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(meteorobs) Clouds and ZHR
Hi folks, I have a question about how cloud cover affects the ZHR.
First of all, I have found many definitions of what ZHR represents in
theory, but very few references which actually contain a formula for
obtaining one from real observations. The one I did find, at
http://www.namnmeteors.org/guidechap8.html , contained one method and
mentioned some variables which were not even included in the formula. One
of these variables was cloud cover.
Let's assume a sky with broken cloud covering about 50% of the sky. If
meteors were point sources, one could expect this cloud to cover 50% of all
meteors. However, meteors are obviously not point sources but arcs of
light. If I see the first half of one meteor disappearing into a cloud, and
the second half of another appearing out of a cloud, I am going to count two
meteors, not two half meteors. So to say half my sky is covered so my
observed rate should be effectively doubled in subsequent calculations would
be entirely incorrect.
Percentage of cloud cover is one variable, but the degree of brokenness is
another which I would think is very difficult to quantify. Small clouds
with numerous gaps would permit observations of more "partial meteors" than
a solid wall of cloud covering a significant section of the sky.
Also, the location of the cloud compared to the radiant would present yet
another thorny consideration. A cloud of, say, 20° diameter covering the
radiant would block more complete meteors than the same cloud 90° away, due
to the much shorter apparent arcs of meteors near the radiant.
Yet another consideration is cloud thickness. Tonight I watched a thin
cloud roll over Cassiopeia, causing the stars in the W to fade considerably
but not quite disappear. Assuming I could see a third magnitude star, but
no fainter, through the cloud, presumably my limiting magnitude was reduced
in that area of the sky to LM = 3.0. The cloud itself was of variable
thickness, and once again I only needed to see part of a meteor shining
through the thinnest potion of it.
I would be very surprised if there is any sort of formula which will deal
with these issues in an entirely satisfactory way. I'm anticipating
experienced meteor observers use some sort of black art. But I'd appreciate
if you'd share your secrets, or at least your thoughts.
regards, Bruce McCurdy, Edmonton, Canada
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