Rare lunar halo – an odd-radius halo – over India

A rare lunar halo

The image above shows a rare lunar halo. Most halos around the sun or moon are common 22-degree halos. They’re caused by ice crystals in the upper air. The ice crystals are typically plate- or column-like hexagonal crystals. But the multiple halo you see in Soumyadeep Mukherjee’s image above is different. It comes from pyramidal ice crystals.

According to sky optics guru Les Cowley, pyramidal crystals tumble more in the air. And so they usually form only rather fuzzy circular halos. Les’ page on these halos, which are sometimes called odd-radius halos, mentions halos of 9-, 18-, 20-, 23-, 24- and 35-degree radius: multiple rings around the moon.

Thank you, Soumyadeep, and thank you, Les!

Here’s another photo of lunar pyramidal crystal halos at Les’ website Atmospheric Optics

Here are two more odd-radius halos, plus a simulation, at Les Cowley’s great website Atmospheric Optics.

Another odd-radius halo, below

Bottom line: Photos and info about multiple halos around the moon – caused by pyramidal ice crystals – called odd-radius halos.