Re: Speckled Light Field in Wing Strobe

[John Denker <jsd@MONMOUTH.COM>]

Hi --

It's virtually certain you were seeing light rain.  I've seen this on a
hundred occasions, mostly in light aircraft.

* The only reason you don't see it on your typical airline trip is that the
airliner spends most of its time above the weather.

* Light rain (or snow) does not noticeably obscure visibility, so your
observation of clear city lights is not inconsistent.  In fact, rain tends
to wash dust and smoke out of the air, typically leaving it clearer than
before.

* The lack of observable moisture on the side windows is also not
inconsistent.  There's very little phase space for drops to hit the window,
and any drops that get lucky get blown away very quickly.

* The lack of rainfall at ground level is also not inconsistent.  It is
quite common to have light precip falling out of a cloud and evaporating
before it hits the ground.  This even has a name: virga.

Cheers --- jsd


At 11:06 PM 4/26/99 -0500, brian whatcott wrote:
> I was looking out of the wing window as the plane approached
> Montreal on wednesday last. When we cleared the thin cloud,
> the night-lights of the city looked strikingly clear.
>
> Then I noticed that as the wing tip strobe flashed, it was
> illuminating a field of prominent light speckles which varied
> randomly each flash. They numbered perhaps fifty, in a visual field
> extending through a circle of diameter perhaps six wing tip chords
> centered on the wingtip.
>
> I estimated the chord at the tapered end
> of the wing at about 1 meter. The short light tracks seemed to be
> about 2 to 6 cm long. Their length seemed constant and appeared to
> decay exponentially which I presumed was due to the flash
> characterisitic.
>
> The sky was obscured by the cloudbase at perhaps 5000 ft.
> The visibility was unlimited.
>
>  It was conceivable we were passing through the freezing level
> at this point.  The speckled field persisted for a minute
> or so during the descent or level approach segment ( not sure which).
>   Walking along a street there after landing, I noticed a cloud
> of fruit flies in a 2 meter sphere about three meters overhead.
> It was not chilly.
>
> I had not noticed this effect before during reasonably frequent
> air trips. Any thoughts?