23 Jan

… something Ray Pierrehumbert once said regarding exo-planets stuck in my mind. Which is that it is hard to imagine an atmosphere without clouds (my paraphrasing).

Atmospheres are expansive things; so condensible species carried in such an environment will inevitably form condensate dispersions — clouds and aerosols. What strikes me as one of the nature’s great mysteries is whether there is a principle that governs how many. More or less, wherever the atmosphere goes up, one finds clouds, and greenhouse atmospheres, by definition, are radiating away their energy, necessitating overturning, and what turns over rises up, at least for a bit. So clouds are to greenhouse atmospheres sort of like peanut butter is to jelly… But while we have an increasingly refined sense of the correct dosage of peanut butter; we really have very little idea as to what sets the condensate burden and hence the optical thickness of the atmosphere. If you think about it, it is kind of funny to realize how we all (myself very much included) are trying to work out how clouds might change by one or two percent with warming, but we can’t really give you a good answer as to why the optical depth is 4-5 rather than 2-3, or 8-9. A humbling thought.

With those motivating words in mind, for today’s stimulus I figured a zoom (through the successive red squares in the attached) might be fun … as on the large scale the clear-sky textures the atmosphere more than the clouds … these places include right where we brilliantly put our observatory (Barbados) and climatological counterparts in the Southern Atlantic. The next image is a zoom that seems consistent with this view, and only when one looks very hard and at very small scales, do the clouds become more interesting than the clear sky.

Which raises two further observations:

1. the little grey patch circled in yellow is the blow-off, or remnant condensate, from a cloud that almost certainly precipitated. We see this all the time, every now and again a cloud gets a bit deeper, rises mightely through the stratification but eventually succumbs, leaving a rather grey condensate shield (of likely big drops) behind to evaporate and condition the atmosphere for subsequent convection … these might be interesting to try and catch with our radar.

2.that we are rather clueless when it comes to the statistics of the clear sky? If, as I am speculating, the most remarkable thing is actually the absence of clouds, why don’t we spend more time studying clear sky? When was the last time you read a paper on the processes governing the clear-sky fraction.

Okay, that is enough fun for a night …

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