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Location: UFOUpDatesList.Com > 2007 > Jan > Jan 7

Re: O'Hare Sighting Cloud Hole Effect - Shough

From: Martin Shough <parcellular.nul>
Date: Sun, 7 Jan 2007 18:30:09 -0000
Fwd Date: Sun, 07 Jan 2007 15:11:15 -0500
Subject: Re: O'Hare Sighting Cloud Hole Effect - Shough


>From: Michael Tarbell <mtarbell.nul>
>To: ufoupdates.nul
>Date: Sat, 06 Jan 2007 13:38:44 -0700
>Subject: Re: O'Hare Sighting Cloud Hole Effect

>>From: Martin Shough <parcellular.nul>
>>To: <ufoupdates.nul>
>>Date: Thu, 4 Jan 2007 16:13:15 -0000
>>Subject: Re: Holes In The Cloud Deck

<snip>


>>Terrific photo of the more usual type of hole in stratified
>>cloud
>>(over Mobile, Alabama, I think) at this blog:

>>http://rustybucket.blogspot.com/2004_09_01_rustybucket_archive.html

>>Another beautiful example from the UK is here:

>>http://tinyurl.com/yexmxf

><snip>

>Great images Martin. It would seem your first example is most
>analagous to the O'Hare phenomenon, aside from size and
>orientation.

>I must admit some puzzlement if the mechanism here is the sudden
>transformation of supercooled water droplets into ice crystals.
>Such crystals would be less dense and have a higher drag
>coefficient than water droplets, so there's no obvious reason
>why they should "rain out", indeed one would guess quite the
>opposite. If the effect derives from the differing light
>transmitting properties of ice and water (their indices of
>refraction are very similar), why do we commonly see high
>altitude clouds composed entirely of ice crystals that have the
>same basic appearance as clouds of water droplets?

>Still hoping for a cloud physics guru to jump in.

Don't have one handy, Mike, but giving it some thought it seems
to me that the idea is not that water vapour in the cloud
spontaneously freezes and then falls as ice, but rather that ice
crystals fall from above into water vapour clouds and nucleate
precipitation, similarly to the way that this often happens in
ordinary rainfall.

Normally when the ice-crystal process starts precipitation it is
happening in the high sub-freezing regions of deep clouds with
mixed ice and supercooled water droplets, and isn't seen.

Water vapour tends to condense on the ice crystals and when mass
becomes more important than drag the larger ones fall, capturing
more droplets on the way and getting still heavier.

I suppose the idea is that when you have very stratified thin
cloud of supercooled vapour with clear air above and below,
exposed to a discrete and localised infall of relatively large
ice crystals from weather processes high above, then this
process is laid open to view. So what we are seeing fall out
would be the result of natural cloud "seeding". Does this make
sense do you think?


Martin






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