photo #4471916

This dramatic arched cloud feature known as an arcus moved in over Villeneuve-d’Ascq, Nord, France. It was a first-time sighting and a memorable souvenir for visiting cloud-lover Ronan Crespin (Member 61,146). An arcus feature appears as a low protrusion of cloud along the front base of an advancing storm, just ahead of the showers arriving. It forms as cool air dragged down by all the falling water in the storm’s showers splays out at the ground, pushing ahead of the storm and lifting up the warmer, humid air near the surface. When this low, stable air is lifted, it cools enough to form a ridge of cloud like a front bumper to the advancing storm. Sometimes, an arcus can have a stacked appearance like Ronan’s here did. The distinct and dramatic striations were likely due to the low air lifted by the winds flowing out from the storm consisting of alternating moister and drier layers. It was a stack of warning for Ronan of the showers about to arrive.