Chapter 10
Optical Phenomena
DGCA CPL/ATPL Study Notes — Aviation Meteorology
Compiled by Capt. Pankaj Pahil
Source: IC Joshi — Aviation Meteorology
1. Introduction
Optical phenomena occur frequently in the atmosphere. They provide useful information about cloud constitution, cloud type, atmospheric turbulence, lapse rate, ice accretion etc. — all important for aviation safety.
flowchart LR
A[Optical Phenomena] --> B[Refraction-based\nRainbow, Halo, Mirage]
A --> C[Diffraction-based\nCorona, Bishop's Ring, Irisation]
A --> D[Reflection-based\nGlory]
A --> E[Emission-based\nAurora, St Elmo's Fire]
A --> F[Scattering-based\nTwilight Colours]
2. Rainbow
Rainbow: A group of concentric arcs with colours ranging from violet to red, produced on a background of water drops (raindrops, droplets of drizzle or fog) in the atmosphere by light from the sun or moon. Sometimes two rainbows are observed — Primary and Secondary.
Primary Rainbow
A circle or arc of circles of coloured light in the sky while it is raining. The circle subtends an angle of 42° at the eyes. Seen opposite to the sun or moon. Its centre is on the line joining the luminary and the observer. The rainbow may form a complete ring when seen from a hill, high tower or from an aircraft. Due to double refraction on entering the rain drop (once on entering, second time on leaving) with one total internal reflection. Colours: VIBGYOR (red outside, violet inside).
Secondary Rainbow
Concentric with the primary. Radius about 52°. Much less bright than primary with breadth almost double. Red is on the inside (radius of arc 50°) and violet on the outside (radius of arc 54°) — colours reversed compared to primary.
| Feature | Primary Rainbow | Secondary Rainbow |
|---|
| Radius | 42° | 52° |
| Red colour position | Outside | Inside (50°) |
| Violet position | Inside | Outside (54°) |
| Brightness | Bright | Much less bright |
| Internal reflections | One | Two |
| Breadth | Normal | Almost double |
Mnemonic: VIBGYOR (Violet Indigo Blue Green Yellow Orange Red) — Primary: Red Outside Violet Inside (ROVIn). Secondary: reversed (VIOlR = Violet Outside, Red Inside).
3. Halo
Halo: A circle of light around the sun or moon. Produced by refraction of light through ice crystals present in Cirrostratus (CS) clouds.
Small Halo (Most Frequent)
Mostly white luminous ring of 22° radius around the sun or moon at its centre. When well developed, the halo round the sun shows a pure clear halo — the red on the outside but other colours usually difficult to recognise. Presence of a halo signifies predominance of ice crystals in the cloud → negligible chances of ice accretion.
Large Halo
A circular halo with a radius of 46°. The portion of the sky inside the ring is conspicuously darker than the rest of the luminous ring. Sometimes a large halo is observed — called the Large Halo. This is less bright.
| Feature | Small Halo | Large Halo |
|---|
| Radius | 22° | 46° |
| Colour | Mostly white, red outside | Less bright |
| Sky inside ring | Normal | Conspicuously darker |
| Cloud type | Cirrostratus (CS) | CS |
| Ice accretion risk | Negligible (ice crystals) | Negligible |
Aviation Significance of Halo: Halo = ice crystal cloud (CS) = negligible icing risk. Fly through CS clouds with halo = safer than flying in clouds with Corona (water droplets).
Exam Tip: HALO = 22° (Small) or 46° (Large). CS cloud. ICE crystals. NO serious icing. Red on OUTSIDE.
Remember: Halo from CS; Corona from AS/AC.
4. Corona
Corona: Luminous one or more (seldom more than three) coloured rings of relatively small radius, not more than 5°, centred on the sun or moon. In each ring, red appears on the outside and violet or blue on the inside. Colours usually dull. Formed due to diffraction of light passing through very small water drops in mist, fog or through a thin cloud (especially Altostratus clouds). Diffraction = slight bending of light wave as it moves along the boundary of an object like a water drop.
