Revealing the Twilight Sky Color by Photographic Observations and Theoretical Calculation

A Master-course student in our laboratory, Masanori Saito, has studied the effects of tropospheric aerosol on twilight sky color based on photographic observations and theoretical calculations. The result has been published in a paper in peer-review journal, SOLA (Scientific Online Letters on the Atmosphere), by him and co-authors on Feb. 28, 2013.

The twilight sky is one of the most beautiful optical phenomena on this planet. We can see the twilight sky before sunrise or after sunset, and the colors of the sky varies day by day (Fig.1). Why is the twilight sky beautiful on clear day? This is because direct solar light is scattered by atmospheric molecules such as nitrogen, oxygen and water vapor, and as a result, twilight sky color beautifully gradates from red near the horizon to blue at the zenith. However, underlying light scattering process is complex due to additional light scattering effects by aerosol particles in the atmosphere. In this study, we observed a phenomenon that twilight sky color near the horizon became blue by photographic observations under a specific condition. With a hypothesis that a main reason of this phenomenon is effects of tropospheric aerosol, we reproduced the phenomena well by a theoretical calculation. Moreover, we also theoretically revealed the mechanism of the phenomena, which is described in the paper. Studious observation and deep consideration sometimes bring a new finding of a phenomenon even if the phenomenon is common. There might be more interesting phenomena in our daily life


Fig. 1.  A photograph of twilight sky.


Published paper:

Saito, M., H. Iwabuchi, and T. Hayasaka (2013), Physical explanation of tropospheric aerosol effects on twilight sky color based on photographic observation and radiative transfer simulations, SOLA, 9, 015–018, doi:10.2151/sola.2013-004