Exploring how solstices and equinoxes can be shown using satellite imagery.
21 February 2023
21 June 2020
Summer solstice in the Northern Hemisphere and the Winter solstice in the Southern Hemisphere takes place in June. Winter solstice in the Northern Hemisphere and the Summer solstice in the Southern Hemisphere takes place in December.
Solstices define the change of astronomical season. To avoid the solstice oscillation in date, meteorological seasons start on the first day of the month, for instance 1 June is the start of the northern hemisphere summer.
During solstices sunglints, direct reflections of the Sun on flat Earth surfaces, like calm oceans, take 'northernmost paths' in the geostationary satellite imagery. In June solstice, the Northern Hemisphere imagery displays sunglints even in the Arctic region.
The animated gif for 20 June (Figure 1), seen from the north pole and based on three geostationary satellites, shows:
- Sunglint in yellow hues, turning to red (higher reflectivity) for the high angles (satellite to Sun, for respective satellite).
- Dust transport over Atlantic in similar hues (similar reflectivity).
- Straylight for Meteosat-8 satellite, visible during respective night hours (light blue tone over the full disc view).
- The full disc (FD) view of Meteosat-8, overlaid by the FD view of other two satellites – where there is a lot of continent mass (Africa), not relevant for sunglint tracking.
Solstices can be applied to monitoring Arctic ice. Figure 3 offers a comparison of channel 0.8µm and the ice concentration product by the Government of Canada. In the sunglint area, waters with thick ice are not reflective, whereas ice-free waters are very reflective. Outside of that area, around 20 minutes after or before sunglint, the reflectivity pattern is the opposite, since ice is not flat and puts light into the satellite. Area A on Figure 3, with only 10% of ice, reflects moderately in both situations. According to Canadian ice products, the amount of ice on Baffin Bay that week was 90% below the 30-year average for areas D and C, as revealed by the strong sunglint at 05:10 UTC. A similar analysis (not shown) explains the reflection pattern in the White Sea, east of Scandinavia, for Meteosat-11 (Figure 6)
Note that spectral channels around 800nm (0.8µm) were preferred for tracking the sunglint position over water bodies, instead of the 600nm region which shows less reflection for land, hence a worse land-sea contrast. Oceans are also more reflective and brighter at shorter solar wavelengths, adding noise to a sunglint detection.
On the geostationary satellite images, the sunglint evolves along an oval during the course of the day (Figure 4). Close to the solstice Sun slant reflection occurs in the forward direction on sea portions without waves, close to 70 degrees north latitude.
Examples are provided on Figures 5-7, first for Himawari around 15:00 UTC on 20 June, then for Meteosat-11 around 00:00 UTC on 21 June, then for GOES-16 around 05:00 UTC.
Curiously, the glint moves eastwards inside each satellite field of view, but follows a westward satellite sequence — the satellite and the Sun being on opposite sides of the Earth.
Also a comparison of two GOES images one year apart (Figure 8) results in no difference, apart from the cloud distribution. Cloud scatters the light, erasing the sunglint.
During the 2020 June solstice the shadow of the Moon, travelling from west to east, appeared in the images of three geostationary satellites. Because the angular diameter of the Moon (the Moon apparent size from the Earth) was smaller than that of the Sun at this time, an annular or ring eclipse occurred.
The shadow of the Moon first appeared in the morning over central Africa, advancing northeast across the south of the Arabian Peninsula over southern Asia to China, and gradually disappearing over the Pacific Ocean (Figure 9).
The movement of the Moon's shadow on the Earth's surface was seen by three meteorological satellites: Meteosat-11 at 0° longitude (Figures 10 and 15), Meteosat-8 at 41.5° E (Figures 3 and 6) and Himawari-8 at 140° E (Figures 4 and 7).
Notice the oval shadow that gradually appears in the animations, always over a different part of the Earth, moving from west to east. The most complete course of this phenomenon is captured in the animation of images from the Meteosat-8 satellite, performing the Indian Ocean Data Coverage (IODC) service.