Aviation Warning: Corona indicates moderate icing — the cloud is above freezing level. Water droplets (supercooled) = icing risk. If cloud is above freezing level = icing risk.
| Feature | Corona | Halo |
|---|
| Radius | <5° (small) | 22° or 46° (large) |
| Cause | Diffraction (water drops) | Refraction (ice crystals) |
| Cloud type | AS, thin AC, mist, fog | CS |
| Icing risk | Moderate (water droplets) | Negligible (ice crystals) |
| Red position | Outside | Outside |
Mnemonic: Corona = Close rings (<5°) = supercooled water = CAUTION (icing). Halo = Huge rings (22°/46°) = ice = Harmless icing-wise.
5. Mirage
Mirage: Light rays are refracted (bent) as they travel from one medium to another of different density. In the atmosphere, density reduces with altitude. Light reflected from a distant object travels in a curved path, keeping lower density on the convex side of the curve. Rising sun or moon thus appears slightly higher than they actually are.
Mirages are due to curving of light rays passing through layers of air in which refractive index changes considerably with height due to density differences. Observed when temperature of earth's surface differs markedly from that of air above.
| Type | Name | Condition | Appearance |
|---|
| Inferior (Lower) Mirage | Lower Mirage | Lapse rate is steep (strongly heated surface, hot road/desert). Air density decreases rapidly upward. | Object appears lower than actual. Pool of water on road. "Water" on hot desert. |
| Superior (Upper) Mirage | Upper Mirage | Inversion (temperature increases with height). Cold sea surface, cold snow field. | Object appears higher than actual. Levitating ships. |
Mnemonic: Inferior Mirage → It looks like Inferior pool (fake water on hot road) → Lapse rate steep. Superior Mirage → Sky reflection, object raised up → Supercooled surface (inversion present).
6. Bishop's Ring
Bishop's Ring: A whitish faint ring, centred on the sun or moon, with a slightly bluish tinge on the inside and reddish brown on the outside. Due to diffraction of light by fine dust particles in the high atmosphere. The dust may also be of volcanic origin. Radius of the ring is about 22°.
Bishop's Ring vs Halo: Same radius (22°) but different cause and appearance. Halo = CS (ice crystals, refraction, bright). Bishop's Ring = fine dust (diffraction, faint, reddish-brown outside).
7. Twilight Colours
Various colours in the sky are produced at sunset and at sunrise by refraction, scattering or selective absorption of light rays from the sun.
8. Irisation
Sometimes due to diffraction of sun light, colours or bands nearly parallel to the margin of the cloud appear, in which pink colour predominates. Occurs in AC lenticularis and other clouds with uniform-sized droplets.
9. Glory
Glory: One or more coloured rings, seen by an observer around his own shadow on a cloud consisting of numerous small water droplets, on fog, or very rarely on dew. The coloured rings are due to reflection — arrangement same as corona. Airborne observers often see a glory around the shadow of the aircraft in which they are flying.
10. Crepuscular Rays
Dark bluish streaks which radiate from the sun. They are the shadows of clouds at or below the horizon.
11. Aurora
Aurora: (Latin: aurora = dawn). A phenomenon in which visible light is emitted by the high atmosphere, at heights varying from 70–100 km, with peak frequency at about 100 km. Aurora are ghostly displays of light in the form of streamers, rays, arches, hands, curtains, draperies, sheets or patches. They appear to shimmer or flit across the sky. Aurora are most common in higher latitudes, centering around magnetic poles. They are greenish white or pronounced red or violet.
| Feature | Aurora Borealis | Aurora Australis |
|---|
| Name meaning | Northern Lights | Southern Lights |
| Hemisphere | N hemisphere (generally over NW Greenland) | S hemisphere |
| Called | Aurora Borealis | Aurora Australis |
Cause: Aurora results from bombardment of gases in the rarefied upper atmosphere by electric particles from magnetic storms. These cause excitation of gases with consequent emission of radiation. They impair radio communication.
