Insolation

Encyclopedia : I : IN : INS : Insolation

Insolation must not be confused with insulation (the latter word has a "u" where the former has an "o").

Annual mean insolation, at the top of Earth's atmosphere (top) and at the planet's surface.

Insolation is the incoming solar radiation that reaches a planet and its atmosphere or, by extension, any object exposed to solar rays, such as a spacecraft or building.

In construction, it is an important consideration when designing a building for a certain climate. It is one of the most important climate variables for human comfort and building energy efficiency. In spacecraft design and planetology it is the primary variable affecting equilibrium temperature and global climate.

The rate of insolation in outer space near 1 AU from the Sun (i.e. at Earth) is also called the solar constant though it does vary slightly. It is close to 1350 watts per square meter of Sun-facing cross section, across the entire electromagnetic spectrum; most of that power is in the visible light portion of the spectrum. On Earth's surface, the Sun's rays are attenuated by the atmosphere. The midday insolation on clear days in temperate latitudes may be estimated as 1000 watts per square meter directly facing the Sun. The actual figure varies with the Sun angle at different times of year (because the sunlight must travel through varying distances of air), and with atmospheric haze and cloud cover. Long-term time-averaged insolation in sunny locations is closer to 250 watts per square meter, taking into account the lower insolation in early morning and evening, and the presence of night.

The insolation into a fixed surface (such as a window or roof) depends on the projected area of the surface perpendicular to the incoming sunlight, not directly on the area of the surface. This is the reason that the polar regions are much colder than equatorial regions on Earth: on an annual average the poles receive less insolation than does the equator, because the Earth's surface at the poles never faces the Sun directly. The projection effect can also be used in architecture to design buildings that are cool in summer and warm in winter, by providing large vertical windows on the equator-facing side of the building (the south face, in the northern hemisphere): this maximizes insolation in the winter months when the Sun is low in the sky, and minimizes it in the summer when the noonday Sun is high in the sky. (The Sun's north/south path through the sky spans over 45 degrees through the year).

See also

• Diffuse insolation
• Direct insolation
• Solar variation
• Effect of sun angle on climate

External links

• [Global Insolation Map]
• [National Science Digital Library - Insolation]
• [Yesterday‘s Australian Solar Radiation Map]

References

• Nall, D. H. (2004-11). Looking across the water: Climate-adaptive buildings in the United States & Europe. In The Construction Specifier, 57, 50 – 56.

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