Sunlight Attenuation

> Without looking at the horizon, would I be able to sense that I was
> on a Habitat? Would I sense any centripetal force? Would the
> sunlight be hotter or brighter on my face? Would I tan easier?
You would sense that you were in a habitat by moving vertically. As I calculated in an earlier post in the Coriolis thread, in an Island One I should feel a sideways tug equivalent to six pounds if I stood up rapidly from a chair. The Coriolis force scales linearly with the rotation, so since the Island Three of O'Neill's book has a quarter of the rotation rate of Island One, I should feel a sideways tug of about one or two pounds.
As for the sunlight, that is a matter of design: how large are the external mirrors, how opaque are the windows? How much cloud do we allow? That would partly be a matter of how much water there is in the habitat, partly on how swiftly the air is cycled through the radiators. Air cycled slowly through the radiators would have time to cool down significantly, causing a large percentage of the humidity to precipitate out, but simultaneously less warm, humid air would be extracted and less cool, dry air would be inserted. Larger mirrors combined with larger radiator surfaces would mean a larger temperature difference between the air being extracted from the habitat and that being inserted into it from the radiators, which again would likely mean less cloud, since more water would have been precipitated out. So there is room for weather design there. Similarly with allowing much sunlight into the habitat at daytime versus not so much, and cycling the air through the radiators more slowly at night, or more swiftly, for cooler nights. These things would influence cloud cover and aerosol content of the air, which again would influence how much direct sunlight shines on you. Remember also that clouds above the window areas of a Three would be sunlit from *below*, and you would see them from the sunlit side, particularly near the edge of a window. As well as the fact that the "sky" would have two bands of predominantly green, this would mean that light entering into a building through windows would have a different colour from what you might be used to on Earth, even if you weren't gazing out through the windows. But the effect may be too subtle to notice compared to different lighting conditions in different neighbourhoods in a single city on Earth.
Very likely the sunlight in a habitat will have less UV light, since any likely glass will tend to block that - or so I believe without knowing it. But any habitat designer would likely prefer to keep the UV light down, to reduce skin cancer among residents allowed to bask in the sun sixteen hours a day or whatever. Though perhaps some UV light would be permitted in - too little to increase significantly the risk of skin cancer, enough for such insects as can see UV light and use it to find flowers. But less UV light would mean less easy tanning.

> Here on Earth, the sunlight that reaches the ground goes through
> many kilometers of atmosphere, smog, water vapor and is reduced in
> intensity by the time it hits my face.
The atmosphere of the Earth is many hundred kilometers thick, but most of the mass is near the surface. The total mass of atmosphere above us is equivalent to ten meters of water. The total thickness of atmosphere above you in an Island Three would be the diameter of the cylinder - six kilometers. But the *mass* of atmosphere that the sunlight would pass through would not be that tiny a fraction - six divided by several hundred. A rough estimate would be this: water has approximately a thousand times the density of air, or somewhat less. So the ten meters of water corresponds to an atmospheric thickness of ten kilometers, or somewhat less, if the Earth's atmosphere were of constant density from the ground and up to some sort of ceiling.
The Island Three is six kilometers in diameter, and the atmospheric pressure at the axis, while not equal to that at the rim, is not significantly lower either. Three kilometers above ground level, it is at the pressure equivalent to one and a half kilometer's altitude on Earth. So the mass of air that a sunbeam will pass through in an Island Three would be lower than the mass of air that it passes through to the surface of the Earth, but not by an order of magnitude.

Jon Lennart Beck.