Short Notes 10 : Optical Phenomena
Corona
- Process: Diffraction of light by tiny, uniform water droplets or ice crystals (sometimes fine solid particles like volcanic dust).
- Appearance: Colored circle or ring around Sun/Moon — blue inside, red outside (opposite of halo).
- Best Producers: Thin, newly formed Altostratus (AS) and Altocumulus (AC) (medium clouds, 6,500–23,000 ft).
- Other Occurrences: Can also form in mist and fog (small droplets near surface).
- Distinction: Caused by diffraction (not refraction); smaller and more vivid than halos.
Halo
- Process: Refraction and dispersion of light through hexagonal ice crystals in Cirrostratus (CS) clouds.
- Cloud Type: High cloud family (16,500–45,000 ft), entirely ice crystal composition.
- Color Order: Red inside, blue/violet outside (opposite of corona).
- Common Sizes:
- 22° Halo — most frequent.
- 46° Halo — large, rarer type (sometimes cited as 42°).
- Significance: Indicates Cirrostratus clouds → approaching warm front, often rain/snow within 12–24 hours.
- Icing Hazard: Negligible — cirri-form clouds lack supercooled droplets.
Aurora Borealis / Australis
- Process: Interaction of charged solar particles (solar wind) with Earth’s magnetic field and upper atmospheric gases.
- Result: Luminous displays (green, red, violet glows) in the thermosphere.
- Aurora Borealis: Northern Lights — visible in high-latitude Northern Hemisphere.
- Aurora Australis: Southern Lights — visible in Southern Hemisphere polar regions.
- Altitude: High atmosphere, above normal flight levels.
Bishop’s Ring
- Process: Diffraction of light through fine solid particles (usually volcanic dust/aerosols in the stratosphere).
- Appearance: Bluish or red-brown ring around Sun/Moon.
- Radius: About 22° (type of corona).
- Occurrence: Seen after major volcanic eruptions (e.g., El ChichĂłn).
Mirages
Inferior Mirage
- Condition: Steep lapse rate — hot surface heats air near ground (warm air below cooler air).
- Mechanism: Light rays bend upward, causing objects to appear inverted or displaced downward.
- Seen: Over hot deserts or roads (“water illusion”).
Superior Mirage
- Condition: Temperature inversion — cold air below, warm air above.
- Mechanism: Light rays bend downward toward dense cold air, making objects appear higher or lifted.
- Seen: Over cold seas or polar regions.
Quick Distinctions Summary
| Optical Phenomenon | Process | Medium | Cloud Type / Condition | Color / Appearance | Key Feature |
|---|---|---|---|---|---|
| Corona ( Not safe ) | Diffraction | Tiny water droplets / ice | Altostratus, Altocumulus | Blue in, red out | Small, colorful ring |
| Halo ( Safe ) | Refraction | Ice crystals | Cirrostratus (high cloud) | Red in, blue out | Large ring (22°/46°) |
| Aurora | Solar particles exciting rare gases | Atmospheric gases | Polar regions | Multicolored glow | High-altitude emission |
| Bishop’s Ring ( Not safe ) | Diffraction | Volcanic dust | Solid particles | Bluish/brown ring | — |
| Inferior Mirage | Upward Refraction | Warm surface ( Hot sand or Road ) | Steep lapse rate | Inverted image | “Water on road” effect |
| Superior Mirage | Downward Refraction | Cold surface (Polar/cold seas) | Inversion | Elevated image | Object appears higher |