Saturday, June 30, 2012

Image of the week : a glorious view

The Terra satellite captured a rainbow-like optical phenomenon called a glory over the Pacific Ocean.
Water droplets within clouds scatter light to produce the effect.


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).
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite acquired the image on June 20, 2012.

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.

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