Sun Photometer Graphs 3 & 4

Figure 3 graphically shows how the energy incident at the top of the atmosphere is affected by the atmosphere. The blue line indicates the reduction in the amount of energy scattered by the air molecules at each wavelength.  The effect is shown as a fraction of the energy (Irradiance) at each wavelength that reaches down to the surface, such that 0 transmission implies complete absorption or scattering by the atmosphere, and 1.0 implying no absorption or scattering by the atmosphere.  The green curve shows the absorption of energy by Ozone molecules that dominates the reduction in energy reaching the surface at the short (ultraviolet) wavelengths.  The red line shows the absorption due to atmospheric water vapor.  The net reduction by the atmosphere (Rayleigh scattering, absorption by water vapor and ozone, and no aerosols) is then shown by the black line.  The bottom illustration of Figure 3 shows how this affects the incident solar energy that is received at the surface at a mid latitude location in the summer (since the amount of sunlight received depends on the incidence angle and the distance from the Sun, the exact amount changes daily at a given location).

 
The discrete wavelengths at which water vapor absorbs is the key to the ability to discern the amount of water vapor in the atmosphere by measuring the amount of radiation at two different wavelengths where the amount of absorption by water vapor is significantly different.  In reality, a narrow band of wavelengths is chosen as the amount of energy in a very narrow range of wavelengths is smaller than in a wider band of wavelengths, so that the differential absorption is easier.  LEDs are commonly available that are sensitive to wavelengths around 820 nm and 920 nm, these can serve as detectors for water vapor. Figure 3 and 4 show the spectral response curves (how much signal can they generate when exposed to "light" of a given wavelength or color).  The water vapor absorption bad centered at 940 nm is much stronger than the one centered at about 820 nm, and this difference is what makes the detection of water vapor possible using commercially available LEDs.  Ideally it would be better to have a detector sensitive to slightly longer wavelengths, but these detectors are not generally as readily available.

 

Figure 3

If there are aerosols in the atmosphere, then the solar energy is reduced further by scattering.  The yellow line in  Figure 4 shows the reduction at each wavelength due to the aerosols.  It is seen that the reduction at wavelengths longer than about 650 nm is almost entirely due to aerosols, whereas at the shorter wavelengths the reduction is also due to Rayleigh scattering by the air molecules.

Two LEDs are used to sense or measure the amount of energy in two bands, one centered at about 820 nm where the amount of absorption by water vapor is small, and another centered at about 940 nm, where the water vapor absorbs strongly.  The Photometer enables the signal generated in those LEDs to be measured as voltage, which is recorded.  The elevation angle of the sun also is needed to calculate the amount of "atmosphere" between the instrument and the Sun.  Using a calibration formula, the ratio of the voltages and  the solar elevation angle, the total amount of water vapor in the atmosphere can be computed.

Figure 4