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> 1. The first was observed several years ago in San Diego, CA,
>USA. There was a thin (but quite visible with some structure) Cirrus
>cloud deck with a long (essentially horizon to horizon) contrail under
>the cirrus (this was obvious from the structure). The sun was
>essentially directly behind contrail from my viewing point. I was in an
>automobile driving along and as I drove, the sun's position changed from
>slightly to the left of the contrail to slightly to the right of the
>contrail. The phenomena observed was a "shadow" on the cirrus that was
>on the side of the contrail opposite from the sun (i.e., it was on the
>right of the contrail when the sun was on the left of the contrail).
>None of this is especially strange except (as I noted above), the
>contrail was BELOW the cirrus deck (hence the quotes around the term
>shadow, since it seemed to be reflected back onto the cirrus deck).
- You state that the contrail was under the cirrus layer, and that this
was
somehow obvious from the "structure". Can you elaborate on this? From
your description of the shadow it sure sounds like the contrail was
above
the cirrus. On the other hand, the explanation may be the same as for
2)
below...
> 2. Just the other day, while driving southe near Portland, OR,
>USA, I saw a long contrail toward the west. It was approximately 2 pm
>local standard time, and although I couldn't see the sun (the car's roof
>was in the way), they must have been fairly close. The contrail was
>essentially north-south, as was the direction of the aircraft, which was
>visible near the head of the contrail in the south. There was cirrus
>present, although none was visible in the vicinity of the contrail
>(although there could have been thin, non-visible cirrus). A thin
>darkening appeared as a thin straight line, essentially co-located with
>the contrail (perhaps slightly offset, but if so not by much). This
>darkening of the sky extended along the contrail and in front of the
>contrail all the way to the horizon.
2) Could you have been seeing the shadow of the contrail in the lower
atmosphere? The shadow would be essentially a plane, and if you were
viewing the contrail from within or nearly within this shadow (i.e. the
sun was behind or very near to the contrail if you could have seen it
through the car roof), the shadow would appear as a line coincident
with
the contrail and extending to the ground ahead of the aircraft viewed
at
a lower altitude than the sun. This visible shadow is in fact the lack
of
light scattering from the aerosols in the air, so is much like the
"rays"
visible radiating from a cumulus cloud with the sun behind it.
Chris Luginbuhl
U.S. Naval Observatory
Flagstaff Station
cluginbuhl_at_nofs.navy.mil
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