SkyPol1.jpg to SkyPol3.jpg: This series shows Cirrostratus clouds, which is optically so thin that it temporarily became invisible. Accordingly the clouds are also called "non-visible" or "sub-visual" Cirrus clouds. They were found at Morro Branco, around 100 km southeast of Fortaleza at the Brazilian Atlantic Coast.

SkyPol1.jpg was taken without polarisation filter and the Cirrus clouds are hardly visible.

SkyPol2.jpg was taken with a circular polarisation filter, optimally reducing the intensity of the blue sky by the setting of polariser and analyser.

SkyPol3.jpg: The same as SkyPol2.jpg, just with lower attenuation of the skylight in the background. Because the magnitude of the polarisation of the light shows significant directionality, the attenuation is not the same everywhere. Therefore the sky seems to be a bit brighter at the left bottom edge.

With.jpg and Without.jpg: The comparison of the two pictures shows that the colours of the hot-air balloon can be clearer noticed and better distinguished using a polarisation filter. The times of exposure are also different using otherwise the same camera parameters (1/125 s without, 1/50 s with polarisation filter). This demonstrates the loss of light intensity at the CCD chip caused by the use of a polarisation filter.

The series SkyPol4.jpg to SkyPol7.jpg displays the described effect once again. Here condensed Cirrocumulus clouds, which were formed as Leewave clouds downstream of the Pyrenees are presented.

Optical Effect

The scattering of light at air molecules can be theoretically explained by the "Rayleigh-Gans Theory". Thereby the interaction of the electromagnetic radiation (i.e. the light) with the molecules is expressed as oscillating dipoles using solutions of the Maxwell equations. Thereby the light that is scattered in forward direction (in direction of the inclining beam) remains non-polarised, if the inclining light is a priori non-polarised. The same is true for the backwards scattering. However, orthogonally to the direction of the incident beam (90 degree scattering) the maximum polarisation of the light occurs. Thereby the electrical field vector is orthogonal to the plane which is formed by the scattered and the inclining beam. If the positions of the pictured cloud, the sun and the photographer form a right angle, the polarised light of the blue sky might be extremely weakened using a linear analyser or a pair of circular polariser-analyser filters in front of the objective.

Thus the background skylight is suppressed and the contrast between clouds and sky is enhanced. Especially for thin Cirrus clouds - where the blue skylight dominates - this effect is useful to support the contrast.

SkyPol1.jpg to SkyPol3.jpg:
S. Borrmann, Morro Branco near Fortaleza, Ceara, Brazil, 22 February 2004, around 3:22 p.m.

With.jpg and Without.jpg: S. Borrmann, Ingelheim, Germany, 16 May 2004, 6:04 p.m.

SkyPol4.jpg to SkyPol7.jpg:
S. Borrmann, Estartit, North Spain, 14 June 2004, 6:25 a.m.

Camera Parameters

Olympus E-1 SLR with 14-54 mm objective. Exposure and Selectivity choice are automatically performed by the camera. The pictures were taken in the sRGB colour range with a resolution of 2560 x 1920 Pixel x 24 colours.