This is the baffling moment an eerie portal to the heavens appeared in the sky over Indonesia.

Footage shows the strange elliptical gap in the blanket of clouds, with white wisps in the centre of the hole, hovering above the Jember Regency in East Java Province on June 4.

Hukama Nur Akmal, a weather forecaster at the Meteorology, Climatology, and Geophysical Agency, said the phenomenon is known as a fallstreak hole - sometimes called hole punch cloud, cavum, or cloud hole.

The gap is formed when a cloud's 'supercooled' water particles - which are below freezing point but have not turned to ice - suddenly freeze and fall to the ground.

The freezing is usually caused by aircraft passing through the cloud layer and triggering ice nucleation. Planes often have reduced areas of pressure near their wings or propellers, which causes air to cool very quickly and produces a trail of ice crystals in the aircraft's wake.

Hukama said: 'When a plane flies through a layer of thin cumuliform clouds it triggers glaciation. The glaciation changes water droplets to ice particles and causes a domino effect, where ice particles descend at high altitudes, and sometimes bend due to different wind speeds.'

He added that cavum clouds can be found in three types of mid to high-level clouds - cirrocumulus, altocumulus and stratocumulus.

Because of their circular appearance, fallstreak holes are sometimes mistaken for UFOs.