Atmospheric Optical Phenomena & Related Meteorological Phenomena

1. Corona

A Corona is a prismatically coloured circle or arc around the Sun or Moon, produced by the diffraction of light by very small particles of relatively uniform size.

Formation

• Process: Diffraction of light.
• Light passes around tiny particles and produces coloured rings.
• The particles are generally small, uniform water droplets.
• The source also notes that small ice particles or fine solid particles can produce similar diffraction effects.

Best Cloud Producers

The best corona producers are:

• Altostratus (AS)
• Altocumulus (AC)

These are medium-level clouds, approximately 6,500–23,000 ft in middle latitudes.

Corona Colour Order

• Blue inside
• Red outside

This is opposite to the colour order of a halo.

Other Possible Sources

A corona may also occur when light passes through:

• Mist.
• Fog.
• Small, uniform water droplets or ice particles.
• Fine solid particles such as volcanic ash.

Key Point

Corona = Diffraction + small uniform particles + Blue inside, Red outside

2. Halo

A Halo is a luminous ring or arc around the Sun or Moon produced by the refraction and dispersion of light through ice crystals.

Formation

• Process: Refraction.
• Produced by hexagonal ice crystals.
• The ice crystals act like small prisms.
• Different wavelengths of light are refracted by different amounts.

Cloud Association

Halos are particularly associated with:

Cirrostratus (CS)

• CS is a high-level cirriform cloud.
• Typical base: approximately 16,500–45,000 ft in middle latitudes.
• It is composed almost entirely of ice crystals.

Colour Order

Because of dispersion:

• Red bends the least → appears on the inside of the halo.
• Blue/violet bends more → appears toward the outside.

3. Size of Halo

There are two important halo sizes:

The source specifically identifies the 22° halo as the most common and the 46° halo as less common.

Exam Point: 22° = common halo; 46° = large/less common halo.

4. Meteorological Significance of Halo

A halo indicates the presence of high-level ice-crystal clouds, especially Cirrostratus.

Frontal Significance

A halo may act as a pre-frontal indication, particularly ahead of an approaching warm front.

The source associates the appearance of halo-forming cirriform cloud with possible rain or snow within approximately 12–24 hours.

Icing

• Cirriform clouds contain predominantly ice crystals.
• Significant supercooled water droplets are generally absent.
• Therefore, structural icing in CS is considered Nil/Trace in the source.

5. Corona vs Halo

6. Bishop’s Ring

Bishop’s Ring is an atmospheric optical phenomenon produced by the diffraction of light through fine solid particles, especially volcanic aerosols.

Formation

• Process: Diffraction.
• Particles: Very fine dust or volcanic particles.
• These particles may be injected into the stratosphere during major volcanic eruptions.
• The source gives El Chichón as an example.

Appearance

• Bluish or red-brown ring around the Sun or Moon.
• Approximately 22° radius according to the source.

Classification

Bishop’s Ring can be considered a type of corona produced by fine solid particles rather than ordinary cloud droplets.

Key Point

Bishop’s Ring = Diffraction + volcanic dust/aerosols + approximately 22° radius

7. Superior Mirage

A Superior Mirage occurs when distant objects appear higher than their actual position.

Atmospheric Condition

It occurs during a temperature inversion:

• Cold, dense air near the surface
• Warmer air above

This commonly occurs over:

• Snow.
• Ice.
• Cold sea surfaces.
• Polar regions.

Mechanism

• Light rays are refracted downward toward the colder, denser air near the surface.
• The observer perceives the distant object as being displaced upward.

Key Point

Superior Mirage = Inversion = Cold below + Warm above = Object appears higher

8. Inferior Mirage

An Inferior Mirage, or lower mirage, occurs when distant objects appear lower than their actual position or may appear inverted.

Atmospheric Condition

It requires:

• Very warm air near the surface
• Cooler, denser air above
• A steep lapse rate

Formation

1. A hot surface such as desert sand or asphalt heats the air immediately above it.
2. The near-surface air becomes very warm and less dense.
3. Light passing through the different-density air layers is refracted upward.
4. Distant objects appear displaced downward or inverted.

A common example is the apparent water-like appearance on a hot road or desert surface.

Key Point

Inferior Mirage = Steep lapse rate = Warm below + Cool above = Object appears lower

9. Superior vs Inferior Mirage

Easy Memory Trick

Superior → object appears Superior/Higher

Inferior → object appears Inferior/Lower

10. Aurora Borealis

Aurora Borealis is the atmospheric luminous phenomenon commonly called the Northern Lights.

Location

• Northern Hemisphere.
• Primarily observed at high/polar latitudes around the Arctic.

Cause

It occurs when:

1. Energetic charged particles from the Sun, carried by the solar wind, interact with Earth’s magnetic field.
2. These particles interact with gases in the rarefied upper atmosphere.
3. The atmospheric gases emit light.

Altitude

• Occurs at very high altitudes in the upper atmosphere/thermosphere.

Southern Counterpart

Aurora Australis = Southern Lights

11. Aurora Australis

Aurora Australis is the atmospheric luminous phenomenon known as the Southern Lights.

Location

• Southern Hemisphere.
• High-latitude/polar regions.
• Associated with Earth’s magnetic polar region.

Cause

• Charged particles from the Sun.
• Solar wind interacts with Earth’s magnetic field.
• Particles interact with gases in the upper atmosphere, producing visible light.

Northern Counterpart

Aurora Borealis = Northern Lights

12. Aurora Borealis vs Aurora Australis

Memory Aid

Borealis → North

Australis → South

13. Important Exam Revision Table

14. High-Yield One-Line Facts

• Corona → diffraction.
• Halo → refraction/dispersion.
• Corona → blue inside, red outside.
• Halo → red inside, blue/violet outside.
• Corona → small, uniform particles.
• Halo → ice crystals.
• Best corona producers → AS and AC.
• Halo cloud → Cirrostratus (CS).
• Common halo → 22° radius.
• Large halo → 46° radius.
• Bishop’s Ring → fine volcanic/dust particles + diffraction.
• Bishop’s Ring → approximately 22° radius.
• Superior Mirage → inversion.
• Superior Mirage → object appears higher.
• Inferior Mirage → steep lapse rate.
• Inferior Mirage → object appears lower/inverted.
• Aurora Borealis → Northern Lights.
• Aurora Australis → Southern Lights.
• Aurora → solar charged particles interacting with the upper atmosphere.
• CS → high-level ice-crystal cloud.
• Halo can indicate approaching frontal weather.