During a night walk in the early hours today, I was treated to the scene above, a beautiful corona. But what is a Corona?
A corona appears as a series of coloured rings closely hugging the sun or moon. Unlike the 22-degree halo, which is born from ice crystals high in the cirrus clouds, the corona is a child of the lower atmosphere. It is formed by diffraction: light waves bending as they graze the edges of tiny, uniform water droplets in thin clouds, such as altocumulus.
The Clues
From a natural navigator’s perspective, the corona is a sign. If you notice the rings are bright and the colours—usually a sequence from blue on the inside to red on the outside—are vivid, it tells you the droplets are of a remarkably consistent size. This uniformity suggests a cloud that has recently formed.
The most vital sign for the outdoor observer is the change in size.
- Shrinking Rings: If the diameter of the corona is contracting, the water droplets are growing. This is a sign of increasing moisture and often heralds approaching rain or a thickening front.
- Expanding Rings: If the rings widen, the droplets are evaporating or becoming smaller, suggesting the air is drying out and the weather may be clearing.
To see a corona is to see the air’s texture. It is a reminder that the sky is not a void, but a fluid medium, reacting to the invisible shifts in temperature and pressure that dictate our journey across the land.
Coronas are often confused with both halos and glories. Here’s how to tell them apart:
The easiest way to distinguish them is to ask: “Where is the Sun?” and “How big is the circle?”
1. The 22-Degree Halo (The Ice Giant)
The 22° halo is the large, majestic ring that surrounds the Sun or Moon.
- Cause: Light refracting (bending) through hexagonal ice crystals in high-altitude cirrostratus clouds.
- Scale: It is huge. If you hold your hand at arm’s length and spread your fingers, the distance from your thumb to your pinky is roughly 20-22°. The halo’s edge will sit right at your pinky.
- Colors: Red is on the inside (nearest the Sun); blue/white is on the outside.
- Meaning: Because ice crystals are high up, a halo often signals the approach of a warm front and rain within 12–24 hours.
2. The Corona (The Water Crown)
The corona is much smaller and more intimate, appearing to hug the light source.
- Cause: Light diffracting (spreading) around tiny, uniform water droplets in mid-level clouds.
- Scale: Usually only a few degrees wide (about the width of three fingers at arm’s length).
- Colors: Red is on the outside; blue is on the inside. This is the opposite of the halo.
- Meaning: It tells you about the texture of the cloud. A shrinking corona means droplets are growing—pack your coat, as rain is likely coming soon.
3. The Glory (The Antisolar Spirit)
The glory is the most elusive of the three because you can never see it while looking at the Sun.
- Cause: Light backscattering off tiny water droplets. You must be positioned between the Sun and a bank of mist or cloud.
- Location: Always at the antisolar point (directly opposite the Sun). You see it around your own shadow (the “Brocken Spectre”) on a cloud deck below you.
- Colors: Like the corona, red is on the outside.
- Meaning: It signifies that you are above a layer of saturated, misty air.
The atmosphere is whispering its secrets. When you see a corona, you are witnessing a delicate, intimate dance between light and water.
You might also enjoy:
Halos, Sun Dogs and Circumzenithal Arcs
The Secret World of Weather – The Book
