Atmospheric
Optics
Tilting at Clouds in La Mancha
The cloud underside is tinted red-brown by sunlight reflected upwards from the region's predominantly red soil.
Imaged around Manzanares, Spain by atmospheric optics expert Stan Gedzelman of Cornell University.
Images ©Stan Gedzelman, shown with permission
Reddened cloud at Consuegra of Don Quixote windmills fame.
The squares show colours spot sampled from the unenhanced image. The background is a neutral grey (RGB, 102,102,102). In contrast the second square from left is (143,138,135) - red enriched.
The cloud underside gets (1) scattered light from the ground, (2) light from the blue sky and (3) some light transmitted right through the cloud. The light from the ground is coloured by the red soil. This cumulus mediocris cloud is optically dense and hardly transmits any sunlight.
Stan Gedzelman mathematically modelled his sighting (Applied Optics 54, B167 (2115)) to find that clouds coloured by the ground beneath are best with high reflectivity ground, an optically thick cloud to give a dark base, a low cloud, low sky cloud cover and a relatively low sun to ensure that the ground beneath the cloud is not shadowed.
Coloured clouds are seen above brightly coloured crops (e.g.
canola). 'Ice Blink', scattered light from ice floes and water leads, were used by Arctic dwellers to find their way.
Water reflects so well that it can form a quasi image on clouds.
