photo #7423554

Akshay Shankar, nephew of Anuja Sarangi (Member 61,801), spotted the cloud feature known as an arcus forming at the front of a Cumulonimbus storm cloud as it loomed over Bangalore, India. An arcus is the broad, low dark ridge or shelf of cloud that can appear at the vanguard of an approaching thunderstorm. It forms as cold air dragged down by the heavy showers of the storm spread out at the ground. This cold air splaying out from the storm is denser than the warm moist air out ahead of the storm, which it lifts as it burrows beneath.As this warmer, moister air is lifted, its moisture condenses into a shelf of cloud attached to the base of the Cumulonimbus like a front bumper. This is the arcus. It’s a cloud feature that makes its intentions clear. Because it forms at the front, an arcus indicates the storm is heading your way rather than retreating and that strong winds, heavy showers, and thunder and lightning could soon follow. Anuja tells us that the Bangalore Met Department had issued an Orange Alert for rain on this day. And for anyone able to read it, this arcus cloud feature was its own dark grey alert, just to make things clear.