The Color Green

Sky Watchers and the Joys of Scattering

Indy Burke (CPR now SGS) has passed on a couple of the CED flashes, and they're great! Could you add me to the distribution list? -- One cloud color point worth mentioning -- green. I've seen only two really green and dark clouds (the bulk of the cloud is dark, but the green is almost luminescent) —- one preceded a tornado funnel in Ohio, and the other preceded a 7 cm storm (in 20 minutes) here in Ft. Collins. Perhaps extreme events depend upon high concentrations of plant derived compounds (chlorophyll?).


This was a bit of fun for me. I searched my memory banks for green clouds and remembered only one occasion. I headed up-elevation from Ft. Collins then north and down toward Cheyenne. A great thunderstorm was just east of Cheyenne. The base of the cloud was greenish, the rest rather black. When I got the e-mail about green clouds, I didn't have an answer. So, I went to the books and asked around. I hit paydirt with a colleague, Bob Davis, who took an atmospheric optics at Penn State more than 10 years ago. He saved his class notes. I like a guy like that. The Prof. in this class would show slides of atmospheric optical phenomena and explain the physics. When test time came, he would show slides and say "What's going on here?" and wait 3 minutes and show the next exam slide. It sounds just like the art appreciation and music appreciation courses the jocks all took!

Anyway, here is the skinny on green, mean clouds.

  1. The sun angle must be very low. This gives a long path length for light scattering.
  2. The sun is in front of you; so, the light that reaches your eyes is by forward scattering (it is brighter than back-scattering (see the blog on mature manure mists.)
  3. There must be lots of haze size particles in the air (the size that scatters in the blue: e.g. biogenic hydrocarbons).
  4. The mean cloud is a collector of the air from near surface that is filled with the biogenic hydrocarbons. If these particles are lifted well up into the clouds, water condenses on their surfaces and rain drops are formed. If they are delivered downward in down drafts around the margins of the cloud, then the drops are evaporated and dried out and the small blue-scattering sized particles are returned.
  5. The blue wavelengths are scatterers to a maximum so in the direction of the sun, the green wavelengths get through to your eye and you see green and are overcome by the kind of jealousy that comes when somebody has already figured it all out and you are a Johnny-come-lately! It is not chlorophyll that makes the mean cloud green, but probably the terpines and hemi-terpines that are made early in stages of photosynthetic carbon fixating and can scatter away all the blue light.

Now for non-mean green clouds. Minnart reports his observations of green clouds only a few hundred meters above the ocean surface over the Indian Ocean where there was a scattering and reflection from a phytoplankton bloom.

Optics again. Sometimes you just can't keep the biota out of the story even if your try. This is a similar to the famous navigation trick by Scandinavian sailors. In ice-bound waters they would sail in the directions of the clouds with the dark bottom, not in the direction of those with bright bottoms. In this case, the strong reflection of light from the ice surface made cloud bottoms bright while the clouds over open water got little light reflection from the ocean surface.


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