A sun candle, also known as a subsun, is a rare atmospheric optical phenomenon that appears as a bright, candle-like spot or short vertical glow directly below the Sun. It forms when sunlight reflects off horizontally oriented, hexagonal ice crystals suspended in the air. While sun candles are most often visible from aircraft or mountaintops looking down on ice clouds, they also appear from the ground under cold, calm conditions when snowflakes or diamond dust are present. The phenomenon belongs to the broader family of ice-crystal optical effects that include sun pillars and sundogs, but it is distinguished by its geometry and reflective mechanism.
Key Takeaways: Sun Candle or Subsun
- A sun candle (subsun) appears directly below the Sun as a bright spot or short vertical glow.
- It forms by reflection, not refraction, from horizontally aligned ice crystals.
- Clouds are common but not required. Suspended ice crystals alone are sufficient.
- Sun candles are visible from aircraft, mountaintops, or the ground.
- Ground-based sun candles often appear shimmering or flickering.
- The phenomenon closely relates to sun pillars and other ice-crystal optics.
What Is a Sun Candle (Subsun)?
A sun candle or subsun is an atmospheric optical phenomenon caused by specular reflection of sunlight from ice crystals located below the apparent position of the Sun relative to the observer. The reflected light produces a concentrated bright spot or short vertical streak aligned with the Sun but displaced downward.
The term subsun literally means “below the Sun,” describing its geometric placement. The name sun candle is descriptive, referring to the candle-flame-like appearance the reflection can take under favorable conditions.
Appearance of a Sun Candle
A sun candle typically appears as:
- A bright white or pale yellow spot directly beneath the Sun
- Sometimes extended into a short vertical glow or streak
- Symmetrical about the vertical line through the Sun
- Variable in intensity, from faint to strikingly bright
Sun candles are usually white or pale yellow because they form by reflection rather than refraction, so they lack the strong color separation seen in sundogs and halos. When observed from aircraft or above a cloud layer, the sun candle often appears steady and well-defined. From the ground, it often looks shimmering, sparkling, or flickering, due to the motion of individual ice crystals.
How the Sun Candle Phenomenon Works
Sun candles form through mirror-like reflection of sunlight.
- Ice crystal shape
The atmosphere contains flat, hexagonal, plate-shaped ice crystals. - Crystal orientation
In calm air, aerodynamic drag causes these plates to fall with their broad faces nearly horizontal. - Reflection of sunlight
Sunlight reflects off the flat basal faces of the crystals, much like light reflecting from a calm water surface. - Observer geometry
When the observer is positioned so the reflected rays reach their eyes, the light appears as a bright spot below the Sun.
This same physical mechanism applies whether the crystals are in clouds, ice fog, or falling snow.
Conditions Required for a Sun Candle
Sun candles do not form randomly. They require a specific combination of atmospheric conditions and viewing geometry that allows sunlight to reflect efficiently from ice crystals toward an observer. Understanding these requirements helps explain both the rarity of the phenomenon and why it is more commonly seen in certain environments.
Atmospheric Conditions
- Hexagonal ice crystals, especially plate-shaped forms which provide reflective surfaces
- Temperatures below freezing, particularly under conditions that promote hexagonal crystal formation
- Relatively calm air, allowing horizontal crystal alignment
Viewing Conditions
- A visible Sun
- Suspended ice crystals positioned to reflect sunlight toward the observer
- Observation may occur:
- From aircraft
- From mountaintops
- From the ground during suitable conditions
Clouds are common but not essential. The essential factor is the presence and orientation of suitable ice crystals, not whether they are part of a cloud.
Ground Observations and Diamond Dust
Sun candles are observable from the ground, particularly in cold environments where ice crystals are suspended near the surface. In these cases, the reflecting crystals may consist of:
- Falling snowflakes
- Diamond dust, extremely small ice crystals suspended in air
- Ice fog in very cold conditions
Ground-based sun candles share the same geometry as aircraft-observed subsuns but typically appear shimmering or flickering. This effect occurs because individual snowflakes and ice crystals are larger, fewer, and more turbulent than those in high-altitude clouds. As these crystals move and rotate, each briefly reflects sunlight, producing a dynamic, sparkling appearance.
Such observations are most common at ski resorts, alpine valleys, and polar regions during calm, sunny, subfreezing weather.
Where and When Sun Candles Are Observed
Sun candles are most frequently observed:
- Over polar and subpolar regions
- In cold continental interiors
- At high altitudes or during commercial flights
- Near the surface during diamond dust events
They are most noticeable when the Sun is low to moderately high in the sky, which enhances contrast and reflection geometry.
Why Sun Candles Are Rare
Although ice crystals are common in cold weather, sun candles require a precise combination of crystal shape, orientation, calm air, and viewing geometry. Flat, hexagonal plate crystals form only within specific temperature and humidity ranges and must remain nearly horizontal to act as mirrors. Even slight turbulence disrupts the alignment, causing the effect to disappear. As a result, sun candles occur infrequently and often last only seconds to minutes.
Comparison With Related Optical Phenomena
Sun candles belong to a broader family of atmospheric optical effects produced by ice crystals, but they are often confused with more familiar phenomena. Comparing a sun candle with related effects such as sun pillars and sundogs highlights the key differences in appearance, viewing angle, and underlying physical mechanisms.
Sun Candle vs Sun Pillar
- Sun candle (subsun)
- Appears below the Sun
- Often seen from above or during ground-based diamond dust events
- Caused by reflection from ice crystals beneath the observer
- Sun pillar
- Appears as a vertical column of light extending above or below the Sun
- Commonly observed from the ground
- Also formed by reflection from horizontally oriented ice crystals
Sun Candle vs Sundog (Parhelion)
- Sun candle
- Located directly below the Sun
- Reflection-based
- Aligned vertically
- Sundog
- Appears to the left and right of the Sun
- Caused by refraction through ice crystals
- Often shows rainbow-like colors
Sun Candle vs Light Pillar
- Sun candle
- Daytime phenomenon involving sunlight
- Light pillar
- Often occurs at night
- Formed by artificial light reflecting off ice crystals
Why Sun Candles Are Scientifically Interesting
Sun candles provide insight into:
- Ice crystal microphysics, including shape and orientation
- Atmospheric stability and turbulence
- The broader physics of halo and reflection phenomena
Because they require precise crystal alignment, sun candles indicate very calm atmospheric layers, both aloft and near the ground.
References and Further Reading
- Barkey, B.; Liou, K.N.; Takano, Y.; et al. (1999). “An Analog Light Scattering Experiment of Hexagonal Icelike Particles. Part II: Experimental and Theoretical Results”. Journal of the Atmospheric Sciences. 56 (4): 613. doi:10.1175/1520-0469(1999)056<0613:AALSEO>2.0.CO;2
- Greenler, Robert (1990). Rainbows, Halos, and Glories. Cambridge University Press. ISBN: 9780521388658.
- Ping-Yü, Ho; Needham, Joseph (1959). “Ancient Chinese Observations of Solar Haloes and Parhelia”. Weather. 14 (4). Wiley: 124–134. doi:10.1002/j.1477-8696.1959.tb02450.x
- Vollmer, Michael; Tammer, Robert (1998). “Laboratory experiments in atmospheric optics”. Applied Optics. 37 (9). The Optical Society: 1557–1568. doi:10.1364/ao.37.001557