Hurricane bertha over Munich

During the passage of the remnants of hurricane bertha across Munich at 10:30 local time on the 11th August 2014, I obtained a spectrum of the relatively heavy overcast sky. The spectrometer fibre was pointed at an altitude of about 60° towards the NE. Using the same process as for the thundercloud reported in a recent post, I can show that the photon pathlength within the cloud in this case is around 30km rather than the over 100km seen in the thundercloud.

The water absorptions, while strong, are also weaker than in the thundercloud. The cloud is still bluer than direct sunlight but the spectrum has a shallower slope in the visible spectrum with a power-law* slope of approximately -1.1 (cf -2 for the thundercloud and -4 for Rayleigh scattering).

The brightness ratio at 600nm of this spectrum to the thundercloud is measured to be 23.5.

Another interesting thing to note in these spectra is that the CaII H & K absorption lines (the strong doublet between 390 and 400nm) appear weakly in 'emission' in the ratio spectrum. This is due to the 'Ring effect'. The light scattering of sunlight from air molecules in the atmosphere is mostly elastic (no wavelength change): the Rayleigh scattering that makes the blue sky. A small fraction of the photons, however, scatter inelastically giving (usually) energy to the molecule and producing scattered light that is a bit redder. This is called Raman scattering - after the first Indian Nobel laureate in physics, Sir C V Raman - which is able to take light from brighter parts of the spectrum and transfer some of it into the absorption lines and so making them shallower. When you take the ratio of this to the direct light from the Sun, the lines show up apparently in 'emission'. This is seen clearly in this and in the thundercloud spectrum.

*although the power-law is not such a good fit in this case (except in the middle of the visible spectrum).

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