Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

tangent In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. More ...

to the

22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunlight o ...

. To produce these arcs, rod-shaped

hexagon In geometry, a hexagon (from Ancient Greek, Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple polygon, simple (non-self-intersecting) hexagon is 720°. Regular hexa ...

ice crystal Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, na ...

s need to have their long axis aligned horizontally.

Description

Upper arc

The shape of an upper tangent arc varies with the elevation of the Sun; while the Sun is low (less than 29–32°) it appears as an arc over the observed Sun forming a sharp angle. As the Sun is seen to rise above the Earth's horizon, the curved wings of the arc lower towards the 22° halo while gradually becoming longer. As the Sun rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.

Lower arc

The lower tangent arc is rarely observable, appearing under and

to a

centred on the Sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the Sun. As the Sun is observed slipping over Earth's horizon the lower tangent arc forms a sharp, wing-shaped angle below the Sun. As the Sun rises over Earth's horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the Sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo. Since by definition, the Sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.

Origin

Both the upper and lower tangent arc form when

al rod-shaped

s in cirrus clouds have their long axis oriented horizontally. Each crystal can have its long axis oriented in a different horizontal direction, and can rotate around the long axis. Such a crystal configuration also produces other halos, including

s and

sun dog A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° ...

s; a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is refracted less strongly than blue light.

References

External links