Moon and Sun Halos

Part of Weather Forecasting

Ice crystal rings, coronas, sun dogs, and lunar brightness as weather predictors.

The 22-Degree Halo

The most important optical weather signal you can observe. A halo is a ring of light around the sun or moon, always at exactly 22 degrees from the center (roughly the span of an outstretched hand at arm’s length from thumb to pinkie tip).

How it forms: Hexagonal ice crystals in high cirrostratus clouds refract (bend) sunlight or moonlight at a minimum angle of 22 degrees. The result is a bright ring — often with a faint reddish tinge on the inner edge and bluish on the outer edge.

What it means: Cirrostratus clouds at 20,000-30,000 feet are almost always the leading edge of an approaching warm front. These thin, sheet-like ice crystal clouds spread ahead of the front by 12-48 hours, forming the halo before lower clouds and rain arrive.

The prediction: A halo around the sun or moon means rain or snow within 24-48 hours. The folk saying “ring around the moon, rain by noon” (or “rain coming soon”) has solid meteorological backing. Studies show this prediction verifies roughly 65-75% of the time — significantly better than chance.

Tracking the halo for better accuracy:

• Halo appearing and sky remaining partly clear for hours — rain likely in 36-48 hours
• Halo appearing and sky thickening within hours — rain likely in 12-24 hours
• Halo disappearing because the sky is getting clearer — the front has weakened or changed track; rain less likely
• Halo disappearing because clouds are getting thicker and lower — the front is arriving; rain within hours

Corona vs. Halo

These are frequently confused but mean different things.

A corona is a small, close ring (typically 1-5 degrees from the moon or sun) with colored bands — often bluish-white inner ring with reddish outer edge. It is caused by diffraction through water droplets in thin clouds (altocumulus or altostratus), not ice crystals.

A halo is a large ring (22 degrees) caused by refraction through ice crystals in cirrostratus.

Weather implications differ:

• Corona = short-term fair or stable. Coronas form in thin mid-level clouds. A bright, sharply defined corona means small, uniform water droplets — stable cloud layer, no imminent change. The corona itself is not a strong weather predictor.
• Shrinking corona (getting smaller over hours) means the water droplets in the cloud are growing larger — the cloud is thickening. This can indicate approaching moisture.
• Halo = change coming. As described above, halos in cirrostratus predict frontal passage within 24-48 hours.

Quick identification: Hold your fist at arm’s length. If the ring is within one fist-width of the moon/sun center, it is a corona. If the ring is two or more fist-widths out, it is a halo.

Moon Brightness and Clarity

The moon’s appearance is a direct readout of atmospheric transparency.

Sharp, bright moon with clear edges — dry, clean air with good visibility. High-pressure system in place. Fair weather continuing.

Fuzzy, haloed moon — moisture at high altitude. As discussed above, likely change within 24-48 hours.

Moon visible but dim, with a diffuse glow — low-level moisture or thin fog developing. Not necessarily a storm warning, but humidity is high. Ground fog likely by morning.

Moon not visible despite being above the horizon — thick cloud layer. Obvious, but note the texture: if you can see the moon’s general location as a brighter area behind clouds, the layer is thin. If the sky is uniformly dark, the cloud layer is deep and rain may be imminent or occurring.

Moon appears unusually large near the horizon — this is the well-known moon illusion and has no weather significance whatsoever. Ignore it.

Color changes in moonlight: A yellow or orange-tinted moon indicates particles in the lower atmosphere — dust, smoke, or pollution. After wildfires or volcanic events, this can persist for days without weather implications. Outside those events, an orange moon at high altitude (not near the horizon) suggests increasing lower-atmosphere moisture.

Sun Dogs (Parhelia)

Sun dogs are bright spots of light that appear on either side of the sun — at the same altitude, typically 22 degrees away (at the same distance as the halo ring). They often appear as concentrated bright spots on the halo, with a slight tail pointing away from the sun.

How they form: Hexagonal plate-shaped ice crystals floating horizontally in cirrus or cirrostratus clouds refract sunlight. Their horizontal orientation concentrates light at the two horizontal points of the 22-degree halo.

Weather prediction: Sun dogs indicate ice crystals in high clouds — the same cirrostratus that produces halos. The same 24-48 hour rain prediction applies.

Sun dogs without a full halo — the ice crystal cloud is patchy rather than sheet-like. Less reliable as a rain predictor, but still worth noting. If sun dogs persist or intensify through the day, the cirrus layer is expanding — frontal approach more likely.

Sun dogs with vivid colors (red closest to sun, blue/green farther away) — well-formed ice crystals in a uniform cloud layer. Stronger indication of organized cirrostratus and approaching front.

Low sun dogs (seen when sun is near the horizon) are less reliable because the sun angle changes the geometry. Mid-day or afternoon sun dogs at moderate sun elevation are more meaningful.

Light Pillars and Arcs

Light pillars — vertical columns of light extending above or below the sun. Caused by flat, plate-shaped ice crystals reflecting light. They indicate ice crystals in the atmosphere but are not a strong weather predictor by themselves. They are beautiful but less useful than halos.

Upper tangent arcs — curved arcs touching the top of a 22-degree halo. Caused by column-shaped ice crystals. Their presence indicates a well-developed cirrostratus layer — strengthens the halo rain prediction.

Circumzenithal arc — a bright, rainbow-like arc high in the sky directly above the observer (near the zenith). Vivid and unmistakable. Requires cirrus clouds with well-formed ice crystals. Like halos, indicates high-altitude moisture that may precede a front.

Practical Observation System

1. Nightly moon check. Step outside after dark. Sharp and clear = fair weather continuing. Halo = rain within 24-48 hours. Fuzzy = increasing moisture.

2. Daytime sun observation. Never stare directly at the sun. Use peripheral vision or shade the sun with your hand to look for halos. Sun dogs are visible without looking directly at the sun — they appear to the sides.

3. Track halo timing. Note when a halo first appears and when it disappears. This gives you the approach speed of the front.

4. Combine with cloud progression. If you see a halo and then notice clouds descending (cirrus to cirrostratus to altostratus to nimbostratus), you are watching a textbook warm front approach. Each cloud stage confirms and narrows the timing of rain.

5. Count the stars. On a clear night, note roughly how many stars you can see. If star count drops on subsequent nights (without moonlight change), atmospheric moisture is increasing. This is an imprecise but useful trend indicator.