Of the total radiation, about 3/4 ultimately reaches the earth. Because most ultraviolet radiation is absorbed from the solar spectrum and does not reach the earth’s surface, the peak of the solar radiation that reaches the earth’s surface is in the visible part of the spectrum. In the visible range, precipitation, clouds, and sandstorms reduce solar radiation. To fully answer the question of how much sunlight the Earth gets, we need to understand more about all the different colors of light emitted by the sun. We’ve given ourselves some tools to talk about and quantify light in a meaningful way. Artificial light, provided by solar simulators, allows you to mimic the sunlight spectra that meets international standards which are needed to have consistent and reproducible experiments. To be able to consistently test photochemical processes in a lab setting, we need to have a standard definition of what is defined as sunlight and a reliable and controllable source of light. Additionally, materials (organic or inorganic) can be designed to absorb light in a broad spectrum of wavelengths even outside of visible light.Īs we can see, the answers to what is sunlight, and how much sunlight we get, are not that straightforward. For example, depending on the type of chlorophyll, plants can absorb wavelengths that range from the UV to the IR to produce useful energy. The position of the Sun throughout the day will also change depending on the season because of the Earth’s axial tilt.Įven though our eyes only perceive the visible portion of the electromagnetic spectrum, other wavelengths produced by the sun (such as UV or IR) also play an important role in many photochemical processes. For example, when the sun is highest at noon in a timezone, it is directly overhead near the equator but more oblique near the poles. Depending on the angle of the Sun, light will have to travel through more or less atmosphere, and the absorption and scatter will change. The atmosphere modifies the spectrum by absorbing and scattering light.Īdditionally, due to the Earth’s curved surface, the sun’s radiation reaching the surface does not strike all areas of the planet at the same angle.
Practically, though, we’re interested in the light that actually reaches the ground, which means light that has to pass through our atmosphere. The sun’s own activity changes result in emission variations of up to 1%.īefore light even enters our atmosphere, then, there’s variation in how much Earth receives. The solar irradiance received by Earth’s atmosphere changes over a year by about 6.6% due to slight variations in the Earth/Sun distance. When talking about the amount of sunlight falling on the Earth, irradiance is the quantity used, which as discussed above is the light power per unit area. Additionally, materials (organic or inorganic) can be designed to absorb light in a broad spectrum of wavelengths even outside of visible light.īecause the Sun emits visible light as well as many other useful and necessary wavelengths, it makes most sense to use radiometry to measure sunlight. For example, depending on the type of chlorophyll, plants wavelengths ranging from the UV to the IR to produce useful energy.
Light-sensitive chemical reactions are also known as photochemical processes, and there are many that humans and all life rely on. Photometry, on the other hand, is only concerned with the measurement of visible light, with a specific view (i.e weighting) toward how strongly or weakly the human eye responds to these wavelengths.Įven though our eyes only perceive the visible portion of the electromagnetic spectrum, other wavelengths produced by the sun (such as UV or IR) also play an important role in many light-sensitive processes. Radiometry is the science of measuring light of any wavelength, that is, in any portion of the electromagnetic spectrum.
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