Singapore
August 2013
There were two rainbow-tinted rings around the moon last night. - TWO!
The rings that encircled the moon arose from light passing through six-sided ice crystals high in the atmosphere. These ice crystals refract, or bend, light in the same manner that a camera lens bends light. The crystals have to be oriented and positioned just so with respect to your eye, in order for the halo to appear. The ring has a diameter of 22° , and sometimes, a second ring, 44° diameter, forms.
Thin high cirrus clouds lofting at 20,000 feet or more contain tiny ice crystals that originate from the freezing of super cooled water droplets. These crystals behave like jewels refracting and reflecting in different directions. Cloud crystals are varieties of hexagonal prisms, (6 sides) and range in shapes from long columns to thin plate-like shapes that have different face sizes.
That’s why, like rainbows, halos around the sun - or moon - are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals making the halo of the person standing a short distance away.
Hmmm that explains why, despite Singapore's small size,
a moon halo in Changi may not be visible by someone staying in Jurong...
Folklore has it that a ring around the moon signifies bad weather is coming. It is believed that the number of stars within a moon halo indicate the number days before bad weather will arrive.
Ok, there were zero stars around the two halos.
A while later, the sky completed cleared up and the craters on the moon can be seen clearly.
So how can rings around the moon be a predictor of weather to come? The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a warm front by one or two days. Typically, a warm front will be associated with a low pressure system which is commonly referred to as a storm.
True enough, this was the sky this morning...
With this weather forecast for the next 12 hours...
Sources
