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(meteorobs) Fwd: Falling crystals


Oops! Here's a follow-up reply to a recent thread on 'meteorobs' about halos,
glories, arcs et al. This if from one of the two co-authors of that fine S&T
article I mentioned before, Les Cowley. Sorry for the delay, Les!

Note also the URL for Les's "Atmospheric Halos" site at the end: this is an
extremely informative site for folks wondering about these phenomena.

Lew Gramer


------- Forwarded Message

From: Les Cowley <lev@dial.pipex.com>
To: <dedalus@latrade.com>
Subject: Falling crystals
Date: Mon, 10 Jan 2000 10:48:28 -0000

>>No, it is not random...the flat crystals align themselves biased toward
>>being parallel with the horizon. Why? I can't tell you that at the
>>moment...more research required, but that is the facts of life.

>According to an excellent article I read in a recent Sky & Telescope about
>possible halos and arcs which might be observed in the atmosphere of Mars (it
>was December's S&T I *think*), flattened ice crystals align themselves parallel
>to the ground (NOT the observer's horizon!) simply because of air resistance.
>
>Drop a sheet of paper from a sufficient height, and it will orient itself the
>same way and gently drift to the ground - or even stay suspended, if it's high
>enough up and winds are at work. Note this is highly sensitive on the SPEED
>at which an object is falling: meteoroids have the tendency to do exactly the
>OPPOSITE, i.e., aerodynamically stablize themselves along their NARROWEST axis.

Lew,

I really should have written 'horizontal' rather than 'parallel to the ground'.
The particular orientations chosen by falling cloud crystals where the
surrounding air flow is laminar are those that have the greatest air resistance,
i.e. plates fall 'face on' and long prisms fall with their long axes horizontal.

Les


Les Cowley
  Halos http://www.lc.dial.pipex.com/halo/halosim.htm

------- End of Forwarded Message


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