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