Insolation or the Incoming Solar Radiations is the main source of energy on the Earth’s Surface. The same energy is responsible for all the climatic phenomenon on the earth’s surface. It is also accountable for all the life on earth’s surface. Although, there is another fact that all the solar energy coming to the Earth’s surface is not utilized. Some of it is reflected back to space, some is absorbed by the atmosphere and some of it is absorbed by earth’s surface. There is a balance in the amount of energy reflected and absorbed which is known as the Earth-atmosphere energy balance or energy budget. To understand more about Earth’s energy budget, watch this video here.
Another important aspect about insolation is its distribution on the Earth’s surface. Due to earth’s shape, its axis tilt and rotation, all the regions doesn’t receive same amount of energy or insolation. This is the reasons for different types of climate in different regions of the world and changing seasons on Earth. We will discuss this in detail using CERES satellite imagery.
CERES or the Clouds and Earth's Radiant Energy System is NASA’s project which provide direct measurements of solar radiation. It has been installed on many important satellites as Terra, Aqua and NOAA-20 to get Earth Radiation Budget (ERB) data at hourly, daily and monthly intervals at spatial coverage of 20 km to global level. The CERES data are used by the climate, weather and applied science researchers to address a range of researches related to energy exchanges between the Earth and space. For more detail about CERES data, click here. The CERES data has been downloaded from here.
The satellite images used here are prepared by the FLASHFlux team, NASA Langley Research Center. FLASHFlux data are produced using CERES observations convolved with MODIS measurements from both the Terra and Aqua satellite. These colored images shows the amount of sunlight (in Watts per square meter). Light shaded areas receive more sunlight and the darkest colors show very less insolation.
| Colour Key of satellite image (From NEO Website - https://neo.sci.gsfc.nasa.gov/view.php?datasetId=CERES_INSOL_M) |
Vernal Equinox (March 21st ) – On march 21st, the sun’s rays are directly on the equator. This is also visible from the CERES image. The brightest region is the region between Tropic of Cancer and Tropic of Capricorn. Maximum insolation is concentrated in this region. The region from the tropics to poles, shows the dark shades, where less amount of insolation is received.
In the CERES image, we can also observe the balance in the insolation received in the tropical and polar regions in both hemispheres. If we don’t consider the difference between the land and water areas, the regions beyond the tropics, in the northern and the southern hemispheres, show similar intensity of insolation. In the tropics of the northern and the southern hemispheres also, similar intensity is there. You will see later on that this is not true in the solstice. During vernal equinox, the days and night are of equal length in all parts of the world.
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| Diagram and CERES satellite imagery of Vernal Equinox |
Summer Solstice (June 21st ) – The revolving Earth around the sun in an elliptical orbit, moves to another position at summer solstice. Due to the tilt of the Earth, the distribution of insolation at this position is changed. Now, on June 21st, the sun’s rays are directly above the tropic of Cancer in the northern hemisphere. The brightest region in CERES image i.e. the region north of tropic of Capricorn, receives the maximum insolation. While the least insolation is received in the regions south of tropic of Capricorn. Due to this unequal distribution of insolation, different seasons are experienced in northern and southern hemisphere. The different season are characterized by unequal length of day and night. As the insolation is focused on northern hemisphere, days are longer than nights. Vice versa happens in south of equator. The north of equator is summer whereas the south of equator experience winter at this time of the year.
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| Diagram and CERES satellite imagery of Summer Solstice |
Autumnal Equinox (September 23rd ) – Earth moves to another equinox position on 23rd September. As it was in Vernal Equinox, the insolation is concentrated in between the tropic of Cancer and tropic of Capricorn. The regions beyond the tropics receive comparably less insolation, as visible in the CERES image. Likewise, the insolation intensity in northern and the southern hemispheres, roughly copies each other. Equal length of days and night are experienced on Autumnal Equinox.
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| Diagram and CERES satellite imagery of Autumnal Equinox |
Winter Solstice (December 21st ) – On Winter Solstice, sun is directly above Tropic of Capricorn in South hemisphere. In the CERES image too, we can see the brightest region being the region beyond tropic of Capricorn. This region receives maximum insolation. As discussed earlier, due to unequal distribution of insolation, there is winters in the northern hemisphere and summers in the southern hemisphere. Now reverse happens, as compared to the summer solstice. There is more sunlight or insolation in south of equator, so the days are longer and nights are shorter. Vice versa happens in the Northern hemisphere.
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| Diagram and CERES satellite imagery of Winter Solstice |
The CERES data clearly shows the amount of insolation received and shift in the incoming solar radiation seasonally. The transition of solar energy shifts can be visualized more clearly by observing the month wise imagery available on NASA’s NEO (NASA Earth Observation) website at link https://neo.sci.gsfc.nasa.gov/view.php?datasetId=CERES_INSOL_M. To know more about CERES data, click here.
Reference -
NASA (n.d.). Home Page. What is CERES? Website link - https://ceres.larc.nasa.gov/
Satellite imagery produced by the NASA Earth Observations team based on FLASHFlux data. Data provided by the FLASHFlux team, NASA Langley Research Center.
NASA NEO Website (n.d.). Solar Insolation (1 Month) CERES data. Website link -- https://neo.sci.gsfc.nasa.gov/view.php?datasetId=CERES_INSOL_M).
