In November 2023, a phenomenon appeared in the sky that transfixed the country. A ghostly, ethereal ring was surrounding the Moon, and it got people talking. What was causing this ‘Moon halo’? Did it really mean bad weather was on the way?
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What is a Moon halo?
A Moon halo appears as a luminous ring around the Moon. It’s usually circular or oval in shape, and adds a somewhat ethereal glow to the night sky. And, if your photos are anything to go by, it’s a brilliant target for lunar photography.
“A lunar halo is a ring of light that appears around the Moon with the inner edge appearing red,” explains Dr Darren Baskill, an astrophysicist from the University of Sussex.
How does a Moon halo form?
Put simply, Moon halos are caused by ice crystals in the upper atmosphere:
“As moonlight passes through randomly orientated hexagonal ice crystals, the light is reflected by at least 22 degrees, and so a ring of light forms in the sky with a radius of 22 degrees,” says Baskill.
This is why the halo is also known as a '22-degree halo' – and it can appear around the Moon or the Sun.
“Halos can form at any time of year and in any climate, as it is always cold enough at high enough altitudes for ice crystals to form. Only water is needed in the form of hazy thin clouds to freeze into ice crystals, which then reflect the moonlight into a halo,” explains Baskill.
What’s the difference between a lunar corona and a lunar halo?
A lunar corona and a lunar halo are both optical phenomena that involve the interaction of moonlight with atmospheric elements, but they have distinct characteristics, as Dr Baskill explains:
“A lunar corona was also visible over the UK last month. Appearing much smaller than a halo, only water droplets are needed, rather than ice crystals, to create the rainbow glow of a corona.”
“White moonlight is made up of all the colours of the rainbow, and as that moonlight passes through the water, it bends – diffracts – into its constituent colours, forming a circle of colours around the Moon.”
“The word corona means 'crown', and scientists often use the word to describe what they are seeing, whether it is a crown around the Moon (a Lunar corona) or on the microscopic level, a crown around a virus (coronavirus),” he says.
Lunar Corona:
- A lunar corona is a colourful, multi-ringed glow that surrounds the Moon.
- It occurs when moonlight interacts with water droplets or small ice crystals in the Earth's atmosphere.
- The size of the corona can vary, and the colours are typically more vivid, ranging from blue on the inner edge to red on the outer edge.
- Coronas are often smaller and more intricately coloured compared to halos.
Lunar Halo:
- A lunar halo is a bright ring of light encircling the Moon.
- It is caused by the refraction and reflection of moonlight through high-altitude ice crystals in the atmosphere.
- Lunar halos are usually larger and less colourful than lunar coronas. They commonly appear as a whitish or pale ring around the Moon.
- The ring is uniform in brightness and lacks the distinct colouration seen in a corona.
Moon halos in meteorology
Although Moon haloes themselves are not direct indicators of imminent rain, as they’re primarily caused by the presence of high-altitude ice crystals in the atmosphere, they are often associated with certain types of clouds.
The types of clouds that typically produce Moon halos, such as cirrus clouds, are sometimes associated with changes in the weather. Cirrus clouds are thin, wispy clouds found at high altitudes and can precede the arrival of a warm front. Warm fronts are associated with the approach of moist air and can lead to various weather conditions, including rain.
So, while seeing a moon halo doesn't directly mean rain is coming, it could be an indirect signal that there are high-altitude clouds in the atmosphere, and weather conditions might change. To predict rain, it's essential to consider other weather indicators and forecasts (for example, dark clouds, a drop in atmospheric pressure, a sudden temperature drop or even your trusty moggy not wanting to go out).
About our expert, Dr Darren Baskill:
Dr Darren Baskill is an outreach officer and lecturer in the department of physics and astronomy at the University of Sussex. He previously lectured at the Royal Observatory Greenwich, where he also initiated the annual Astronomy Photographer of the Year competition.
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