Expert: Steven LJ Russo - 9/7/2009

Question
QUESTION: I was standing in my back garden around midnight tonight watching how brightly the moon was shining through the thin clouds passing it by high up in the atmosphere. Whilst a large thin cloud was passing the it by, the moon was completely eclipsed by a dark area (slightly larger than the moon itself) within the cloud. The moon was not visible for 1-2 seconds before becoming visible again as the cloud continued to pass it by.

I was quite shocked at first as I thought it was a very dark object passing the moon travelling at the same speed as the cloud. I continued to watch the moon and approximately 10 seconds later another large transparent cloud passed over it containing a another, slightly larger 'dark space'. This dark area also eclipsed the moon, this time however for a slightly longer period of time, whilst also giving the appearance of travelling with the cloud or at the same speed.

I continued to watch for several minutes after I saw this but didn't spot any more dark areas within the clouds that passed the moon. This has intrigued me deeply, however in the 45 minutes since I witnessed this phenomenon, the little bit of research that I carried out on the internet has produced no explanations.

Hopefully you will be able to help put my mind at ease as to what caused this.

Louis Jenkins
Berkshire, UK
7/9/09

ANSWER: Lots of times you have thin cirrus clouds which are visible in the Moon light, and darker (nimbostratus possibly)clouds which are not visible.  So the Moon "disappears" behind the darker cloud for a few seconds, or sometimes minutes.

The thin cirrus clouds are about 6 - 8 miles up, and the darker clouds are about 1 - 2 miles up.

I have seen the thing you describe many times.

steve


---------- FOLLOW-UP ----------

QUESTION: It was (and still is) a very clear night, around 10-15% cloud cover, and the only visible clouds were the thin cirrus clouds. How is it that the darker clouds, which are lower and presumably thicker, are completely invisible to the eye?

Answer
It could have a lot to do with YOUR location.  If you don't have a lot of "light pollution", the lower clouds will not be illuminated that much.  Also, the lower clouds are "thicker" than the high cirrus clouds and wouldn't let that much light pass through.

Steve