Aurora may be compared with the coloured commercial sign displays in which high voltage electric discharge is passed through tubes containing particular gas under low pressure, from which the colour light characteristic of the gas is then emitted.
12. Atmospheric Electricity and Lightning
In fine weather, the earth is negatively charged and the electric potential is directed towards the earth and has a mean value of 100 volts/m. During a thunderstorm, the direction of current is locally reversed. Atmosphere carries a positive charge towards rain, and air to ground lightning flashes convey predominantly negative charge to ground.
Types of Lightning
| Type | Description |
|---|
| Ground Discharge (Thunderbolt) | Lightning between cloud and ground. Usually branches downward from distinct main channel (streak or ribbon lightning). Generally between 10–20 m diameter. A luminous ball observed soon after a ground discharge and disappears with a violent explosion = ball lightning. Moves slowly in air/on ground. |
| Cloud Discharge (Sheet Lightning) | Lightning takes place within the cloud. |
| Air Discharge | Lightning from cloud to the air — does NOT strike the ground. |
13. Saint Elmo's Fire
Saint Elmo's Fire: Sometimes when flying in CU/CB clouds, an aircraft may experience bluish or greenish luminous discharge due to strong electrical field in the cloud. The continuous luminous glow is also accompanied by cracking sound. Intensity of the glow is weak to moderate.
Aviation Warning: Saint Elmo's Fire = strong electrical activity in cloud = in or near CB/CU → precursor to lightning → AVOID the cloud. If experienced, immediately divert.
14. Practice Q&A
Q1. Aurora Australis occur in the (a) S hemisphere (b) N hemisphere (c) Equator
✅ Answer: (a) S hemisphere
Aurora Australis = Southern Lights = Southern hemisphere. Aurora Borealis = Northern Lights = N hemisphere.
🎯 Australis = Australia = South. Borealis = Boreal (northern forest) = North.
Q2. Aurora Australis are called ......... Lights (a) Northern (b) Southern (c) Temperate
✅ Answer: (b) Southern
Aurora Australis = Southern Lights. Aurora Borealis = Northern Lights.
🎯 Straight definition question — memorise the pairing.
Q3. Aurora Borealis occur in the (a) S hemisphere (b) N hemisphere (c) Equator
✅ Answer: (b) N hemisphere
Aurora Borealis = Northern Lights, generally over NW Greenland in the N hemisphere.
🎯 Borealis → Boreal → Northern. Common exam pairing.
Q4. Aurora Borealis are called ......... Lights (a) Northern (b) Southern (c) Temperate
✅ Answer: (a) Northern
🎯 Fixed pairing: Borealis = Northern; Australis = Southern.
Q5. Corona occur in ......... clouds (a) AS (b) NS (c) Both AS and CS (d) Any one of all these
✅ Answer: (a) AS (Altostratus — and thin clouds/mist/fog)
Corona forms in AS, thin AC, mist or fog — wherever small uniform water drops exist. Not in NS (too thick) or CS (ice crystals produce Halo, not Corona).
🎯 Corona = AS. Halo = CS. This is a common exam distinction.
Q6. Bishop's ring is due to the diffraction of light by fine particles of (a) water (b) dust (c) ice
✅ Answer: (b) dust
Bishop's Ring = diffraction of light by fine dust particles (possibly volcanic) in high atmosphere. Not water drops (that's Corona) or ice crystals (that's Halo).
🎯 Bishop's Ring = DUST diffraction. Radius = 22°. Reddish-brown outside.
Q7. The radius of the Bishop's Ring is about (a) 32° (b) 22° (c) 42°
✅ Answer: (b) 22°
Bishop's Ring radius is about 22° — same as Small Halo radius but different cause and appearance.
🎯 Both Bishop's Ring and Small Halo share 22° radius — distinguish by cause: dust vs ice crystals.
