Spectrum of a thundercloud_3

This is a further developed representation of my thundercloud spectrum. The main addition is the transformation of the (green line) ratio spectrum by a division of it by the nitrogen dioxide transmission model fit (shown as the orange line spectrum shifted upwards by 0.4 units for clarity) and also by the power law function* that fits the bluening of the cloud with respect to direct sunlight.

I have marked the lightning emission lines/bands which originate - from left to right - from the nitrogen molecule (N2), the CN molecule in the UV and in the red/nir. a small blip (unmarked) at 617nm due to neutral oxygen (OI), and then OI, OI, NI and finally NI at 869nm.

The light blue spectrum is a HITRAN-derived transmission model ( www.cfa.harvard.edu/hitran/ ) for the Sun on the horizon which includes the ozone Chappuis absorption but does not include either Rayleigh or aerosol scattering. I added this to show the strength of the ozone absorption at twilight and its weakness when the Sun is at an altitude of 45° as it was during this lightning storm. The reason for this is that, during twilight, the incident Solar radiation takes a long tangential path through the ozone layer which lies between heights of about 12 and 40km, producing strong Chappuis absorption. At high Solar altitudes, the Chappuis absorption is so weak as to be almost invisible and the long scattered pathlength taken by light is the thundercloud happens at a much lower altitude where there is little ozone. The absorption bands of water and the oxygen Collisionally Induced Absorption (CIA) complex (labelled here as O4) are, however, of similar strength since they are all produced low in the atmosphere.

I also point out the Raman scattering phenomenon - the Ring effect - discussed in an earlier post. Another little twist is the presence of an enhanced continuum in the gap between water bands around 750nm. I think this is due to the vegetation on the ground reflecting from the underside of the cloud at a wavelength just above the 'chlorophyll red-edge'.

As I have already mentioned, lighting storms are one of the dominant contributors, along with fossil fuel combustion and the burning of forests etc., to the nitrogen oxides found in the atmosphere.

* That is (Lam/Lam_0)^-2.3, i.e., blue



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