Notes

Close Encounters of the First kind
Visual sightings of an unidentified flying object seemingly less than 500 feet away that show an appreciable angular extension and considerable detail.[10]

Close Encounters of the Second kind
A UFO event in which a physical effect is alleged. This can be interference in the functioning of a vehicle or electronic device; animals reacting; a physiological effect such as paralysis or heat and discomfort in the witness; or some physical trace like impressions in the ground, scorched or otherwise affected vegetation, or a chemical trace.[10]

Close Encounters of the Third kind
See also: First contact (anthropology)
UFO encounters in which an animated creature is present. These include humanoids, robots, and humans who seem to be occupants or pilots of a UFO.[11]

Close Encounters of the Fourth kind
A UFO event in which a human is abducted by a UFO or its occupants.[13] This type was not included in Hynek's original close encounters scale.[14]

Hynek's erstwhile associate Jacques Vallee argued in the Journal of Scientific Exploration that a CE4 should be described as "cases when witnesses experienced a transformation of their sense of reality", so as to also include non-abduction cases where absurd, hallucinatory or dreamlike events are associated with UFO encounters.[15]

The film The Fourth Kind makes reference to this ranking.

Close Encounters of the Fifth kind
A UFO event that involves direct communication between aliens and humans.[13] This type of close encounter was named by Steven M. Greer's CSETI group and is described as bilateral contact experiences through conscious, voluntary and proactive human-initiated cooperative communication with extraterrestrial intelligence.[16] [17]

Close Encounters of the Sixth kind
Death of a human or animal associated with a UFO sighting, although some might consider this as a more severe example of a second-kind encounter.[18][19]