By Bernie Connell and Erin Sanders
Sun glint is an optical phenomenon that can be seen in visible and near-IR satellite imagery over water features such as ocean, lakes, and rivers. Its presence depends on the geometry between Sun, Earth, and satellite viewing angle. When sunlight is reflected off water features towards the satellite sensor at nearly equal angles, this type of reflection is called specular reflection.
How far north can we see sun glint? Sun glint is a common occurrence in the tropics and is typically seen in daytime satellite imagery between 30ºN and 30ºS. The following examples show sun glint at latitudes higher than 30ºN during northern hemisphere summer using visible imagery in the red part of the spectrum (0.64 μm). The first animation shows imagery from the GOES-17 satellite located at 137.2°W in the early hours between sunset and sunrise on July 21, 2021. From this perspective, a seasoned eye may discern the pattern of sun glint areas appearing to travel north and eastward with the changing angle of the sun.
GOES 17 – Arctic sunset and sunrise on July 21, 2021 from 8:30-10:30 UTC.
Let’s zoom in on a few of these regions and take a closer look. From this perspective the regions of sun glint are easier to spot. As the Sun, water surface, and satellite geometry changes, areas of brightness appear and then disappear in less than an hour.
GOES 17 – Arctic sunset on July 21, 2021 from 8:10-9:20 UTC.
These areas of sun glint indicate relatively calm waters with a smooth surface to reflect sunlight, much like a mirror. The increase in reflected solar energy that reaches the satellite sensor can saturate the image as seen by the bright white swaths. However, not all water surfaces reflect sunlight equally. Winds cause rougher water and waves that will scatter sunlight in multiple directions and appear darker, as less sunlight will be directed to the satellite sensor. Cloud cover will also obscure any water surface below. Both of these can lead to patchy areas of sun glint in satellite imagery. Can you differentiate between calm water, rough water, and cloud cover in the next two examples?
GOES 17 – Arctic sunrise on July 21, 2021 from 9:00-9:50 UTC.
GOES 17 – Arctic sunrise on July 21, 2021 from 9:20-10:20 UTC.
The following set of animations show sun glint detected by GOES-16 located at 75.2°W on July 26, 2021. We will again start with the full view from sunset to sunrise, and then focus on two examples.
GOES 16 – Arctic sunset and sunrise on July 26, 2021 from 3:30-6:30 UTC.
Zooming in over the Pacific Ocean off the western coast of the United States shows the contrast between the extensive bright glint area and darker surrounding areas of cloud. The sun glint can be seen well above 40ºN.
GOES 16 – Arctic sunset on July 26, 2021 from 3:30-4:20 UTC.
Sun glint highlights areas of calm water and can reveal details about the water surface and its reflective properties. In this next example, it also shows the ocean-land boundary with Greenland to the north. Sun glint appears on the water surface but not land, although land can certainly influence wind patterns over the water surface and affect how the light is reflected.
GOES 16 – Arctic sunrise on July 26, 2021 from 4:40-5:30 UTC.
So how far north can we see sun glint in GOES satellite imagery? It is clearly seen above 75ºN on this day! The latitudinal extent of where sun glint can be seen has a seasonal dependence. In the northern hemisphere it can be spotted between the vernal equinox (~20 March) and the autumnal equinox (~22 September). The summer solstice (~20 June) is when we would expect sun glint to be seen the farthest north of the equator based on the positioning of the Sun, Earth, and satellite.
Notice the unexpected.