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(meteorobs) Re: Clouds and ZHR

Bruce and All,

If the solid cloud cover within your field of view exceeds 20%, the IMO 
suggests you suspend recording due to the uncertainty of correction 
factors involved under such conditions. Personally, I would not quit 
altogether, but rather isolate this section of time and the meteors seen 
until conditions improve. If the clouds are thin then I would suggest 
using no obscuration factors, but rather estimate these lower limiting 
magnitudes at 10-15 minute intervals during the times of interference. 
If only a small cloud is involved then estimate the LM inside the cloud 
and two other areas free of interference.

I hope this helps!

Bob Lunsford


Bruce McCurdy wrote:

> 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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