Definition
An optical phenomenon appearing as a luminous ring around the sun or moon, caused by the refraction of light through ice crystals suspended in high-altitude cirrus or cirrostratus clouds. The presence of a halo often indicates moisture aloft and can signal the approach of a warm front or deteriorating weather within the next 24 hours.
Plain English
A bright ring of light seen around the sun or moon when their light bends through ice crystals high in the atmosphere. Pilots and weather observers note halos because they often hint that wetter, cloudier weather is on the way.
Context Anchor
Pilots may notice a halo during preflight weather observation or while looking ahead at sky conditions in flight.
Derivation
From the Greek halos, meaning 'a disk' or 'a ring of light around the sun or moon.' The original Greek sense already described exactly what pilots observe today, which is why the word carried over directly into meteorology.
Why Pilots Care
Signals the likely presence of high ice-crystal clouds that often precede a warm front or deteriorating weather.
Grounding Statement
If you see a pale ring around the Sun or Moon, you are seeing light shaped by ice crystals high above you.
Intuition Check
A halo is not a cloud by itself and it is not a sign around the aircraft. In weather use, it is the ring of light caused by ice crystals high in the sky.
Example Sentence 1
During the evening preflight briefing, the pilot noticed a halo around the moon and checked the forecast for an approaching front.
Example Sentence 2
A 22-degree halo often appears when cirrostratus clouds are overhead, prompting a closer look at the forecast.