It took place at 15:54 UTC on 21 June. Although the Natural RGB from 21 June at 12:12 UTC (Figure 16) 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 sunglint on satellite imagery. Figure 17 shows an area close to the Baffin Bay. GOES-16 through its 0.47µm VIS channel captured a strong sunglint 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 sunglint 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.
Another effect of the high Sun in the NH around the solstice are the parallax cloud shadows on satellite imagery. Figure 18 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 19 for a practical explanation.
On 21 June 2018 the solstice took place at 10.07 UTC (Figure 20).
In the ABI Cloud Type animation for 19 June from 02:00 to 08:00 UTC (Figure 21), corresponding to midnight in North America, the Sun obliquely shines from the other side of the Earth and reflects almost flat into the GOES-16 solar sensors.
Usually, the most reflecting parts are non-frozen portions of the ocean, but there seems to be little chance for that after the cold winter. The strongest reflection is circled in black and corresponds to cloud covered pixels, usually stratus made of reflective water droplets. The sunglint area moves from west to east, corresponding to the Sun moving east to west on the other side.
The chart on ice extent from the government of Canada (Figure 22) gives a view for 20 June of the few ice-free areas inside the image, not affected by sunglint in this example.
The solstice can fall on different dates from year to year, between 20 and 22 June. This year, a leap year, it was on 20 June at 22:34 UTC.
The solstice also happened during a full Moon, which in June is known as a Strawberry Moon — a rare astronomical occurrence.
This solstice marks the shortest night in the Northern Hemisphere and the longest night in the Southern Hemisphere. In the Arctic region, above 66.6 degrees latitude, the Sun does not set in 24 hours. The Meteosat-10 imagery, 20 June 22:00 UTC–21 June 01:45 UTC around midnight shows the Sun-illuminated region in the upper part, close to the Earth's upper boundary, as a result of maximum Earth tilting at this time of the year.
The grazing sunlight reaching the satellite is enhanced by reflection on flat surfaces, as lakes or calm ocean areas. The evolution of the sunglint area on water surfaces can be followed in the animation, progressing from west to east.
The sensors for solar radiation saturate for strong signals and can 'blind' neighbouring pixels to the west. The cloud-free areas in the Norwegian sea are pictured with a better resolution by the VIIRS instrument on SNPP, without sunglint. This image (Figure 25) uses VIIRS Day-Night Band (DNB) channel at 00:00 UTC on 21 June.
The polar orbit reaches further north than Meteosat, and allows a glimpse into the almost clear Kara Sea and the area east of Svalbard.
The June solstice took place at 16:39 UTC. A particular geometry between the Sun, reflective surfaces like quiet waters and the Meteosat satellite, produces sunglint, which propagates, if strong, to the pixels west of the reflecting surface. In the animation you find these occurrences to the north and west of Iceland and Norway, the Bothnian Bay, White Sea and some Russian and Finnish lakes.
This 15-min time step, Meteosat-10 HRV loop, 21 June 00:00 UTC–22 June 00:00 UTC shows the movement of the illumination from the Sun, over the north pole, throughout the day of the solstice. If you stop the animation at 16:30 UTC, this will be the moment when Sun reached the highest point in the northern hemisphere in 2015. Sunglint can also be clearly seen in this animation. This graphic shows the location of the Meteosat sunglint centre for any time of the year.
Experts from the Hungarian Meteorological Service tracked the 2014 summer solstice using Meteosat-10 imagery.
Figure 27 (above right) shows full disc Natural Colours RGB image at 06:00 UTC on 21 June. In the illuminated area we can clearly see the surface features and clouds. The black colour shows the area where Sun had not yet risen, so it was still night. The boundary line between day and night (the so-called terminator line) and the rotation axis of the Earth has a 23.5 degree inclination on that day.
The 24-hour animation of images taken every 15 minutes, shows how the Northern Hemisphere was illuminated by the Sun over the summer solstice (21 June 12:00 UTC to 22 June 12:00 UTC). Download animation
On Figure 28 (Meteosat-10, 21 June, 23:45 UTC) we can see that the Sun never set over the North Pole on that day.
