An unusual cloud formation on Thursday in the town of Bintulu in Sarawak has caused excitement among residents.

Many were curious about the long white horizontal streak that appeared in the dark grey sky that morning, and took the opportunity to post videos and pictures online.

“Really beautiful... like a staircase to heaven,” wrote Safiah Ros on Facebook, while Jeya Lee said it was “beautiful and extraordinary”.

Some were excited by the unusual cloud appearance, while others wondered if it signalled an imminent thunderstorm.

“Hopefully, it will not be like Sibu, which was hit by heavy winds and rain. A lot of trees collapsed and roofs went flying,” said Este Bah on Facebook, in reference to a thunderstorm that hit parts of the nearby town on April 5.

The Malaysian Meteorology Department (MetMalaysia) said the phenomenon is known as an arcus cloud formation.

According to the British Met Office, arcus clouds are low-level, wide-ranging clouds typically associated with powerful storm clouds and thunderstorms.

“In tropical areas, this cloud formation exists if there are cumulonimbus clouds, or clouds that bring rain or storms,” Borneo Post quoted a MetMalaysia spokesman as saying. “Although the arcus or roll clouds are harmless, they may bring heavy rain, strong winds and lightning strikes.”