Trend in the seasonal numbers of Moscow NLC nights. (a) The number of NLC nights according to the Moscow data for 1990 to 2013 without a weather correction factor. (b) Relative (weather-corrected) numbers of NLC nights. The script in the upper left corner shows the 2-D linear regression model versus time and solar Ly α flux. The contribution of both arguments is given by the blue line, the contribution of the time-trend is shown by the black thick line, and the thin red curves denote 95% confidence limits.