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Re: Refraction. was: Bubble Horizon Altitude Corrections
From: Fred Hebard
Date: 2004 Jul 12, 00:06 -0400
From: Fred Hebard
Date: 2004 Jul 12, 00:06 -0400
OK ladies & gentlemen. We have the case of a mirage, where the image rises above the desert floor. Are there any counter examples of an image sinking below the horizon? Fred On Jul 9, 2004, at 3:58 PM, Trevor J. Kenchington wrote: > A couple of days ago, I wrote: > >>> Temperature inversion in the San Fernando Valley, California. >>> Bob >> >> I stand to be corrected but I think that is a temperature inversion, >> not >> a density inversion. I think (but again no more than that) that the >> density of air is significantly affected by humidity, as well as >> temperature and pressure, such that warm, humid air can be stable >> under >> colder, drier air. >> >> Time to step aside from his topic and leave it to atmospheric >> scientists >> -- which I am not! > > I'm still no atmospheric scientist but, since everyone else has been > kind enough to ignore the obvious error in the above exchange, I guess > I > should point it out. (At least, the error should have been obvious but > it took me a day or so to notice it.) > > The normal pattern of temperatures in the atmosphere is one of > decreasing temperature with increasing height (though pressure also > decreases with height, such that density falls despite the lower > temperature). In a temperature inversion, there is an _increase_ in > temperature with altitude and thus a sharper than normal decrease in > density -- the exact opposite of Bob's suggestion that density > increases > with height in an inversion. > > This sharp drop in density at a temperature inversion can often be seen > when small-scale ones form on misty mornings: Smoke will rise from > sources near the ground but then flatten out at the height of the > inversion. Its density is low, as a result of the high temperatures > from > whatever fire is making the smoke, but no low enough to penetrate the > low-density warm layer above the inversion. Hence the smoke gets > trapped > near the ground (which, on a much larger scale, is why temperature > inversions in the San Fernando Valley lead to smog problems. > > > To return to my original point: I don't think that high density air > overlying low density can be stable. George has suggested mirages as > examples but those are very local and very unstable (hence the > shimmering often seen with them). Intense solar heating keeps a very > shallow layer of air warm, even though the heated air is rising and > dispersing while cooler air is descending into contact with the land > where it is rapidly heated its turn. > > > Trevor Kenchington > > > -- > Trevor J. Kenchington PhD Gadus@iStar.ca > Gadus Associates, Office(902) 889-9250 > R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 > Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 > > Science Serving the Fisheries > http://home.istar.ca/~gadus >