Insolation

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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.

US annual average solar energy received by a latitude tilt photovoltaic cell.

Synonyms: Flux, and Power density.

Insolation is a measure of solar radiation energy incident on a surface. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter (kW•h/m²). A related term, Irradiance, is the amount of solar power received on a given area measured in kilowatts per square meter (kW/m²).^[1]

The surface may be a planet or a terrestrial object inside the atmosphere, or any object exposed to solar rays including spacecraft. It may result in radiant heating^[2]of the object, or reflection, depending on the object's reflectivity or albedo.

The rate of insolation above the earth's atmosphere in outer space, is also called the solar constant, although it does vary slightly with the Sun's activity. It is measured by satellite to be roughly 1366 watts per square meter +/- 0.2%.^[3] The radiant power is distributed across the entire electromagnetic spectrum, although most of the power is in the visible light portion of the spectrum. The Sun's rays are attenuated as they pass though the atmosphere, thus reducing the insolation at the Earth's surface. The midday insolation on clear days in temperate latitudes may be estimated as 1000 watts per square meter (angled toward the Sun).^[1] The actual figure varies with the Sun angle at different times of year, according to the distance the Sunlight travels through the air), and depending on the extent of 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.^{[citation needed]}

1 The Projection Effect
2 Applications
3 See also
4 References
5 External links

[edit] The Projection Effect

The insolation into a surface is largest when the surface directly faces the Sun. As the angle increases between the direction normal to the surface and the direction of the rays of Sunlight, the insolation is reduced in proportion to the cosine of the angle. This is known in optics as Lambert's cosine law. This 'projection effect' is the main reason why 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 at the poles, the Earth's surface is angled away from the Sun.

[edit] Applications

In spacecraft design and planetology, it is the primary variable affecting equilibrium temperature and global climate.

In construction, insolation is an important consideration when designing a building for a particular climate. It is one of the most important climate variables for human comfort and building energy efficiency.^[4]

The projection effect can 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 47 degrees through the year).

Insolation figures are used as an input to worksheets to size solar power systems for the location where they will be installed.^[1]^[5] The figures can be obtained from an insolation map or by city or region from insolation tables that were generated with historical data over the last 30-50 years.