Some researchers recommend that observations be classified according to the features of the phenomenon or object that are reported or recorded. Typical categories include:
Saucer, toy-top, or disk-shaped “craft” without visible or audible propulsion. (day & night)
Rapidly-moving lights or lights with apparent ability to rapidly change direction & then suddenly stop, impossible for conventional aircraft.
Large triangular “craft” or triangular light pattern
Cigar-shaped “craft” with lighted windows (Meteor fireballs are sometimes reported this way).
Other: chevrons, equilateral triangles, spheres, domes, diamonds, shapeless black masses, eggs, & cylinders.
[edit] Hynek system
J. Allen Hynek developed another commonly used system of description, dividing sightings into 6 categories. It 1st separates sightings into distant- & close-encounter categories, arbitrarily setting 500 feet as the cutoff point. It then subdivides these close & distant categories based on appearance or special features. The three distant-encounter categories are:
Nocturnal Lights (NL): Anomalous lights seen in the night sky.
Daylight Discs (DD): Any anomalous object, generally but not necessarily “discoidal”, seen in the distant daytime sky.
Radar/Visual cases (RV). Objects seen simultaneously by eye and on radar.
Subgroups of the distant category of sightings correlate with evidentiary value. RV cases are usually considered to have the highest value because of radar corroboration, whereas NL cases have the lowest because it is so easy to mistake lights seen at night for prosaic phenomena such as meteors, bright stars, or aircraft. RV reports are also fewest in number, while NL are most common.
Hynek also defined three “close encounter” (CE) subcategories:
CE1: Strange objects seen nearby but without physical interaction with the environment.
CE2: A CE1 case that leaves physical evidence (e.g. soil depressions, vegetation damage) or causes electromagnetic interference (see below).
CE3: CE1 or CE2 cases where “occupants” or entities are seen. (Hence the title of Steven Spielberg’s movie Close Encounters of the Third Kind.)
From UFO Casebook:
CE4+: aliens communicate with the observer, even abduct, experiment on the observers, others. UFO Casebook lists additional categories, in which the UFO and/or alien is captured/destroyed by military forces and/or civilians.
Like the RV cases, CE cases are considered higher in evidentiary value because they include measurable physical effects, & because objects seen up close are less likely to be the result of misperception. Like the RV cases, these tend to be relatively rare.
Hynek’s CE classification system has since been expanded to include such things as alleged alien abductions and cattle mutilation phenomena.
[edit] Vallee System
Jacques Vallee has devised a UFO classification system which is preferred by many UFO investigators over Hynek’s system as it is considerably more descriptive than Hynek’s, especially in terms of the reported behavior of UFOs.
Type - 1 (a, b,c, d)- Observation of an unusual object, spherical discoidal, or of another geometry, on or situated close to the ground (tree height, or lower), which may be associated with traces - thermal, luminous, or mechanical effects.
a - On or near ground.
b - Near or over body of water.
c - Occupants appear to display interest in witnesses by gestures or luminous signals.
d - Object appears to be “scouting” a terrestrial vehicle.
Type - 2 (a, b,c) - Observation of an unusual object with vertical cylindrical formation in the sky, associated with a diffuse cloud. This phenomenon has been given various names such as “cloud-cigar” or “cloud-sphere.”
a - Moving erratically through the sky
b - Object is stationary and gives rise to secondary objects (sometimes referred to as “satellite objects”)
c - Object is surrounded by secondary objects
Type - 3 (a, b,c, d,e)- Observation of an unusual object of spherical, discoidal or elliptical shape, stationary in the sky.
a - Hovering between two periods of motion with “falling-leaf” descent, up and down, or pendulum motion
b - Interruption of continuous flight to hover and then continue motion
c - Alters appearance while hovering - e.g., change of luminosity, generation of secondary object, etc.
d - “Dogfights” or swarming among several objects
e - Trajectory abruptly altered during continuous flight to fly slowly above a certain area, circle, or suddenly change course
Type 4 (a, b,c, d) - Observation of an unusual object in continuous flight.
a - Continuous flight
b - Trajectory affected by nearby conventional aircraft
c - Formation flight
d - Wavy or zig-zag trajectory
Type 5 (a, b,c)- Observation of an unusual object of indistinct appearance, i.e., appearing to be not fully material or solid in structure.
a - Extended apparent diameter, non-point source luminous objects (“fuzzy”)
b - Starlike objects (point source), motionless for extended periods
c - Starlike objects rapidly crossing the sky, possibly with peculiar trajectories
based on the Wikipedia materials