A strange looking cloud that appeared to the west of Cobar last Wednesday evening and then vanished in an instant, has been explained by the Bureau of Meteorology as a rare lens shaped formation or in technical terms an 'Altocumulus Lenticucaris'.

Green Street resident Denise Kosef said she and her mother were watching the unusual cloud at about 7.45pm when it disappeared in a flash.

"We couldn't believe what happened," she said.

"One moment the cloud was reflecting the last rays of the sun, then whoosh, it was gone. We had never seen anything like it before and were amazed.

"For a while we thought, oops, the aliens have arrived in Cobar."

Another Green Street resident who was watering his garden, also witnessed the lens-shaped cloud and was stunned when it vanished in less than a second.

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©Cobar Age
Green Street residents Denise Kosef and her mother Edna Neale re-enact their sighting of a strange happening in the sky west of Cobar last Wednesday evening.

The resident said the cloud was the only object in the vast expanse of clear late evening sky west of the town and its sudden disappearance created an uncanny feeling.

Duty Cobar meteorologist Nigel Smedley said lenticular cloud formations occurred when condensation took place in the waves created by strong mid-level winds.

Although the winds are strong, the clouds appear to remain practically stationary but can abruptly disappear due to a lack of moisture and the air ascending in a wave and dissipating at the leading edge.

Mr Smedley said during his nine-year career as a meteorologist he had witnessed several similar events in other parts of New South Wales, but last Wednesday's observation by local residents was the first reported for Cobar.