Why does the water look strange during a strong wind? (Glint or glint phenomenon.)

Among other things, during strong winds, the water may look gray or otherwise strange in color. In this case, it may be a glint, i.e. the flickering of sunlight caused by the waves. Optically, it is a matter of directional reflection.

You can see the glint, e.g., in true-color images when the water surface reflects sunlight directly onto the satellite instrument. This situation is mostly due to a wave field raised by a strong wind in the appropriate direction. In this case, there seems to be a gray cover on the surface of the water, under which it can be difficult to distinguish the real features of water quality (see the first example below).

The glint is also recognized by a peculiar phenomenon in which the islands seem to leave long shadows in the downwind direction (see another example below). This is especially noticeable in lake areas. It is not an optical shadow area, but the effect of the island on the wave.

More information about the phenomenon can be found in English on NASA's Earth Observatory website.

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An example of the glint phenomenon in the high seas north of Åland. A prolonged strong north wind has lifted a wave in the open sea, causing flickering visible as a gray overlay in the true color image of the Sentinel-2 satellite. The link to the image in the Tarkka service.

Example of a glint phenomenon in Isojärvi, Pomarkku. The islands seem to leave a 'shadow' on the downwind side. The link to the image in the Tarkka service.

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An example of the effect of the glint effect on color tones in the exact Sentinel-3 true color image. Due to the flickering phenomenon, the color tones turn reddish in a large area in the Gulf of Finland. The link to the image in the Tarkka service.