In RoundSun1.jpg, RoundSun2.jpg and Degas.jpg the sun looks like a circle, whereas it seems to be diffuse in DiffuseSun1.jpg and DiffuseSun2.jpg. The sun is also pictured somewhat blurred in SunCirrus1.jpg and SunCirrus2.jpg.

Cirrus clouds consisting of aspheric, angular ice particles with definite cristal structure generate the blurred border of the sun. If a cloud consists of liquid, that means spherical water droplets the sun can be recognized as a distinguished circle through the cloud layer. Spherical droplets scatter the incident sunlight preferentially in forward direction towards the observer. Ice particles generate several reflections in different directions.

Therefore light can be also reflected to the eyes of the observer at smooth surfaces of ice-crystals which are farer away from the line sun - observer. This happens in dependence of the orientation of the particles in the air. The border of the sun appears then blurred. So shape and appearance of the sun shining through a cloud layer indicates if the cloud consists of solid ice particles or liquid and possibly under-cooled droplets.

Also watching the sun from an aircraft it also seems to be blurred when even higher Cirrus clouds occur (SunCirrus1+2.jpg taken at an altitude of 10 km). The images have a structure in diagonal direction because of scratches on the window pane of the aircraft. Cirrostratus3.jpg, Cirrostratus4.jpg and Cirrostratus2.jpg (look also to Cirrostratus) were taken from the ground. Because of the ice particles in the Cirrus clouds they show a diffuse sun. In chapter Objects in clouds the characteristic sizes of droplets and ice particles are summarized.

Cirrus1-5.jpg were recorded in the tropes at an altitude of 11.3 km and ambient temperatures of -54 deg C. Therefore all clouds which are found above the aircraft are iced and the sun appears diffuse through them. Especially Cirrus4.jpg and Cirrus5.jpg show an almost cloudless, blue sky. However, the few ice particles generate a blurred picture of the sun. In order to avoid overexposure the image was taken as the winglet widely covered the sun. The lens aperture was manually adjusted. It was very small to avoid overexposure. Therefore the whole image appears rather dark and the cloud cannot be very well noticed anymore.

RoundSun1.jpg, RoundSun2.jpg: S. Borrmann, Pulpit Rock, Carinthia, Austria, 1 January 2003 at noon-time

DiffuseSun1+2.jpg: S. Borrmann, Pulpit Rock, Carinthia, 2 January 2003 around 2:40 p.m.

SunCirrus1+2.jpg: S. Borrmann, flight from Frankfurt to Birmingham, 19 December 2003

Cirrostratus2.jpg: R. Weigel, S. Borrmann, Forli, North Italy, APE-Infra Field Campaign, October 2002 (see also chapter Cirrostratus)

Cirrostratus3+4.jpg: S. Borrmann, Ingelheim, Germany, 27 April 2004, 7:06 p.m. to 7:08 p.m.

Cirrus1-5.jpg: S. Borrmann, flight Varig RG2375 from Fortaleza to Sao Paulo in Brazil, 2 March 2004, 4:39 p.m. to 4:54 p.m.

Degas.jpg: Edgar Degas "Cheveaux de Course", Oil Painting, 107 cm x 73 cm (1869 - 1872), Barber Institute of Fine Arts, University of Birmingham, United Kingdom

Camera Parameters

Except Cirrostratus2.jpg all pictures were taken with a SLR (14-54 mm objective), whereby zoom and dimming were changed from picture to picture. Exposure and selectivity choice (100 ASA) was performed by the program automation. Colour range (sRGB) and resolution (2560 x 1920 Pixel x 24 colours) was the same for all images.