Sunglint during September equinox

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In 2016, 22 September was the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere.

Sun glint during September equinox
Date & Time
22 September 2016 06:00 and 14:27 UTC
Satellites
Meteosat-8 and 10
Instruments
SEVIRI
Channels/Products
Natural Colour RGB

By Jose Prieto and Ian Mills (EUMETSAT)

Equinox is either of two astronomical moments in the year when the Sun hits vertically on the Earth's equator. The one at 14:21 UTC on 22 September 2016 marks the start of the astronomical autumn in the northern hemisphere.

Figure 1 (above, right) is the Meteosat-8 Natural Colour RGB, taken as close to the actual time of the equinox as possible — on 22 September at 14:27 UTC.

Meteorologically the image at the equinox is of no special value, apart from the fact that the sunglint area, which characterises the ocean area mirroring solar radiation straight into the Meteosat sensors, is centred on the equator too, moving west roughly half the apparent speed of the Sun.

Sunglint areas indicate, in the absence of cloud, where winds are stronger (darker brown pixels in the enhanced zoom of Figure 2) and weaker or calm (brighter brown pixels). This applies in a radius of roughly 500 km around the point of specular reflection.

Enhancement on the sunglint area
Met-10 Natural Colour RGB,22 September 14:15 UTC Enhanced Met-10 Natural Colour RGB,22 September 14:15 UTC
Figure 2: Comparison of Meteosat-10 Natural Colour RGB images showing the sunglint.

Meteosat-8 at 40 °E (Figure 1) and Meteosat-10 (Figure 2) show different sunglint areas. For Meteosat-10, the area is further to the west.

Image scanning with Meteosat takes 12 minutes, and the equator area was scanned at the equinox precise minute.

Figure 3
 
Figure 3: Meteosat-10 Natural Colour RGB, 22 Sept 06:00 UTC

Figure 3 is the Meteosat-10 Natural Colour RGB from earlier in the day, at 06:00 UTC, and shows the terminator straight along the zero degree meridian (equal day and night).

See also:

Solstices & Equinoxes (Time and Date website)

 
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