Altostratus clouds are a mid-level cloud layer that look like a blue-gray sheet across the sky. The sun can shine through, such as in this picture, giving them the name Altostratus translucidus. They typically form as a stable air mass is gently lifted, which tends to happen ahead of a front. This is common along the West Coast (as in this picture from Washington) where frontal systems frequently move onshore from the Pacific Ocean creating widespread, light rain during the fall, winter, and spring months.
Location: Bellingham Bay, Washington
Date: October 2012