Colour Interpretation Guide


sun glint

date
MetOp

length
Cloud RGB - Recipe

author
Red: Channel 1 (0.63 micron)
Green: Channel 2 (0.87 micron)
Blue: Channel 4 (10.8 micron)

Sun glint

A sun glint appears yellow in the AVHRR Cloud RGB images, varying from dark blue with some yellowish tones (weak sun glint) to bright yellow (strong sun glint).

The open water surfaces are usually dark blue in AVHRR Cloud RGB images. An exception is the area of sun glint. A sun glint is the mirror-like reflection of the Sun from a water surface. Sun glints occur on cloud-free ocean, sea, river, lake surfaces at a given geometry of the sun and satellite viewing directions. In the sun glint area increased amount of solar energy is reflected from the water surface. Smooth ocean water becomes a "silvery mirror", while rougher surface waters (with big waves) appear less bright. As the ocean and sea is never perfectly smooth or calm, the Sun’s reflection gets blurred giving the water surface a washed out appearance.

On the right panel of the image below a sun glint can be seen both over the Black Sea and the Mediterranean Sea (north and south of Turkey) with variable strength. A very bright yellow patch south of the Turkish coast (north-east of Rhodes Island) shows very intense sun glint.

The left panel shows an image of the same area without sun glint, taken 50-minutes earlier. This image is presented to make it easier to distinguish sun glint from clouds on the right panel, which is not obvious, as the low- and medium-level clouds are also yellowish. The typical wash out appearance of sun glint helps to distinguish sun glint from clouds.

Sun Glint Sun Glint

AVHRR Cloud RGB images of the Black Sea and eastern Mediterranean region for 22 June 2016 at 07:19 (left) and 08:04 (right) UTC

Some more examples are presented below.

Sun Glint Sun Glint

Sun glint over the Mediterranean Sea, AVHRR Cloud RGB images for 24 May 2016 at 08:58 UTC (left) and 08 May 2016 at 07:49 UTC (right)

Explanation of the colours of a sun glint (see also the recipe on the left side):
• The open water surface without sun glint is dark blue: the red and green components are almost zero, while the blue component is somewhat stronger but still low.
• In case of sun glint the red and blue signals increase, while the blue component does not change. As the reflection is similarly intensive in the VIS0.63 and NIR0.87 channels, the sun glint appears yellowish in the AVHRR Cloud RGB images.