If you ventured out tonight and glanced up towards the sky, you might well have noticed that the moon looked a bit - how shall I put it - out of focus. The reason for this is that the near full moon was shining through a thin vale of cirrostratus clouds, giving it that fuzzy appearance. But, if you were lucky enough, you would have noticed something else about the moon - it was surrounded by a large glowing halo.
Cirrostratus clouds can produce a range of atmospheric optical effects and halos. During the day, these optics can be visible with slight but very recognisable colours - although it's clear you are not looking at a rainbow as the features are the wrong shape in the wrong part of the sky. Given tye fact that they are suspended high in the sky there's definitely no chance of this Irish man finding a little persons with a life changing pot of gold in his possession.
The first time I photographed a halo, the rainbow colours were clearly visible and inevitably the old mnemonic about the vanity of a certain king's military exploits popped into my head as I singledout the colours one by one.
But if these halo features aren't rainbows, what causes them? They are similar to rainbows in that it's got to do with the refraction of light. But in this case, the light is being refracted through the ice crystals that the cirrostratus clouds are made up from. For the full circular halo to be formed, the ice crystals must be hexagonal in shape, like short unsharpened pencils - or basaltic columns! (I wonder if anyone photographed the halo at the Giant's Causeway tonight - that would have been some juxtaposition!). And this shape causes the light to be refracted through normally on average at 22° - hence this feature is known as a 22° halo. It occurs typically on 100 days during the year
In actual fact, in the images I took tonight, I think I can make out a smaller fainter ring inside the larger one. That, along with the larger size of this halo, makes me think the main halo might actually be a rarer and much larger 46° halo, but I've contacted some weather experts on Twitter to check this out for me. If it is, it'll be the first time I've captured this particular halo.
Both the 22° and 46° halos are most common with cirrostratus clouds as these tend to form a reasonably uniform milky coverage across the sky, allowing the unbroken circles to be formed. In folklore, they are often associated with unsettled weather approaching. This is actually based on some sound meteorology. The rising milder air associated with a warm front at first can form cirrus and cirrostratus clouds, harbingers of coming frontal rain and blustery winds. And, sadly, in the case of tonight's cloud, that is indeed the case as a depression lines up to pass our way on Thursday.
But in the meantime, hopefully you got to enjoy something of the wonder of tonight's moon halo shining through the milky cirrostratus draped as a veil across the whole sky. Tomorrow, the only ring you'll be seeing above you if you're out and about is the big sphere of an umbrella!