|
meteoptic-l [ät] ursa.fi
message archive
This is meteoptic-l [ät] ursa.fi message archive.
Note, your can reply the messages on this page only if your
are already subscribed the list.
» To the end of the list/message
> James Lowery: "Glory on the Ground" (Nov 13, 18:11):
>> This week while I was flying a search and rescue mission with the Civil
>> Air Patrol, our plane (a single engine, high-wing Cessna 172) made a
>> nice glory on the ground. We were flying at an altitude of 1200 to
1500
>> feet above sea level which meant that we were even closer to the ground
>> than that most of the time. It was the first time I had been in a
plane
>> that made a glory on the ground rather on the tops of the clouds.
>>
>> The interesting thing that I noticed was that the brightness of the
glory
>> increased as it passed over *lighter* color terrain. I was expecting
it
>> to be brighter over darker terrain because my logic was that the bright
>> "light" of the glory would show up more against a dark "background."
>> This difference in the "brightness" of the glory is probably not
noticed
>> as much when they are projected on the cloud tops because the clouds
are
>> more of a uniform color.
>>
>> Does anyone have an explanation for why the glory was brighter when it
>> was projected onto lighter color terrain?
>
> Were there any fog at the ground where you observed this phenomenom? The
> reason I ask is that glory is result of light scattering in spherical
> water drops, such as fog or cloud droplets. If there were no fog and the
> phenomenom was observed at the ground, then you probably saw
> heiligenshein (hopefully I got it right), which is just a brightening of
> the terrain around your shadow. Glory, on the other hand, seem very much
> like corona around your shadow, with multiple rainbow colored rings
> around your shadow.
>
> If there was a fog layer at the ground where you saw the glory, then
> indeed one would expect the glory to be brighter against dark
> background, if that is the only difference in observational conditions.
Heligenschein requires water drops ON THE GROUND. It works like
this: the water drops focuses incident sunlight on the grass (or
whatever) below. The grass reflects this focused light diffusively,
but the reflected light is then once again focised by the water drops,
towards a direction opposite the original direction.
Heligenschein can be simulated quite neatly if you buy a number of
small glass balls about 1 cm in diameter (of the kind that can be
found in many toy shops) and then spread them out on e.g. a piece of
paper. Illuminate this with a lamp and look at it as close to the
lamp as you can get, then look at it more from the side. There'll be
a quite dustinguishable difference in brightness, and it"ll be obvious
why it gets so brighter when viewed as near the lamp as possible.
There's still another effect to consider, which I call the
"opposition effect". This effect requires no water drops neither on
the ground nor in the air above the ground, but it does require a
rough surface. This rough surface will produce a lot of small
shadows when illuminated from a direction different from directly
behind. These shadows are not individually visible, but they
decrease the brightness. When illuminated directly from behind, all
these small shadows disappear and the brightness increases quite
noticeably. This effect can be very obvious when flying over a forest
and the shadow of the airplane falls on the forest.
This "opposition effect" is also responsible for the full moon being
about ten times brighter than the half moon, and not merely twice as
bright as could be naively expected, or PI times as bright as if it
would be if the lunar surface was a perfect diffusor (=a "Lambertian"
reflector).
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch_at_saaf.se psr_at_home.ausys.se
|