Crepuscular Rays and Light Scattering

An astronaut took this photograph while the International Space Station (ISS) was cruising over the Arabian Peninsula and through the terminator, or day-night boundary. The oblique photo captures some cumulus clouds with their tops flattened into an “anvil” formation due to atmospheric conditions at the tropopause. The anvil clouds cast elongated shadows due to the angle of the setting Sun.

Two atmospheric optical phenomena are visible in the photo: crepuscular rays and Rayleigh scattering. Both effects are produced by the low angle of the Sun and the atmospheric scattering of sunlight. Non-selective scattering is also at work in the water droplets of the clouds, making them appear bright white.

Extending behind the clouds, the bright streaks of crepuscular rays occur when incoming light is partially obstructed by a cloud or tall feature on the horizon. Small gaps in the clouds allow sunbeams to pass through, causing rays of light to appear in bright columns oriented away from the Sun. Dust particles suspended in the atmosphere at the time of the photo helped make the sunbeams more clearly visible.

As light passes through the atmosphere, it collides with particles and gases causing changes in its direction, a phenomenon called Rayleigh scattering. This effect is especially noticeable during low Sun angle conditions at sunrise or sunset because the incoming light must pass through much more of the atmosphere than at high noon. As a result of this longer path, the atmosphere scatters more of the shorter blue wavelengths of sunlight compared to longer orange and red wavelengths. In the scene above, the golden hue of the clouds matches the color of the sunset.

Astronaut photograph ISS063-E-53859 was acquired on July 19, 2020, with a Nikon D5 digital camera using a focal length of 50 millimeters. It is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by a member of the Expedition 63 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by Cadan Cummings, Jacobs, JETS Contract at NASA-JSC.