Upper tangent arc
Encyclopedia
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
, an atmospheric optical phenomenon
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
which appears over and tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
to the 22° halo
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
around the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
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 sun forming a sharp angle. As the sun rises, 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
Lower tangent arc
A lower tangent arc is a rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun.Just like upper tangent arcs, the shape of a lower tangent arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc...
to form the circumscribed halo
Circumscribed halo
A circumscribed halo is a type of halo, an optical phenomenon that circumscribes a related phenomenon, the 22° halo centred on the sun.When observed, a circumscribed halo is normally oval in shape and, at the points directly below and above the sun, lies tangential to the 22° halo...
.
Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus cloud
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...
s have their long axis oriented horizontally, while otherwise rotated in any direction. This orientation of the crystals also produces other halos, including 22° halo
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
s and sun dog
Sun dog
A sun dog or sundog, scientific name parhelion ; , also called a mock sun or a phantom sun, is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.Sundogs may appear as a colored patch of light to the left or right of...
s, but a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like many other halos, upper tangent arcs grade from a red inner edge to a blue outer edge because red light is refracted
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
more strongly than blue light.
External links
- www.paraselene.de - Tangent Arcs (including several HaloSim simulations.)