June solstice 2019

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The June solstice is the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere. It took place at 15:54 UTC on 21 June.

June solstice 2019
Date & Time
21 June 2019 05:00 UTC and 12:12 UTC and 27 June 11:00 UTC
Satellites
Meteosat-10, GOES-16, Sentinel-3
Instruments
SEVIRI, ABI, OLCI
Channels/Products
Natural Colour RGB, Visible, OLCI channels 15, 10 and 8

By Jose Prieto and Vesa Nietosvaara (EUMETSAT)

Solstice marks the shortest night of the year in the northern hemisphere (NH). Above 66.6° latitude, the sun is above the horizon 24 hours a day. Over the North Pole, the sun describes a full circle at constant elevation for the duration of the day. That elevation decreases towards the horizon to reach it at the September equinox. For other polar locations the solar path is tilted and, in the course of the weeks goes under the horizon for a fraction of the 24 hours cycle.

Although the Natural RGB from 21 June at 12:12 UTC (Figure 1, top right, click to expand) has no special meaning in astronomical terms, by chance it shows a front near the western coast of South Africa as a herald of the starting of the winter season in the southern hemisphere.

Often around the solstice, the particular geometry between the sun, satellites and reflective surfaces generates sun glint on satellite imagery. Figure 2 shows an area close to the Baffin Bay. GOES-16 through its 0.47µm VIS channel captured a strong sun glint due to grazing reflection from the sun onto the satellite at 05:00 UTC on 21 June, a time close to midnight in most of the Americas.

Strong reflection occurs on flat surfaces, in this case mostly frozen sea waters. For any geostationary satellite, similar sun glint is present on midnight imagery at latitudes higher than 66.6° for the longitude of the geostationary satellite. In this example, the patches of sunglint show around 80°W, 74°N.

Image comparison
Image from 05:00 UTC Image from 05:30 UTC
Figure 2: Comparison of GOES-16 visible images during the solstice, showing the sunglint.

Another effect of the high sun in the NH around the solstice are the parallax cloud shadows on satellite imagery. Figure 3 shows Meteosat (left) and Sentinel-3 (right) images a few days after the solstice on 27 June at 11:00 UTC, over central Spain.

Figure 3
 
Figure 3: Meteosat-11 (left hand side) and Sentinel-3 (right) views on the same cloud system over central Spain, 27 June 11:00 UTC. Meteosat shows dark pixels south, Sentinel OLCI instrument north-west of the cloud.
 
 
Figure 4: The dark boundary is explained as a different perspective from the two satellites. Scissors indicate the sun rays direction. Copper coin is the actual shade on the ground, the golden is the physical cloud. On the left hand side (Meteosat view), the shade appears "south" of the cloud

Shades can be seen at the north-west cloud boundaries in the OLCI RGB, but at the southern boundaries in the Meteosat-11 image.

The dark boundaries are shaded pixels. Meteosat can see the ground in those pixels close to the southern cloud boundary, because it is located at a lower elevation than the sun for that time.

However, the Sentinel-3 OLCI instrument, passing almost vertical above the cloud, at a higher elevation than the sun, sees shadows on the north-west of the cloud. See Figure 4 for a practical explanation.

 
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