If you’re wondering about that large ring around the Moon and what it means, you might be interested in the following. It’s what I discovered about the cause and relevance of Moon halos for weather prediction.
I’ve always held the belief that when you see a ring around the Moon it means rain is on the way. My dad told me this as a kid and it somehow stuck with me ever since. I guess it fits with what I’ve observed.
But is there any physical reason for the connection between Moon halos and weather. I found out the nitty gritty…but I’ll start with what this phenomenon represents…
What is a Moon halo?
A Moon halo is an optical illusion! Yep, just like rainbows and desert mirages it doesn’t exist at the point of where it appears to be located. Similar are Sundogs and Diamond Dust.
There’s really no ring around the Moon, but rather the Moon shining through our atmosphere produces this illusion. It’s not the Moon but rather the atmospheric conditions that’s different to normal.
What you see is this hazy ring circling the Moon. Sometimes it has a rainbow effect. It’s not always as large as in the above image. It can be smaller and tighter.
Here’s a different image…
Of all the Moon phases, the one associated with halos is the ‘full Moon’ or days on either side of the full Moon as this is when the Moon reflects the most light.
Halos are fairly common compared to Blood Moons and Blue Moons, which are considered rare.
What causes rings around the Moon
What causes Moon halos is the presence of ice crystals, which refract the Moon’s light.
Cirrus clouds are involved. These are the wispy feathery type high above that are also known as ice clouds. They contain ice crystals because the air at that height (altitude of 5.5 km, ~ 20,000 ft) is extremely cold. Any moisture freezes.
These ice crystals have a hexagonal face (these are mini 6-sided prisms with 60° sides) and as the light passes through the face and out the side of numerous ice crystals suspended in the atmosphere it creates this illusion of a ring around the Moon.1
To better understand how light refracts, think of light passing through a prism.
Halos around the moon indicates rain
It wasn’t just my father who held this idea about Moon halos heralding rain. Age-old beliefs of various cultures around the world exist with similar stories about the phenomenon.
Cultural astronomy: First peoples of Australia recognized the significance and relationship between Moon halos and weather. Some traditional clans spoke of the size of the halo indicating the amount of rain to come.1 This has to do with the size of the ice crystals.
- Larger ice crystals = large halo = large amounts of rain
- Smaller ice crystals = small halo = small amount of rain
Association with rain
The Moon halo tells us that ice crystals are in the atmosphere due to cirrus clouds. The increasing presence of cirrus clouds indicates a low-pressure system (rain) is on the way.
When high-level cirrus or low-level stratus clouds increase in a region, the region can expect to see precipitation from an incoming low-pressure system.
National Geographic, “Cloud Cover“
Moon circle and wind
Torres Strait islanders could tell of approaching winds by how the halo around the Moon was positioned, centered or off-center.1
Also for them, coupled with wind, the halo indicated that rain was imminent, but with a calm outlook, rain was days away.1
Coastal and islander folk used this knowledge to assess their opportunities for fishing and venturing across seas.
Bottom line
Cirrus clouds are what cause rings to appear around the Moon as seen from Earth.
Cirrus cloud formation is the physical reason for the halo effect. It’s also the meteorological connection to trailing rainy weather conditions associated with the phenomenon.
Indigenous peoples used their naked eyes and intuitive thinking (pattern based) to understand the sky, land, and sea relationships, an example is with Moon halos and weather.
Their sky knowledge preceded telescopes and binoculars of modern day astronomy. To their advantage, they had the opportunity of gazing at dark skies wherever they went.
Info sources
- Astronomy: Sky Country (2022) by Noon and De Napoli shares the first knowledge of the night skies of Australia.
