are mean values and corresponding lower and upper quartiles of gridded Δf_oF₂, respectively. The parameter r is the correlation coefficient between Δf_oF₂ and <ap>_24 h, and the number in the parentheses is the corresponding P-value indicating statistical significance of the correlation coefficient.