Q8. Superior Mirage occurs in (a) Lapse (b) Isothermal (c) Inversion
✅ Answer: (c) Inversion
Superior (Upper) Mirage occurs when there is temperature inversion. Cold sea surface → inversion → light bends upward → object appears higher (superior) than actual.
Lapse rate steep = inferior mirage. Inversion = superior mirage.
🎯 Superior = Inversion (temperature inverted = superior mirage). Inferior = steep lapse (hot surface).
Q9. Inferior Mirage occurs when there is (a) Lapse (b) Isothermal (c) Inversion
✅ Answer: (a) Lapse
Inferior (Lower) Mirage occurs when lapse rate is steep — strongly heated surface, hot road or desert. Surface air is very warm, density rapidly decreasing with height.
🎯 Inferior Mirage → I = Image appears below = Intensive heating = Insolation-driven lapse.
Q10. Corona are formed due to ......... of light (a) Refraction (b) Diffraction (c) Scattering
✅ Answer: (b) Diffraction
Corona = diffraction of light by very small water droplets. Halo = refraction by ice crystals. Rainbow = refraction + internal reflection.
🎯 Corona = Diffraction (water drops). Halo = Refraction (ice). Rainbow = Refraction + Reflection.
Q11. Corona are formed due to light passing through (a) Mist only (b) Fog only (c) small water or ice particles only (d) Any one of all these
✅ Answer: (c) small water or ice particles only [primarily water]
Corona forms in mist, fog, or through thin cloud (AS, AC) with very small water drops. Textbook answer is (c) — small water drops primarily (ice rarely).
🎯 Corona = SMALL WATER DROPS via diffraction. Radius <5°.
Q12. Halo is produced by (a) Refraction (b) Diffraction (c) Scattering
✅ Answer: (a) Refraction
Halo = refraction of light through ice crystals in CS cloud. This is a fundamental distinction in DGCA exams.
🎯 H for Halo → H for Higher clouds (CS) → H for refraction (Halo = refraction).
Q13. Halo is produced when light passes through (a) water particles (b) ice crystals (c) both
✅ Answer: (b) ice crystals
Halo = light refracted by ice crystals in Cirrostratus. Exclusively ice crystals.
🎯 Halo = ice crystals in CS. If you see a halo, no icing risk in that cloud.
Q14. Halo occur in the cloud (a) AS (b) NS (c) CS
✅ Answer: (c) CS
Halo occurs due to ice crystals in Cirrostratus (CS) — a high cloud. Not AS (medium, water drops → Corona) or NS (low, thick).
🎯 Halo = CS. Corona = AS. Permanent exam distinction.
Q15. Halo is a luminous ring of ......... radius (a) 22° (b) 32° (c) 42°
✅ Answer: (a) 22°
Small Halo = 22° radius. Large Halo = 46° radius. Primary rainbow = 42°. Secondary rainbow = 52°.
🎯 All the angles: Halo=22°/46°, Rainbow=42°/52°, Bishop's Ring=22°, Corona=<5°.
Q16. Halo round the sun shows a pure clear ......... on the outside (a) Red (b) Yellow (c) Violet
✅ Answer: (a) Red
Both Halo and Primary Rainbow have Red on the outside. This is because red light is refracted least and appears at the outer edge.
🎯 Red outside: Halo AND Primary Rainbow both. Secondary rainbow reverses: Violet outside.
Q17. Halo signifies predominance in the cloud of (a) Supercooled water drops (b) Ice crystals (c) Both
✅ Answer: (b) Ice crystals
Halo = ice crystals predominance. Hence negligible icing chances.
Supercooled water drops → Corona, not Halo.
🎯 Halo = ice crystals = fly safely. Corona = water drops = ICING CAUTION.
Q18. The cloud which cause Halo has ......... chances of ice accretion (a) negligible (b) maximum (c) medium
✅ Answer: (a) negligible
Halo = CS cloud = ice crystals = negligible icing. Ice crystals do not accrete on airframes the way supercooled water droplets do.
