Sunglint

Sunglints appear blurred yellowish grey in VIIRS Cloud Phase RGB.

Open water bodies outside sunglint areas usually appear black, in some rare cases dark blue. Sunglint usually appear blurred yellowish grey over this dark background. Its brightness and extension depend on the wave characteristics. Within the sunglint areas there are often darker regions where the surface roughness is different (for example due to weaker wind, lower waves).

Figures 1, 2 and 3 shows sunglint over the Mediterranean and the Aegean Seas. For comparison, True Colour RGB images are presented, where the sunglint is even better seen. (In True Colour RGB the water is usually dark blue outside the sunglint region, while sunglint appears greyish, whitish).

Fig. 1: Cloud Phase and True Colour RGB images for 30 May 2021, 11:59 UTC.

Fig. 2: Cloud Phase and True Colour RGB images for 12 June 2021, 12:05 UTC.

Fig. 3: Cloud Phase and True Colour RGB images for 10 August 2021, 11:08 UTC.

Note that in figure 3 smoke is also seen in True Colour RGB (in grey colour) over and close to the western coast of Greece.

Explanation of the colours of open water bodies in sunglint areas (see the recipe):

Outside the sunglint areas:
• The reflectivity values of both near-infrared channels are extremely low, almost zero (water absorb strongly at these wavelength). In the visible channel the reflectivity is somewhat higher but still very low. Therefore, open water bodies usually appear black.

Within the sunglint areas:
• In case of sunglint the reflectivity values of all three channels increase. The VIS0.67 reflectivity is usually the highest one, but since the near-infrared reflectivity data are highlighted in the 0-50% range in Cloud Phase RGB, the red and green signals usually become stronger than the blue signal. Additionally, since the red signal is somewhat stronger than the green one, the sunglint area appear yellowish grey over the dark background.