Distribution of Solar Energy Geographical, Seasonal and Spectral

2 Distribution of Solar Energy (Geographical, Seasonal and Spectral) 

Outside the terrestrial atmosphere, the total flux of solar energy is approximately 1.4 kW/m2 in a plane perpendicular on the direction to the Sun. This value oscillates a few percent for various reasons. Because of the elliptic orbit excentricity of the Earth, the distance to the Sun varies by 3.3% and consequently, the insolation is 6.6% larger at perhelion than at aphelion. Besides the well-known 11 year periodicity of sun-spots, it appears that the Sun is an irregular variable star with a small luminosity amplitude. 

The dashed curve on Fig. 1 is the standard spectrum of an opaque object23 traditionally called a black body at the absolute temperature T = 5750 K. This corresponds quite closely5 to the solar spectrum outside the terrestrial atmosphere in the wave-number range considered on Fig. 1. It was discussed23,25 that the maximum of such a standard spectrum as a function of the wave-length occurs at 4.9651 kT where the Boltzmann constant k = 0.695 cm-1. The constant 4.9651 is the root of the transcendent equation for C = 5 in 

If the standard spectrum of an incandescent opaque object is plotted as a function of wave-number, the maximum occurs already at 2.8215 kT (corresponding to 11270 cm-1 on Fig. 1) which contains the root of Eq. (1) for C — 3. The reason why the two spectra look so different, is that 1 nm corresponds to 10 cm-1 times (1000 nm/A)2. Finally23,251 if the spectrum is plotted as number of photons (not energy) per unit of wave-number, the maximum occurs at 1.5936 kT where the constant is the root of Eq. (1) for C = 2. This would be close to 6330 cm 1 for T = 5750 K, to be compared with 19 840 cm-1 or 504 nm in the A-representation with C = 5. 

Liquid water, also in the form of droplets in clouds, have a much broader type of vibrational bands than the gaseous molecule. H20 shows many overtones and combination frequencies unexpectedly strong, compared with the three fundamental frequencies. The best known effect is perhaps the blue colour of liquid water, mainly due to a weak band close to the sodium lines at 17000 cm 1 (and absent in D20). With a spectroscope, it is easy to see the narrow lines of gaseous H20 and a broad band of the liquid in the daylight from the sky. 

---