Optical Phenomena

Meteorology · Chapter 10 · DGCA CPL and ATPL

Rainbows, halos, coronas and glories are clues: each tells a pilot what the cloud is made of, and whether it will ice up the aircraft.

Remember

Rainbow: refraction twice and one internal reflection in raindrops, opposite the sun. Primary 42°, red outside; secondary about 52°, fainter, colours reversed.
Halo: 22° ring round the sun or moon, by refraction in the ice crystals of cirrostratus. Ice means negligible icing. The rarer large halo is 46°.
Corona: small coloured rings (5° or less), by diffraction in tiny water drops of altostratus, mist or fog. Moderate icing if above the freezing level.
Inferior mirage over hot ground (steep lapse rate); superior mirage over cold surfaces (inversion). Bishop's ring: about 22°, diffraction by fine (often volcanic) dust.
Aurora borealis = northern lights, aurora australis = southern lights. St Elmo's fire: a bluish glow on the aircraft in a strong electric field (CU/CB).

What is the sky telling you?

Light meets water

Five cards with real photographs. Swipe or tap Next.

Opposite the sun, in falling rain A rainbow appears on a curtain of rain, drizzle or spray when the sun or moon is behind you. Its centre is on the line from the sun through your eyes, so from a hilltop or an aircraft it can be a full circle. The primary bow has a radius of 42°, red on the outside and violet inside.
Bent in, bounced, bent out Each colour of sunlight bends by a slightly different amount as it enters a raindrop, reflects once off the back, and bends again as it leaves. That splits white light into VIBGYOR. A secondary bow (about 52°) comes from two reflections: it is fainter, wider and its colours are the other way round.
Mirages Light bends as it passes between air of different density. Over a scorching road or runway the air just above the surface is much hotter than the air above: a steep lapse rate. Light from the sky curves upward to your eye, so you see a 'pool of water' that is really the sky: an inferior mirage. Over a cold sea or snowfield there is an inversion instead; light curves downward and distant objects appear lifted above the horizon: a superior mirage.
Twilight, irisation and rays At sunrise and sunset the light crosses a long path of air; scattering and absorption leave the reds and oranges. Irisation is pink-dominated colour along cloud edges, by diffraction. Crepuscular rays are dark bluish streaks fanning from the sun: shadows of clouds near or below the horizon.
Which way round? You see two rainbows, one above the other, after an evening shower over Bhopal.

Takeaway: Bows and rings come from sunlight bent (refracted), bounced (reflected) or spread (diffracted) by drops and crystals. Which one you see tells you what the cloud is made of.

Why is a rainbow always opposite the sun?

How a rainbow works

One drop, then millions.

Sunlight meets a drop A rainbow needs the sun (or moon) behind you and falling drops of rain, drizzle or fog in front.
Bent, bounced, bent Light is refracted on entering the drop, totally reflected once at the back, and refracted again on leaving. Each colour (violet, indigo, blue, green, yellow, orange, red) bends by a slightly different amount, so white light fans out.
The primary bow Drops at 42° from the point directly opposite the sun send you their colours: a circle of 42° radius centred on the line from the sun through your eye. Red is on the outside, violet inside. From a hill, a tower or an aircraft you may see the whole circle.
The secondary bow Sometimes a second bow appears outside the first, at about 52° (red at about 50°, violet at 54°): two reflections inside the drop, so the colours are reversed (red inside), and it is fainter and nearly twice as broad.

Memory hook: 42°, red out. 52°, red in.

Takeaway: A primary rainbow is sunlight bent entering a drop, reflected once inside and bent again leaving it; each colour bends a little differently. It is a 42° circle round the point opposite the sun, red outside. A secondary bow (two reflections) is at about 52°, fainter and wider, red inside.