Solar Radiation

February 28th, 2019

Solar Radiation

Solar radiation is radiant (electromagnetic) e

The electromagnetic spectrum encompasses all types of radiation. The label “Solar Spectrum” includes the electromagnetic energy released from the Sun

Energy from the sun

The Sun emits 3.864 X 1026 Watts(W)



Solar radiation, or light, travels in waves. We measure these waves by their wavelength (usually in nanometers) or their frequency (usually in Hertz)

Ultraviolet radiation(UV) is one of the three types of radiation emitted from the Sun and is the most electromagnetically energetic of the three. However, UV only makes up 8% of the Sun’s total radiation that reaches Earth’s surface. UV is separated into 3 wavelength ranges: UV-A, UV-B, and UV-C

UV-C (100-280nm) is the most harmful to organisms’ DNA. However, UV-C only accounts for 0.5% of the total solar radiation as most of the UV-C radiation is absorbed by the ozone

UV-B (280-320nm) radiation is only partially absorbed by the ozone. UV-B radiation is known for causing human skin cancer and impairing photosynthesis in plants

UV-A (320-400 nm) is not absorbed by ozone in the atmosphere. However, it can be blocked from the surface by cloud cover. UV-A is responsible for sunburns in humans and is also more inhibiting than UV-B in regards to photosynthesis. Studies have shown that UV-A can significantly decrease photosynthesis by more than 70%

Nearly all of UV-C, half of UV-B and some of UV-A radiation is absorbed by ozone in the stratosphere before it can reach the surface.

Infrared radiation (IR) is the least electromagnetically energetic radiation emitted from the Sun at >700nm. However, it does account for 49.4% of the solar energy that reaches Earth’s surface

IR is absorbed mainly by water vapor and carbon dioxide in the atmosphere and converted to heat energy. Thus IR is responsible for warming Earth’s surface

If IR, along with greenhouse gases, were not available to warm Earth’s surface, Earth’s average temperature would plummet to -18 C (0 F) compared to the current average temperature of 15 C (59 F)1

Infrared radiation is responsible for warming Earth’s surface and atmosphere.

Photosynthetically Active Radiation (PAR), also known as visible light, is the wavelength range (400-700nm) that is best fit for photosynthesis to occur. PAR encompasses the electromagnetic spectrum from visible blue/ violet to red

PAR is the only wavelength range considered “photosynthetically active” because it has the perfect amount of energy to excite the electrons needed to start photosynthesis, but not enough energy to damage DNA or break chemical bonds

Optimal photosynthetically active radiation is the band from 400-700 nm, which encompasses the visible light spectrum

Solar Radiation can be affected by many factors

  • Time of Day
  • Cloud Cover
  • Air Pollution
  • Hole in the Ozone Layer
  • Latitude(Angle of Sun)

In the summer, solar radiation (measured by irradiance) will be greatest over the equator and the hemisphere tilted toward the sun.

The further a city is from the equator, the more the solar radiation received will fluctuate throughout the year.

The angle of the sun determines solar irradiance. The greater the angle, the lower the solar intensity.

Measuring the amount of solar radiation available within a water column is useful for scientists to make inferences about water quality and estimates of algal biomass

PAR sensors give scientists the ability to quantify the amount of photosynthetically active radiation that is available to algae within a water system

These PAR sensors can be placed above water and underwater to get a measurement of the upwelling radiation (radiation reflected from below) and downwelling radiation (radiation from above), respectively. The combination of these two measurements provides an accurate measure of the PAR available in the water column3

Direct solar radiation affects the temperature of the water column, which allows for fluctuations in photosynthetic activity

PAR sensors use a silicon photodiode, which converts photon energy to electrical current, and a glass optical filter to contain the wavelength absorption between the preferred range of 400-700nm2

Some PAR sensors also contain a diffusive sphere that allows for an enhanced field of view for sensing a wider angle of radiation. The sphere directs light through the optical filters to the photodiode4

To maintain accuracy, the sensors’ cosine response accounts for indirect radiation. This indirect radiation can originate from scattered light caused by the turbidity of the water column, or sunlight received when the Sun is at a low solar angle5

  1. Ma, Q. (1998, March). Greenhouse Gases: Refining the Role of Carbon Dioxide. Retrieved February 25, 2019, from
  2. Light Measurement. (n.d.). Retrieved February 25, 2019, from
  3. Underwater PAR. (n.d.). Retrieved February 25, 2019, from
  4. Light Measurement. (n.d.). Retrieved February 25, 2019, from
  5. Light Measurement. (n.d.). Retrieved February 25, 2019, from

All references can be viewed in the reference section of the Fondriest Environmental Website “Learning Center” under “References”


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