The rotational axis of the Earth is not perpendicular to the plane of the orbit. The difference between the rotational axis and the perpendicular is 23.5 degree (see Figure 29).
In half of the annual orbit the Northern Hemisphere gets more sunlight, while in the other half it's the Southern Hemisphere that gets more sunlight. At the equinox both hemispheres get the same amount of radiation. This is the reason for the seasons' progress. The effect described above can be clearly seen on satellite images in Figure 30. In the first row we can see the images corresponding to the autumnal equinox (September 2013), in the second row to the winter solstice (December 2013), in the third row to the vernal equinox (March 2014), in the fourth to the summer solstice (June 2014) of the North Hemisphere at 06:00, 12:00, 18:00 and 24:00 UTC. In June and December we can see the midnight Sun.
The animation shows those areas that SEVIRI can see which are lit by the midnight Sun. At 23:30 UTC the sunglint is prominent north of Iceland.
For a period of some days around the solstice it is possible for our geostationary satellites to see what is known as the ‘midnight Sun’. Because of the position of the Sun, the sunlight is reflected off the northern polar region and is seen by our Meteosat Second Generation satellites, as shown in Figure 32 (right).
Figure 2 shows the change in the Sun's illumination of the Earth due to the position of the Earth relative to the Sun.
Satellite solar images from geostationary satellites show sunglint areas close to the polar circle around the time of the lowest Sun, as seen in the animation from HRV channel of Meteosat-11 at 0 degrees (Figure 34).
The first strong sunglint shows on the western side of the image at 20:45 UTC, then it moves eastwards (the Sun being on the other side of Earth) to the ocean-free area east of Argentina. Sunglint appears as an artificial bright artifact for three/four consecutives time slots, not to be confused with cloud. GOES-16, at 75 °W, picks up different areas of strong direct reflection from the Sun during the low-Sun period of the day. On Figure 35, the sunglint area shifts at roughly 2000 km/h. We see a sudden glare at 04:30 UTC on the more liquid and flat parts of the ocean in the Antarctic peninsula.
Also, Sentinel-3, through its OLCI instrument at 300 m resolution, shows Antarctic areas in 24 hour solar illumination. In Figure 36, some shades indicate the presence of high ice slopes, for instance under the North arrow. Drifting ice combines icy and liquid parts in the sea in the northern part of the image.
Solstice started on 21 December at 16:27 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 animated loop (Figure 37) shows the Sun reflecting on the Antarctic around 0:00 UTC on 15-21 December, 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 (sunglint) quickly moves from west to east, a bit surprisingly. The location of the sunglint 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 sunglint 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 ).
The Meteosat image at 12:00 UTC on 21 December (Figure 38) 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.
View of Earth at the time of the solstice in December 2015, as seen by Meteosat-7 and Meteosat-10.
That difference can be seen by how much of the Earth is visible in the Meteosat images. The Sun illuminates more of the Southern Hemisphere at this time of year, and the North Pole remains in darkness.
Figure 39 (above) shows the view of the Earth from the Meteosat-7 satellite, covering the Indian Ocean, at 05:00 UTC just after the solstice at 04:48 UTC.
Figure 40 shows the view at the same time but from Meteosat-10 using the Natural Colour RGB. Even at Greenwich midnight, grazing reflection on calm water surfaces produces a strong signal close to the Antarctica, where the Sun is 24 hours above the horizon. In particular this graphic shows the location of the Meteosat sunglint centre for any time of the year.
Evolution of the natural RGB, based on channels 3, 2 and 1 from Meteosat-10 during the northern hemisphere (NH) winter solstice. This is a rare case of Meteosat sunglint at polar latitudes. A similar effect is observed on calm waters in the Arctic region in June during the NH summer solstice.
Symmetry suggests similar features east of Greenwich, however, these were not found. The animation, from 21 December 22:27 UTC–22 December 00:27 UTC and the comparison, below identifies the brightest sunglint occurring on icy waters.
The March and September equinoxes mark the times in the year when day and night are of equal length across the globe.
