So what is this meterological phenonoma called? I'm in Phuket, it's almost midday with the sun overhead. There is a very lagre and completely circular 'cloud' through which the sun is shining in the center (see my picture). I've never seen something like this - is the sky going to fall down??

Simon

We had this in Pattaya last year, all the locals thought it was the end of the world.

After alot of searching it turned out is is something to do with ice crystals forming in the upper atmosphere, refracting the light to form a perfect ring around the sun, quite impressive to look at. Sorry can't remember the exact name of the phenomenon.

OK, it's called a sun halo.. googled it, very nice effect...

Simon

This is a strange coincidence, but yesterday's "Astronomy Picture of the Day" was of just such a phenomenon. Have a look at this link, it describes the halo, and gives a load more links for further information.

http://antwrp.gsfc.nasa.gov/apod/ap080516.html

Incidentally, this is a very well run and informative site with a new picture every day, and some of them are absolutely stunning. Check out the archives.