Lower tangent arc
Encyclopedia
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 arc
s, 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 forms a sharp, wing-shaped angle below the sun. As the sun rises above over the 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
. The lower tangent arc is rarely visible as it only appears when the sun elevation exceeds 22°. Most observations thus are from elevated observation points such as mountains and planes.
To produce lower tangent arcs rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.
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 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....
appearing under 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 a 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...
centred 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...
.
Just like upper tangent arc
Upper tangent arc
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
s, 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 forms a sharp, wing-shaped angle below the sun. As the sun rises above over the 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
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...
. The lower tangent arc is rarely visible as it only appears when the sun elevation exceeds 22°. Most observations thus are from elevated observation points such as mountains and planes.
To produce lower tangent arcs rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.
External links
- Atmospheric Optics - Alaska Lower Tangent Arc - Great photo by Ryan Skorecki.
- Atmospheric Optics - Lower Tangent arc - A photo taken from an aeroplane.
- Lower Tangent Arc Sep 25, 2005 - Photos of a lower tangent arc.