Abstract
The color of the sky in day-time and at twilight is studied by means of spectroscopy, which provides an unambiguous way to understand and quantify why a sky is blue, pink, or red. The colors a daylight sky can take primarily owe to Rayleigh extinction and ozone absorption. Spectra of the sky illuminated by the sun can generally be represented by a generic analytical expression which involves the Rayleigh function , ozone absorption, and, to a lesser extend, aerosol extinction. This study is based on a representative sample of spectra selected from a few hundred observations taken in different places, times, and dates, with a portable fiber spectrometer.
Highlights
Two major causes of the extinction of light by the atmosphere, extinction by air molecules and ozone absorption, were discovered in the second half of the XIXth century when scientists1 questioned why the sky is blue
Agree that the blue color of the sky in day-time is due to Rayleigh scattering and that blue nuances at twilight, when the sun is low on the horizon, require an additional ozone absorption [11,12,13,14,15,16]
The red color of the sky in these observations results from Rayleigh extinction which diminishes the blue part of the spectrum, and from ozone absorption which isolates the red bump on its blue side and shifts its central wavelength towards the red
Summary
Two major causes of the extinction of light by the atmosphere, extinction by air molecules (nitrogen mainly) and ozone absorption, were discovered in the second half of the XIXth century when scientists questioned why the sky is blue. Fujii had exactly the shape of occultation spectra of the sun (which I knew from [20]) multiplied by 1/λ4 They should be interpreted as sunlight extinguished by the atmosphere Rayleigh-scattered by air-molecules in the direction of the observer. The obvious advantage is to free the conclusions from arbitrary inputs, either parameters such as column densities of nitrogen or ozone along the path of light, which are precisely the quantities observation should retrieve, or the specific geometry of the scattering and the structure of the atmosphere, which are always difficult to ascertain.
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