Purple Light and Twilight Arch

December 14, 2009


Claudia Hinz
Summary Authors: Claudia Hinz; Jim Foster

The photo above showing a regal purple sky and twilight arch was observed on the morning of November 17, 2009, from Mt. Wendelstein, Germany in the Bavarian Alps. I was quite astonished to see this intense purple sky about half an hour before sunrise (Sun elevation at -6 degrees). The unusual light was a result of dust particles from the Sahara Desert. In fact, two separate streams of Saharan dust were positioned overhead; a lower one at an altitude of about 5 miles (8.5 km) with dust from the western parts of the Sahara; and a higher one at about 7 miles (11 km) containing dust from the eastern Sahara. This dust, entrained in two different but related atmospheric currents, buckled north from the Saharan region of Africa and happened to overlap just above the Alps.

As the optical thickness of the atmosphere increases, more light is scattered. When no clouds are present, as was the case this morning, in order for the twilight sky to show coloration a significant portion of sunlight must be scattered. Dust particles readily scatter sunlight. In general, scattering is more pronounced when smaller dust and aerosol particles (100-1,000 nanometers in diameter) are present, producing deep purple sunrises and sunsets. Purple twilights are best observed when the Sun is between about 2 and 6 degrees below the horizon. Note the faint crepuscular rays emanating from the horizon. It was observed that these rays stretched across the entire sky.

Mt. Wendelstein coordinates: N47.7033, E12.0129