Colors of Scattering and Diffraction Observed in an Altocumulus Cloud

November 17, 2013

Mario-Freitas-July-14-2013-EPOD (2)

PhotographerMario Freitas
Summary Author: Mario Freitas

A deep blue region in the daytime sky indicates that local atmospheric conditions are free (are nearly so) from water droplets and aerosols. Their presence causes the sky to appear whitish. Shorter wavelengths of sunlight are more intensely scattered by molecules such as nitrogen and oxygen (Rayleigh scattering). On the other hand, all visible wavelengths are equally scattered by water droplets and dust, due to the larger size of these particles (Mie scattering), providing the contrast between a blue background sky and bright white clouds. However, clouds aren't always white. This fact has long been portrayed in art history by painters who tried to better depict what they observed by adding shadowy blues and grays to their white surfaces. Doing so provided a more realistic, tridimensional appearance. Yellow and pink hues were included to suggest sunset and twilight.

The photo above was taken in the courtyard of Oscar Niemeyer Museum in Curitiba, Brazil. The filaments along the edge of an altocumulus cloud exhibit bright nonspectral, metallic colors, characterizing the optical effect known as iridescence. These colors result from the diffraction of white sunlight by similarly-sized, spherical water droplets that compose the cloud. Note that the Sun has been shielded behind a tree-like work of art by Brazilian sculptor E. Titton in the museum’s garden. Always use caution when looking at the brighter portion of the sky, and use extreme caution when looking toward the Sun. Picture taken in July 14, 2013.

Photo Details: Camera Maker: Panasonic; Camera Model: DMC-LX5; Focal Length: 13.9mm (35mm equivalent: 77mm); Aperture: f/8.0; Exposure Time: 0.0006 s (1/1600); ISO equiv: 80; Software: ACDSee Pro 6.