A layer of stratocumulus clouds over the Pacific Ocean served as the
backdrop for this rainbow-like optical phenomenon known as a glory. Glories generally appear as concentric
rings of color in front of mist or fog. They form when water droplets within
clouds scatter sunlight back toward a source of illumination (in this case the
Sun). TheModerate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired the image on June
21, 2012. The image was saturation-enhanced to make the glory effect more
visible.
Although glories may look similar to rainbows, the way light is
scattered to produce them is different. Rainbows are formed by refraction and reflection; glories are formed by backward diffraction. The most vivid glories form when an
observer looks down on thin clouds with droplets that are between 10 and 30
microns in diameter. The brightest and most colorful glories also form when
droplets are roughly the same size.
From the ground or an airplane, glories appear as circular rings
of color. The space shuttle Columbia observed a circular glory from space in 2003. In the image
above, however, the glory does not appear circular. That’s because MODIS scans
the Earth’s surface in swaths perpendicular to the path followed by the
satellite. And since the swaths show horizontal cross sections through the
rings of the glory, the glory here appears as two elongated bands of color that
run parallel to the path of the satellite, rather than a full circle.
Glories always appear around the spot directly opposite the Sun,
from the perspective of the viewer. This spot is called the anti-solar point. To visualize this, imagine a line
connecting the Sun, a viewer, and the spot where the glory appears. In this
case, the anti-solar point falls about halfway between the two colored lines of
the glory.
Glories are usually seen against a background of white clouds.
Clouds are white because the sunlight is scattered many times by multiple
droplets within the clouds. The white light often obscures details of glories,
but without them in the background, the glory would not be visible.
Another notable feature in this image are the swirling von karman vortices that are visible to the right of the
glory. The alternating double row of vortices form in the wake of an obstacle, in
this instance the eastern Pacific island of Guadalupe.