Captain Meteo Answers Your Atmospheric Science Questions

Q: Why is there Air?

The original atmosphere of our planet was almost certainly lost to space in the early excitements of planetary formation and bombardment. The air we have today can be considered to be the accumulated out gassing of the planet - Terran afflatus, so to speak. The rocks out of which our planet formed contained small amounts of various gasses which have been gradually released by physical and chemical volcanic processes.

Q: Why is the sky blue?

The blueness of the sky, where the sky still is blue, is evidence for the molecular composition of the atmosphere. Because the atmosphere consists of myriads of these little tiny guys bouncing around randomly, their random motions produce random fluctuations in the density of the air at very small scales. The expected size distribution of these fluctuations can be calculated, and it turns out that most of them are several times as small as the wavelength of visible light. Light scatters from density fluctuations (or, more precisely, from the index of refraction fluctuations resulting therefrom) and the expected amount of scattering can be calculated.

It is found that for the appropriate size fluctuations, scattering varies with the fourth power of frequency (inverse fourth power of wavelength) so that shorter wavelengths are scattered a lot more. Blue light is the shortest, and is scattered the most. The blue light of the sky is a haze of light scattered from the fluctuations in molecular density.

Ultraviolet light is shorter still, and scattered even more, which is one reason that you can get a sunburn outside even if you are sitting in the shade of an awning - there is enough scattered UV to give you a burn.

If visible light was several times shorter in wavelength, or the density variations several times larger, scattering would be more democratic among the wavelengths

Q: Where do clouds come from? Were there clouds in ancient times?

If you spend a bit of time watching the sky you notice that some clouds appear to come from somewhere over the horizon, but others seem to form out of thin air. Well, all clouds do form out of thin air, but some last long enough to blow from one side of the horizon to another.

The way clouds form out of thin air is that water vapor in the air condenses to form tiny drops of water, or, sometimes, if it's cold enough, tiny particles of ice. Nearly everyone has seen the first process - if you watch steam condense from a tea kettle, or a steam engine, or breathe out on a cold day, or in a walk in freezer. If the air is clean enough, it's really quite hard to form these little droplets or (especially) ice crystals. Water vapor likes to have a little bit of "dirt" of some kind or other to condense on - something with some electrical charges sticking out to attach to the polar water molecules.

Bits of such "dirt," suitable for the formation of water droplets, called cloud condensation nuclei, or CCNs, are ubiquitous in the lower atmosphere. Tiny grains of sea salt from breaking wave, the chemical exudates of trees and factories, and dust from the ground all work well. Nuclei suitable for the condensation of ice crystals (ice condensation nuclei) must meet a tougher standard, because ice crystals are very picky about how they form. When water vapor reaches the critical amount in the atmosphere, there is usually enough CCNs for clouds to form. For ice clouds, this isn't necessarily so. That's the reason that we often see giant contrails form in the wake of jet aircraft. There might be enough water vapor for a cloud to form, but not enough ice condensation nuclei. The exhaust of the jet engine seeds the upper atmosphere with enough condensation nuclei to produce sometimes giant clouds - the US cleared rater noticeably in the days after 9/11 when no planes flew.

So, how about our second question? Were there clouds way back when? For sure there were, since we see eroded sedimentary rock even from extremely ancient times, rock that could only have been produced by running water, which had to get there from the sky, in the form of rain or snow from clouds.

Any Questions?