The line that separates the portions of the Earth experiencing daylight from the portion experiencing darkness is known as the 'terminator line'.
The equator was in the focus on 20 March because the Sun shone the brightest there on that day. The equinox happened at exactly at 16:15 UTC. After that time Sun starts to shed more energy on the northern hemisphere for the next six months.
Peak reflection of the Sun in the equator area is best observed through a sunglint. The Meteosat-11 Natural Colour animated loop (Figure 44) shows peak reflection of the Sun in the morning hours from the water surfaces of Indian Ocean.
Sunglint over Indian ocean was also observed with the MODIS instrument on the Terra satellite (Figure 45). The peak of the glint expands to the north (towards Sri Lanka) and towards the south as well.
Due to the advanced resolution of the MODIS visible channels (500m) the contours and shape of the glint are more obvious than on the Meteosat satellite imagery (max. 1km for the broadband HRV channel, 3km for the animated Natural Colour RGB). Roughness of the water surface leads to less Sun reflection back to the imaging instrument. This roughness is mostly affected by local winds.
Essentially the difference between MODIS and Meteosat-11 stems from their orbit. Sun-synchronous orbits see the sunglint as a vertical column, where the the max intensity is found at the latitude where the Sun is positioned at the back of the satellite, i.e. where the Sun falls vertically. From the GEO orbit the sunglint is almost circular, the position depending on the Sun-satellite geometry.
The Meteosat-10 Natural Colour RGB shows at the moment of equinox a roughly symmetrical distribution of frontal structures in northern and southern hemispheres, with the inter-tropical convergence zone in the middle, as also described in the case study Atmosphere mirroring around the equator.
Note also that the ice over Greenland and over Antarctica is seen with similar contrast.
This event in 2016 continues its shift towards an earlier time in our official calendar on leap years. It is now the earliest in the year since 1896. This trend will only be broken in 2100, because it will not be a leap year, thus delaying its occurrence by almost one day.
The Sun crosses the celestial equator from south to north, so it falls perpendicular on the Earth's equator, as can be noticed in the strong sunglint on the Indian Ocean in the Natural Colour RGB at 04:30 UTC (Figure 47).
The 2015 vernal equinox took place on 20 March at 22:45 UTC.
Around the date of the equinox the Sun heats above and below the equator equally and, therefore, we can consider, approximately, that the atmosphere should have symmetrical behaviour north and south of it. One example of this is the symmetric development of two low pressure systems north and south of the equator as can be seen in Figure 48 over the Atlantic ocean. However, many factors create local asymmetries such as global land/sea distribution, paths of warm/cold ocean currents, global atmospheric circulation, difference in the heat capacity of the air, land and water, etc.
Therefore, the better reference to the ‘temperature equator’ is Intertropical Convergence Zone (ITCZ). This zone can be depicted best as a band near the equator where the convective activity is at a maximum. Usually this is a bright area in satellite imagery. The ITCZ cloud band can be seen in the SEVIRI visible 0.6µm channel (Figure 49). The same figure shows an overlaid field of geopotential height of 500hPa surface. It is obvious that, for instance, the isoline of 5880gpm is equally distant from the ITCZ (around the eastern tip of Brazil).
The 22 September 2016 took place at 14:21 UTC. Figure 50 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 51) and weaker or calm (brighter brown pixels). This applies in a radius of roughly 500km around the point of specular reflection.
Meteosat-8 at 40 °E (Figure 48) and Meteosat-10 (Figure 51) 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 52 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).
The Meteosat-10 Natural Colour imagery (Figure 53) from 06:00 UTC and the Meteosat-7 Visible imagery (Figure 54) from 02:00 UTC show the Sun's illumination of the Earth due to the position of the Earth relative to the Sun on the morning of the equinox.
Meteosat-7 could take this similar looking snapshot four hours earlier than Meteoesat-10 due to it being 57° further east.The moment of equinox occurred slightly later at 08:22 UTC. Figure 55 is the Meteosat-10 Natural Colour RGB taken closest to the time of equinox.
Note the sunglint moving along the equator.
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