Optical Effects
Rainbow I
Rainbow II
Rainbow III
Polarisation Rainbow
Halo I
Halo II
Sun Pillar
Sun through clouds
Crepuscular Rays



All images point from Brezzo di Bedero over the Lago Maggiore to the Monte Rosa Massive. The images show the visibility in dependence of different atmospheric conditions. Visibility6.jpg and Visibility7.jpg were taken after several rainy days and display a very clear atmosphere. In contrast the atmosphere is hazy in Visibility3.jpg. In the images of the top row the first, dark mountain chain is around 6 km away from the observer, whereas the Monte Rosa Massive is located in 60 km linear distance. The Monte Rosa Massive consisting of four characteristic peaks is presented in Visibility4.jpg to Visibility7.jpg (plus Visibility10.jpg and Visibility11.jpg). Thereby the dark mountain chain in the front is the same as the one in the images Visibility1.jpg to Visibility3.jpg.

The visibility is limited by the scattering of light at aerosol particles and air molecules. Knowing the extinction coefficient the range of visibility can be calculated using Koschmieder's law. Without any aerosol particles visibility is limited by light scattering at air molecules to 100 to 300 km. Objects along the line of vision become brighter and brighter, until they reach the brightness of the horizon; in the top row of pictures this is demonstrated by mountain chains of different distances to the observer.

At this point the contrast is so low that the human eye cannot distinguish the object from the sky anymore. The more distant the objects (e.g. mountain chains) are, the more light travelling from the objects towards the observer is scattered out of the light path by aerosol particles (and air molecules). Thereby direct "light from the object" is lost. At the same time sun light which does originally not travel towards the observer is scattered into this light path as "external light". Because of the external light the image of the object becomes brighter. The loss of light from the object and the simultaneous gain of external light (without any information on the object) reduces the signal-to-noise ratio. This results in a reduction of contrast between object and environment. Aerosol particles in the diameter range between 0.1 µm and 1 µm scatter light most effectively. Their effect can be also noticed in the chapter "Smog over Crete".

Visibility1-11.jpg: S. Borrmann, Brezzo di Bedero, Region around Lago Maggiore, Northern Italy, August 2004

Visibility1.jpg: 8/11/2004, 6:22 p.m.
Visibility2.jpg: 8/11/2004, 6:23 p.m.
Visibility3.jpg: 8/11/2004, 7:51 p.m.
Visibility4.jpg: 8/13/2004, 07:07 a.m.
Visibility5.jpg: 8/13/2004, 7:56 p.m.
Visibility6.jpg: 8/14/2004, 6:23 a.m.
Visibility7.jpg: 8/27/2004, 11:29 a.m.
Visibility8+9.jpg: 8/27/2004, 4:28 p.m.
Visibility10.jpg: 8/27/2004, 4:29 p.m.
Visibility11.jpg: 8/27/2004, 5:26 p.m.