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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
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