Figure 2.

Diel changes in the colour of skylight on two separate occasions in south-eastern Wisconsin. (A) Change in dominant wavelength over the course of a day; red line = summer, blue line = winter. (B – D) Spectral composition of skylight at three representative points of the summer sky; (B) pre-dawn/post-sunset night sky; (C) sunrise/sunset; (D) mid-day. While taken from the summer morning, these spectra are representative of both the summer and winter data, and also characterize the events surrounding sunset.