Low level water clouds
Water clouds are whitish in the Natural RGB images, because their reflectivity values are high and close to each other in the 0.63, 0.87 and 1.6 micron channels.
The image below (MetOp AVHRR Natural Colour RGB image from 08 March 2014 10:06 UTC) shows the Strait of Gibraltar with the Mediterranean Sea to the East, and the Atlantic Ocean to the West. The low level wind was easterly. The region was mainly cloud free except a cumulus cloud band elongated from the strait to the East. In this band low level cumulus clouds formed due to the orographic forced convergence and lifting. West from the strait the clouds disappeared.
MetOp AVHRR Natural Colour RGB image from 08 March 2014 10:06 UTC
Explanation of the white tone of low-level water clouds in the Natural Colour RGB (see the recipe):
• With an equal contribution of the red, green and the blue colour beam over low-level water clouds, the resulting colour is white.