Balloon Flight Over Central Italy with Blue and Black Sky
November 03, 2011
The photo above was taken onboard a high-altitude balloon flight (StratoSpera 3 — the Italian StratoSpera Project) above Tuscany, Italy on September 10, 2011. The altitude is 129,892 ft (39,591 m). This photo is perhaps the highest ever captured from a non-scientific balloon. The view here is toward the south – the Tyrrhenian Sea is at top center and Lake Bolsena is at the upper left.
Note the blue color of the sky along the Earth’s limb and the black above it. This blue color is due to scattering of white sunlight – Rayleigh scattering. Scattering by very small particles (molecules of air) is greater at the violet end of the electromagnetic spectrum than at the yellow or red portions of the spectrum. Because our eyes aren’t as sensitive to the violet wavelengths as they are to blue colors, the sky takes on a bluish hue. At sufficiently high altitudes there are simply too few air molecules to effectively scatter sunlight and the sky is therefore mostly devoid of color. Passengers riding in the Concorde, altitude of 60,000 ft or (18,000 m), were able to observe a mostly black sky during daytime trans-Atlantic crossings.