Winter solstice 2017

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As precisely forecast, the northern hemisphere winter solstice (summer solstice in the southern) started on 21 December at 16:27 UTC.

Date & Time
21 December 2017 00:00 UTC & 12:00 UTC
Natural Colour RGB

By Jose Prieto

This is the start of the shortest season in the year (just 89 days), the Earth being closest to the Sun. Meteorologically, the ephemeris is of little consequence, other than the start of longer daylight periods.

Figure 1
Figure 1: Animated gif of Met-10 Natural Colour RGB, 15-21 Dec, 00:0 UTC

The stronger illumination in the north will not convert into higher atmospheric temperatures until several months later, due to cloud, air and ocean thermal inertia.

The Meteosat-10 Natural Colour RGB aninated loop (Figure 1) shows the Sun reflecting on the Antarctic around 0:00 UTC on 15-21 Dec, from Meteosat-10, with bright flares south of 66°S where light slant touches the cold waters in between widespread cloud cover. These show as very bright patches appearing every night, but only meaningful in relation with the Meteosat-10 optics.

Around midnight in Greenwich, the glaring reflection (sun glint) quickly moves from west to east, a bit surprisingly. The location of the sun glint at different times is given in this diagram.

SEVIRI provides the distinction of cloud, ice surface and water based on the readings for channels at 3.9µm, 10.8µm and 0.8µm, as basic channels. The difference 3.9µm–10.8µm is high for cloud when the scene is lit by the sun, and negative for ice and water surfaces, except on the area of almost mirror reflection, where the 3.9µm sensors get saturated.

Other sources like METNO's sea ice area fraction product, confirm the presence of ice close to the coast line, which reduces the sun glint on it. Surface water temperatures are still well under 0°C, even under the freezing point of salty waters, but keep fluid due to their dynamic (see Sea Surface Temperature product from CMEMS).

Figure 2
Figure 2: Met-10 Natural Colour RGB, 21 Dec 12:00 UTC

The Meteosat image at 12:00 UTC on 21 December (Figure 2) shows an extreme dark northern part, only lit under 66.6°N. A subtropical convergence zone is tilted to the south, crossing Africa through Tanzania and the Congos.

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