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Location: UFOUpDatesList.Com > 2007 > Jan > Jan 5

Re: Holes In The Cloud Deck - Tarbell

From: Michael Tarbell <mtarbell.nul>
Date: Thu, 04 Jan 2007 11:20:41 -0700
Fwd Date: Fri, 05 Jan 2007 07:56:36 -0500
Subject: Re: Holes In The Cloud Deck - Tarbell

>From: Michael Tarbell <mtarbell.nul>
>To: ufoupdates.nul
>Date: Wed, 03 Jan 2007 14:25:11 -0700
>Subject: Re: The '06 O'Hare UFO

>>From: Martin Shough <parcellular.nul>
>>To: <ufoupdates.nul>
>>Date: Wed, 3 Jan 2007 16:31:53 -0000
>>Subject: Re: The '06 O'Hare UFO


>>It's a good point though that _if_ we had reliable data on a
>>volume of cloud evaporated then it might be possible to do this
>>calculation. I recall there was case, I believe in the northern
>>US or Canada in 1947, where a bolide-like UFO was photographed
>>apparently cutting a swathe through cloud and Projectr Sign
>>noted the possibility of doing a similar calculation (though
>>I've always wondered in that case whether the apparent path
>>through the cloud might not be shadow cast by a trail far above,
>>having seen similar effects caused by high jet contrails). But
>>anyway it's interesting.

>An obviously critical parameter is the "excavated" volume of
>cloud. I have emailed Peter Davenport off-list to see if has is
>aware of any data regarding the thickness of the cloud layer, as
>well as the angular width of the hole.


Peter Davenport was kind enough to reply with additional
information. To quote him directly: "The hole was reported to us
to have been almost exactly the same diameter as the object's
diameter." He apparently did not have any data for the thickness
of the cloud layer, although in a thank-you note I reiterated to
him my continued interest in this parameter.

This additional information regarding the angular width of the
hole has implications beyond the physics of how the hole was
generated. Combined with cloud layer thickness data, it also
places geometric constraints on where witnesses would have to be
located to even see clear sky through the hole.

Specifically, if H is the height of the base of the cloud layer,
T is the thickness of the layer, and D is the diameter of the
hole, then for an observer to see clear sky through the hole,
he/she must be within a ground range R of a point directly
beneath the hole, where

R = D(0.5 + H/T)

(Assuming the ground and cloud layer planes are parallel and the
hole axis is normal to them).

For the O'Hare case, the base of the cloud layer is reported to
be 1900 ft. The object diameter is reported to be anywhere from
6 to 24 ft (although larger estimates are consistent with the
data, e.g., one witness estimates the angular width as "a
quarter at arm's length", another estimates an altitude of 1500
ft, which gives an object diameter of ~53 ft). The cloud layer
is described as "thick", but I have no idea what that means. If
we adopt the values T = 150 ft and D = 25 ft for cloud thickness
and hole diameter respectively, then observers must be within a
ground range R of about 330 ft. to see the clear-sky effect.

It's not obvious whether observers beyond the clear-sky range
would notice the hole in the cloud layer, particularly if they
weren't looking for it. In any case, it is reasonable to
conclude that the area of observability was potentially quite


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