A Glint of Understanding

NASA researchers and data offer new insights into the impish and interesting visual phenomenon of sunglint in satellite imagery.

If you look into the sky on a sunny day you may occasionally see a bright spot or vertical line on either side of the Sun. These halos are called Sun dogs and are caused by the refraction of sunlight encountering clouds of ice crystals floating in the upper-middle atmosphere. If the same clouds were viewed from space, the reflected light would likely show up as a very bright spot, or "sunglint," over land or water.

This is an example of a sunglint captured by the Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) spacecraft. The inset image in the lower left shows the glint-affected area in detail. The image in the lower right shows the glint viewed through the EPIC Glint Product mask. Different colors in the glint mask indicate which wavelength(s) in the EPIC observations are affected by glint from ice clouds or smooth water surfaces; grey color indicates areas where no glint was detected. Credit: NASA EPIC Team.

The glints are certainly neat to see, and their brightness can both hinder and help researchers investigating clouds, weather, climate, and other intriguing topics. NASA scientists are actively learning about sunglints, gaining an understanding of their nature, and now even offering a first-of-its-kind dataset identifying them in Earth-observation imagery.

Like a Floating Mirror

Glints are caused by the reflection of sunlight from highly reflective objects, such as the ocean or other water surfaces and clouds. Some of the first glints spotted from space were in 1990 by astronomer Dr. Carl Sagan in data acquired by NASA's Galileo spacecraft when the probe captured images of Earth on its flyby to Jupiter. Glints had also been previously observed by people in aircraft and from mountaintops.

Recently, Dr. Tamás Várnai and Dr. Alexander Marshak from NASA's Goddard Space Flight Center in Greenbelt, MD, have been examining sunglints through their work as atmospheric and remote sensing scientists with the joint NASA/NOAA/U.S. Air Force Deep Space Climate Observatory (DSCOVR) mission. DSCOVR is a solar wind-detecting spacecraft one million miles from Earth. The spacecraft carries the Earth Polychromatic Imaging Camera (EPIC), which captures images of the sunlit part of the planet each day. EPIC also can measure ozone, aerosols, cloud reflectivity, cloud height, vegetation properties, and UV radiation. Várnai and Marshak have found that EPIC is great for identifying sunglint as well.

"I saw one for the first time in 2016 when we were examining seasonal changes in the average reflectance of the entire Earth as seen by EPIC," said Marshak. "They had a shape of a bird and we first called them 'birds in clouds.'"

Now that Marshak and Várnai know what to look for, they see glints all the time.

"We can spot small glints every day and see a bright one every two or three days," said Várnai.

After seeing the first glint in 2016, Marshak and Várnai began working with their colleague, physicist Professor Alex Kostinski from Michigan Technological University in Houghton, MI, to determine what exactly could be causing the bright spots. Together, they figured it out and published a paper explaining that cloud sunglints are made by floating, hexagonal platelet-shaped crystals that are about 1/10 of a millimeter large and form in freezing air. The crystals by nature of their form and physics tend to line up horizontally in the air, creating a sort of composite reflective surface that can bounce light like a mirror. From space, the sunglint clouds appear very bright and can have a distinct red-green-blue appearance in EPIC images.

Similarly, ocean or lake water that is flat and waveless can also produce sunglints that can be seen from space. NASA's Worldview satellite imagery exploration tool team created an explainer describing how water-based sunglints appear and what they can reveal in satellite imagery.

Too Bright or Just Right

Sunglints are so bright that they can cause spikes in satellite optical measurements that fool data processing algorithms or even saturate and blind instruments.

"Sunglint from ice causes problems mainly in estimating the properties of clouds, which can appear thicker or higher in the atmosphere than they really are because glints make the cloud appear so bright," said Várnai.

Sunglints in water can be troublesome as well. Like clouds, they reflect large amounts of light, making it harder to accurately measure the levels of plankton and other substances in water based on their brightness or to visually assess the physical appearance of nearby objects.

At the same time, sunglints do have their benefits.

For one, the presence of a glint tells researchers that the cloud is made up of horizontally aligned crystals, which is helpful for atmospheric researchers studying the composition of clouds. Additionally, scientists can see how much light the cloud crystals reflect and estimate their cooling effect on the atmosphere. Overall, researchers can track how the plume of crystals behaves and use this to learn more about the lifecycle of clouds, and, through this, discern how human activity effects the lifespan of clouds.

Another benefit is that the intensity of glints makes them a convenient marker for geolocation purposes. If an object of interest is far out in the ocean, it can be difficult to precisely position it without a nearby coastline or other major fixed point to reference. However, the position of glints can be accurately determined and then used as reference points to help geolocate the object in an image.

New Glint Dataset

Given the usefulness of studying glints for atmospheric and other research, the EPIC instrument team has started offering a glint data product. The Level 2 product identifies points in EPIC images where the alignment of solar and viewing directions is suitable for sunglint observations. The products include surface-type flags showing whether the area of a pixel is covered mainly by water, desert, or non-desert land; the glint angle tells how favorable the EPIC view direction is for glint detection; and the glint mask indicates whether glint has been detected. The EPIC Glint Product can be found by visiting NASA's Atmospheric Science Data Center (ASDC) or through NASA's Earthdata Search.

"In the future, we hope to expand the product to include measurements on how much brighter the glint is because of the ice, which could help users estimate how many ice crystals are present and other details," said Várnai.

Learn More

Whether seen from the ground or space, sunglints are interesting and helpful—albeit impish—atmospheric phenomena. And like most shiny things, they easily get our attention. If they have yours and you'd like to learn more, check out these other sunglint image features and articles:

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