Cloud Fingers Over Tracy, California
September 27, 2011
On the photo above, finger-like altocumulus clouds are seen stretching across an azure blue sky. These billow clouds (undulatus) formed in atmospheric waves. They're oriented more or less perpendicular to the wind direction. Slots between the clouds appear where the wave dips (trough). The air temperature in the wave crest (where the wave climbs) is slightly cooler than in the trough, so atmospheric water vapor more readily condenses here. Cloud droplets evaporate; however, where the wave moves the cloud into the trough. Photo taken in the late afternoon of July 31, 2011, from Tracy, California.
Photo details: Olympus E-5 camera; f/11; 1/1000 sec. exposure; ISO 200; 14 mm focal length.