Solar Insolation

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Solar insolation (not to be confused with insulation) is the amount of radiation arriving from the sun that collides with a flat surface. This includes the direct (beamed) radiation from the sun, scattered (diffused) radiation from the sky, and reradiated (or reflected) radiation from the surroundings. Both direct and diffused radiation depend on a number of factors such as the day of the month, the time of day, and the weather. The reflected radiation depends on the topography of the surrounding area, the amount of shade, and the reflectivity (also called albedo) of the earth. Albedo varies significantly with the type of material and whether the surface is exposed or covered by water, snow, or vegetation. Atmospheric conditions affect the intensity of the light as it reaches the collector. Factors affecting the light intensity are cloud cover, humidity, and atmospheric composition.

Figure 1 Summer and winter sun paths in the northern hemisphere.
Figure 1 Summer and winter sun paths in the northern hemisphere.
Figure 2 Earth travels around the sun along an elliptical orbit about once every 365 days.
Figure 2 Earth travels around the sun along an elliptical orbit about once every 365 days.

It is a common experience that the sun rises higher in the sky in summer than winter. It may also be noted that, in the northern hemisphere, the sun rises south of due east in winter and north of due east in summer. As winter proceeds into spring and summer, the sun rises earlier in the morning, moves higher in the sky, reaches its highest point at noon, and sets later in the afternoon (See Figure 1). The opposite is true in the southern hemisphere (a).

Unlike what some might suspect, the total solar energy incident on a south-facing window is greater during the winter than in the summer in spite of the fact that days are longer in the summer. Two factors contribute to this:

1) Although the sun rises earlier in the summer, it remains north of the window before it sets in the afternoon. In winter, the opposite is true -- sun rises and sets south of the window; therefore, south-facing windows are subject to incident sunlight for the entire length of the solar day. For example, a house in Long Beach, California located at 33° 48’ latitude in the northern hemisphere experiences about fourteen hours of daylight (5:30 am to 7:30 pm) on Summer Solstice (June 21) and ten hours of daylight (7:30 am to 5:30 pm) on Winter Solstice (December 21). The south-facing windows in this house receive sunshine for only seven hours (8:30 am to 3:30 pm) during summer and receive the full ten hours of sunshine during the winter.

2) The summer sun is higher in the sky than the winter sun. In the winter, the lower sun strikes the windows more directly than in the summer when the sun is higher. For example, every square meter of the window of a house in Long Beach receives, on average, 220 watts of solar energy per hour of daylight in the winter, whereas the same window receives 40% less energy in the summer. Furthermore, because of the higher angle of incidence in summer, more of the sunlight is reflected off the glass than would be during the winter.

Of all the radiation emitted by the sun, only a very small fraction is intercepted by the earth’s atmosphere. This amount is called the solar constant and represents the energy from the sun, per second, that falls on a surface perpendicular to the sun’s rays and just outside the earth’s atmosphere. The solar constant is not truly “constant”, however. Because the actual orbit of the earth around the sun is elliptic, the average distance varies by about 3.4% during the year and the solar constant varies between 1,300-1,400 W/m2. Like the sun, the earth is also emitting radiation, but at a much lower rate. The earth’s (terrestrial) emission is in the infrared region of the spectrum and averages at around 172 W/m2.

Question: The data collected on the motion of the earth around the sun shows that the distance between the earth and the sun is slightly shorter in the winter than in summer (Figure 2). Why then are summers warmer than winters?

Answer: The earth’s rotational axis is not perpendicular to its orbit around the sun, but tilts by 23.5 degrees. In the northern hemisphere, the angle of the earth’s rotation tilts toward the sun in the summer and away from it in the winter. In southern hemispheres, the earth’s tilt is in the other direction and the seasons are reversed (b).

To calculate the amount of energy reaching a surface, we not only need to know the position of the collector plate, but also its tilt – the angle that the collector makes with a horizontal plane. It is obvious that the highest amount of solar energy will be received by a collector that is perpendicular to the sun’s rays. The sun is, however, rarely directly overhead, and its position changes throughout the day, limiting the solar power averaged over a 24-hour period to only one quarter of the solar constant value, or 340 W/m2. Due to the absorption and scattering by particles and clouds, only 18% of this amount actually reaches the earth. Therefore, for maximum collection efficiency, many designs employ tracking systems that move the collectors to directly face the sun at all times. Tracking systems, however, can be very costly and are used only in special applications.



(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) From now on, we will limit our discussions to the northern hemisphere.

(b) The seasonal variation of the lengths of days and nights can be explained by noting the relative position of the sun with respect to the earth. Twice a year on the vernal (first day of the spring) and autumnal (first day of the fall) equinoxes, day and night become equal in length. The vernal equinox has been celebrated throughout history as the time of rejuvenation and rebirth, thus marking the start of a New Year in the Zodiac calendar. The summer solstice occurs around June 22, and represents the longest day of the year. The shortest day of the year falls on winter solstice, at or around December 22 in the Northern Hemisphere.

Further Reading

Markvart, T., and Castanar, L., Solar Cells: Materials, Manufacture and Operation, Elsevier Publishing Company, 2005.

Galloway, T., Solar House, Elsevier Publishing Company, 2004.

Stine, W. B., and Harrington, R. W., Solar Energy Systems Design, John Wiley and Sons, Inc., 1985.

Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society, covers solar, wind and biomass energies.

External Links

National Renewable Energy Laboratory: Solar Research (http://

Energy Efficiency and Renewable Energy: Solar Energy, US Department of Energy (

American Solar Energy Society (

Solar Electric Power Association (

California Solar Center (