Next message: Trevor J. Kenchington: "Mirages, was: Refraction"
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 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
>
