Strange observation.

[ollypenrice]

I've just stepped out (well after sunset) on a windy night of patchy cloud, moving swiftly across the nearly full moon. The moon was showing clearly through a patch of bright, moonlit cloud. It was obvious that the cloud would clear the moon, leaving a patch of clear sky, then the next bank of cloud would cover it again. Now here's what actually happened; the cloud drifted past the moon (which was brilliant despite the cloud) and as the 'clear' sky passed in front of it the moon disappeared entirely. When the next bank of cloud passed in front of the moon it reappeared, bright as you like!

Now I live at a dark site and spend a lot of time under the stars. However, I can honestly say that I've never seen this before. I have several guesses as to what's going on but before I prejudice anyone I wonder what you think of this?  Is it something you've all seen dozens of times and I've just been too daft to notice? Or is it odd to you, too?

Olly

[Stu]

Have you been at some of that cheap French plonk again Olly? :-)

Sounds very interesting, but not something I've seen before!

Stu

[ollypenrice]

Have you been at some of that cheap French plonk again Olly? :-)

Sounds very interesting, but not something I've seen before!

Stu

Nope.

(It was quite expensive!)

No, really, I actually repeated this observation. It's consistent tonight .Does that mean it strikes you as odd as well?

Olly

[Stu]

Yes, it does seem a bit strange. I assume no stars appeared in the 'clear sky'?

Stu

[Paul M]

Perhaps what looked like a break in the clouds was actually obscured by a higher overcast?

[ollypenrice]

Perhaps what looked like a break in the clouds was actually obscured by a higher overcast?

I'm sure this is so. However, was it just a coincidence that the higher overcast was placed where there was no lower cloud or is there a systematic connection? This is what intrigued me.

Yes, it does seem a bit strange. I assume no stars appeared in the 'clear sky'?

Stu

None. However, there is an almighty amount of moon so that's to be expected.

Olly

[Paul M]

I'm sure this is so. However, was it just a coincidence that the higher overcast was placed where there was no lower cloud or is there a systematic connection? This is what intrigued me.

Olly

If there is a significant difference in the height of the cloud layers then when seen at a low elevation, features that coincide with your line of sight won't actually share the same column of air. They will be displaced in the horizontal as well as the vertical so any physical connection in terms of gaps coinciding is unlikely.

But you never know. Local topography and laminar wind flow can create some semi-static cloud features to repeat from time to time. Example being Cumulus Lenticularis, but they tend to have a "stacked" appearance through a considerable height. 

[brantuk]

I've seen this before - it's deceptive - all that happens is a much thicker bank of cloud passes in front of the moon totally obscuring it. It's so black it looks like clear night sky - but it isn't - it's a thick black patch of cloud. I would worry if you'd seen stars in it though.

[stevepeverall]

That is truly a strange observation Olly.

The only way I can rationalise it is that low cloud under moonlight is visible and high cloud is not because of the dispersion of the light.

I've been out on many a nights and not seen a cloud in the sky but also not seen any stars either.

Brantuk's point of "I would worry if you'd seen stars in it though" is very pertinent.

[ollypenrice]

I've seen this before - it's deceptive - all that happens is a much thicker bank of cloud passes in front of the moon totally obscuring it. It's so black it looks like clear night sky - but it isn't - it's a thick black patch of cloud. I would worry if you'd seen stars in it though.

I think you have it. The odd bit, though, is that the high cloud didn't block the moonlight reaching the low cloud. It can only have been a chance alignment of high cloud lying over a gap in low cloud.

Olly

[Zakalwe]

Interesting.

On a side note, it does go to show how even an experienced person can be confused by the night sky. And Olly is a guy that spends a huge amount of time under night skies. It's no wonder that people that rarely look up get confused and apportion all sorts of natural occurrences to supernatural sources.

[brantuk]

"chance alignment of high cloud lying over a gap in low cloud"

Yup - you'd have to be an astrolloger to predict cloud movement accurately lol.

"people that rarely look up get confused and apportion all sorts of natural occurrences to supernatural sources"

Yup - usually "small village" people in the middle of a desert somewhere - and I don't mean YMCA guests hehe

[Cath]

Darn it !

Don't you just hate it when a perfectly rational explanation spoils a perfectly good mystery 

[wxsatuser]

A possible cause, may be that with cloud would help cause this effect.

Here are two images from METOP A/B polar weather satellites.

These images show the south of France and the red areas are particulates.......particulates are light absorbing.

The red areas can be dust, volcanic ash, rubbish from fires or other stuff that may be thick enough to cause a problem.

pic1

pic 2

[Joe_L]

Being a cloud watcher (well what else do you do when the stars aren't out?) in daylight I've observed something similar - a patch of high cloud roughly matching a hole in the lower cloud, a sort of inverted Fallstreak Hole  (http://cloudappreciationsociety.org/gallery/photo-08044-5/). Sometimes they align but wind at different levels can alter things. 

Joe

[ollypenrice]

I don't think I like particulates!!

On the other hand Fallstreak holes look marvellous.

Good posts, thanks guys.

Olly

[Beulah]

Nail on head, Kim. 

Probably proves the point that there is no light pollution where you live Olly. All the clouds in my area are usually tinged with dirty orange...

[chiltonstar]

I've seen this before - it's deceptive - all that happens is a much thicker bank of cloud passes in front of the moon totally obscuring it. It's so black it looks like clear night sky - but it isn't - it's a thick black patch of cloud. I would worry if you'd seen stars in it though.

Yes, my conclusion. I've seen it quite often and sometimes looked with bins at the "sky" - it really is just a darker cloud with it's darkness emphasised because of the lighter clouds . (perhaps ry decent Italian red rather than cheap French plonk).

Chris