Halos, Nacreous Clouds and Aurora

Halos, nacreous clouds and aurora can be told apart by three numbers: how high the light is, how cold the air is there, and which wavelength is emitted.

Three rare sights share one identification method: ask for altitude, temperature and wavelength. Once those three are pinned down, the rest is pattern matching.

The 22° halo is the lowest of the three. Cirrus clouds at -20 to -40 °C are made of hexagonal ice prisms with a 60° refracting angle, and light passing through two faces of those prisms is bent into a ring 22 degrees from the sun or moon. A spread hand at arm's length spans it: thumb on the sun, little finger on the ring.

Nacreous clouds sit far higher, at 15-25 km in the stratosphere, and form only where the temperature falls below -78 °C. Lit from below after sunset, they shine with iridescent mother-of-pearl colours that ordinary low clouds never show.

Aurora works at 100-300 km, where the air is thin enough for excited atoms to glow undisturbed. Oxygen's green line at 557.7 nm has a 0.7 s lifetime, so green curtains move and flicker quickly; the red line at 630.0 nm has a 110 s lifetime, so red glows build and decay slowly.

All three sit above the layer that makes ordinary sky colour — about 75% of the atmosphere's mass lies below 12 km. For that everyday blue and red, see Why the Sky Is Blue and Why Sunsets Turn Red and Orange.

• Measure the radius: a ring at arm's-length span around the sun or moon means a 22° halo in cirrus.
• Check the timing: pearly colours well after sunset, from clouds far above the weather, point to nacreous clouds.
• Watch the motion: fast green curtains with slow red fringes mean aurora, not cloud.
• Note the place and season: stratospheric cold below -78 °C and high-altitude aurora both favour polar regions.

Further reading

• Why the Sky Is Blue
• Why Sunsets Turn Red and Orange