🎯 Ice crystals bounce off airframe; supercooled WATER freezes on contact = icing. Halo cloud = safe.
Q19. Sometimes a halo with a radius of ......... is observed, called Large Halo (a) 32° (b) 42° (c) 22°
✅ Answer: (b) 42° — Wait, textbook states 46°. Closest option = (b) if options are 32/42/46
Large Halo radius = 46°. If options given are 32°, 42°, 46° — answer is 46°. The textbook explicitly states 46° for Large Halo.
🎯 Small Halo = 22°. Large Halo = 46°. Don't confuse with rainbow (42°) or Bishop's Ring (22°).
Q20. Halo occurs from ......... cloud (a) Low (b) Medium (c) High
✅ Answer: (c) High
Halo occurs from Cirrostratus (CS) — a HIGH cloud. High clouds are composed of ice crystals that refract light to form halo.
🎯 High cloud (CS) = Halo. Medium cloud (AS) = Corona.
15. Master Reference Tables
All Angular Values
| Phenomenon | Angle/Radius |
|---|
| Primary Rainbow | 42° |
| Secondary Rainbow | 52° (arc 50° for red, 54° for violet) |
| Small Halo | 22° |
| Large Halo | 46° |
| Corona | <5° |
| Bishop's Ring | ~22° |
Optical Phenomena Master Comparison Table
| Phenomenon | Cause | Mechanism | Cloud/Medium | Icing Risk | Key Distinction |
|---|
| Rainbow | Water drops (rain/drizzle) | Double refraction + 1 total internal reflection | Rain, drizzle, fog | N/A | VIBGYOR; red outside primary |
| Halo (22°) | Ice crystals | Refraction | CS (high cloud) | Negligible | Red outside; 22°; fly safely |
| Large Halo (46°) | Ice crystals | Refraction | CS | Negligible | Sky inside darker |
| Corona | Small water drops | Diffraction | AS, mist, fog, thin AC | Moderate | <5° radius; icing warning |
| Bishop's Ring | Fine dust particles | Diffraction | High atmosphere (volcanic) | None | Reddish-brown outside; 22° |
| Mirage (Inferior) | Steep lapse rate | Refraction (curved ray) | Hot surface | N/A | Image below real; fake water |
| Mirage (Superior) | Inversion | Refraction | Cold surface | N/A | Image above real |
| Glory | Water drops | Reflection (backscatter) | Fog, cloud | N/A | Around observer's own shadow |
| Aurora | Solar particles | Emission | 70–100 km altitude | N/A | Impairs radio; Borealis=N; Australis=S |
| St Elmo's Fire | Electrical discharge | Luminous discharge | CB/CU clouds | High (in CB) | Blue/green glow + cracking sound |
Answer Key — Textbook Q&A
| Q | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
|---|
| A | a | b | b | a | a | b | b | c | a | b | c | a | b | c | a | a | b | a | b | c |
Quick Revision Summary — Chapter 10:
• Rainbow: water drops; primary 42° (red outside); secondary 52° (colours reversed, red inside)
• Halo: ice crystals in CS; 22° (small) or 46° (large); refraction; negligible icing
• Corona: water drops in AS/mist; <5°; diffraction; MODERATE ICING RISK
• Bishop's Ring: fine dust; 22°; diffraction; reddish-brown outside
• Mirage: inferior = hot surface (steep lapse); superior = cold surface (inversion)
• Glory: observer's shadow on cloud/fog; reflection by water drops
• Crepuscular Rays: shadows of clouds at/below horizon
• Aurora: 70–100 km altitude; magnetic storms; impairs radio; Borealis=N; Australis=S
• St Elmo's Fire: in CB/CU; blue-green glow + cracking; electrical field; AVOID
• KEY EXAM: Halo = CS = ice = no icing. Corona = AS = water = ICING WARNING.
Capt. Pankaj Pahil