Solar constant, radiant flux

The sun radiates approximately as a blackbody, with an effective temperature of about 6000 K. The total solar flux is 3.9 x 10 W. Using Wien s law, it has been found that the frequency of maximum solar radiation intensity is 6.3 x 10 s (X = 0.48 /rm), which is in the visible part of the spectrum 99% of solar radiation occurs between the frequencies of 7.5 X 10 s (X = 4/um) and 2 x 10 s (X = 0.15/um) and about 50% in the visible region between 4.3 x 10 s (X = 0.7 /rm) and 7.5 X 10 s (X = 0.4 /Ltm). The intensity of this energy flux at the distance of the earth is about 1400 W m on an area normal to a beam of solar radiation. This value is called the solar constant. Due to the eccentricity of the earth s orbit as it revolves around the sun once a year, the earth is closer to the sun in January (perihelion) than in July (aphelion). This results in about a 7% difference in radiant flux at the outer limits of the atmosphere between these two times. 

Essentially all of the energy for life originates in the form of electromagnetic radiation from the sun. In radiometric units the radiant flux density of solar irradiation (irradiance) perpendicularly incident on the earth s atmosphere—the solar constant — is about 1366 W m-2. The solar constant varies by up to 3.4% from the average due to the earth s elliptical orbit. The value given is for the mean distance between the earth and the sun (the earth is closest to the sun on January 3, at 1.471 x 10s km, and furthest away on July 4, at 1.521 x 108 km). There are additional variations in solar irradiation based on changes in solar activity, such as occur for sun spots, which lead to the 11-year solar cycle (Pap and Frolich, 1999). In Chapter 6 (Section 6.5) we will consider the solar constant in terms of the annual photosynthetic yield and in Chapter 7 (Section 7.1) in terms of the energy balance of a leaf. 

Solar constant - The mean radiant energy flux from the sun on a unit surface normal to the direction of the rays at the mean... 

Flux of radiant energy at the earth (Solar Constant) =... 

The solar constant 4 represents a radiant flux E (energy area ... 

Reactions (1) and (2) essentially convert solar radiant energy into thermal energy. The parameters which determine the rate of ozone formation (UV photon flux, atomic and molecular oxygen number density and the total gas number density) are not constant with altitude and so the ozone concentration and hence Tg varies with altitude. The net result is that Tg increases thoughout the stratosphere until a maximum is reached at the stratopause whence Tg begins to decrease again. 

The sun is an immense ball of gases heated by internal nuclear reactions that produce radiant energy. The sun s electromagnetic radiation is emitted into space in all directions and is constantly bombarding the earth. A continuous input of sunlight energy, to the extent of about 1.3 kW/m (19 kcal/min the solar flux ). 

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