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Hynek devised a six-fold classification for UFO sightings. They are arranged according to increasing proximity:

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Lights in the night sky

Daylight Discs: UFOs seen in the daytime, generally having discoidal or oval shapes

Radar-Visual: UFO reports that have radar confirmation—these supposedly try to offer harder evidence that the objects are real, although radar propagation can be occasionally discredited due to atmospheric propagation anomalies

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Close Encounters of the First Kind

Visual sightings of an unidentified flying object, seemingly less than 500 feet (150 m) away, that show an appreciable angular extension and considerable detail.

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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]

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Close Encounters of the Third Kind

UFO encounters in which an animated entity is present—these include humanoids, robots, and humans who seem to be occupants or pilots of a UFO.

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Bloecher Close Encounter Subtypes

UFO researcher Ted Bloecher proposed six subtypes for the close encounters of the third kind in Hynek's scale:

A : An entity is observed only inside the UFO

B : An entity is observed inside and outside the UFO

C : An entity is observed near to a UFO, but not going in or out

D : An entity is observed—no UFOs are seen by the observer, but UFO activity has been reported in the area at about the same time

E : An entity is observed, but no UFOs are seen and no UFO activity has been reported in the area at that time

F : No entity or UFOs are observed, but the subject experiences some sort of "intelligent communication."

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Close Encounters of the Fourth Kind

A Close Encounter of the Fourth Kind is a UFO event in which a human is abducted by a UFO or its occupants. This type was not included in Hynek's original close encounters scale. Hynek's erstwhile associate Jacques Vallee argued in the Journal of Scientific Exploration that the "Fourth Kind" should refer to "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.

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Close Encounters of the Fifth Kind

A Close Encounter of the Fifth Kind is a UFO event that involves direct communication between aliens and humans. 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.

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Close Encounters of the Sixth Kind

Death of a human or animal associated with a UFO sighting (though this could be considered a more severe example of a second-kind encounter).

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Close Encounters of the Seventh Kind

The creation of a human/alien hybrid, either by sexual reproduction or by artificial scientific methods.