halo

i. The luminous effect produced by the refraction and reflection of light by prismatic ice crystals present in the cirrus and cirrostratus clouds. It is a white circle or prismatic arc of a circle, with the sun or the moon at the center. They are of two different radii: in the first case, the halo sub-tends an angle of 22°, whereas, in the other case, it subtends an angle of 46°. The coloration of a halo changes from red inside to blue outside. This is just the opposite of that in a corona-green and blue are usually too weak to be seen.

ii. A high-altitude, low-opening (HALO) paradrop system. The paratroopers jump from aircraft at very high altitudes and open their parachutes at very low heights. This technique ensures rapid infiltration and tactical surprise. The latter is achieved as the paratroopers can jump from the aircraft, which is flying some distance away from the drop zone.

iii. The reflection of cockpit instruments seen in a canopy at night.

iv. The colored ring seen on clouds in the direction away from the sun (i.e., with the aircraft’s shadow at the center). Also called a pilot’s halo.

v. A bright ring around the spot produced by a beam of electrons striking the fluorescent coating in a cathode-ray tube.