June solstice 2019
21 June 2019 05:00 UTC and 12:12 UTC and 27 June 11:00 UTC
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.
11 November 2020
21 June 2019
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) 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
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.
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.
