Deceptive Skies

April 08, 2013

P3130038 2B

Photographer: John Adam
Summary Authors: John Adam; John’s Web site; Les Cowley (Atmospheric Optics)

April 2013 Viewer's Choice

The sky is not always what it seems. It was late afternoon on a chilly mid-March day in the village of Bildeston, Suffolk, England. Surely the solid looking cumulus congestus near the horizon is casting its shadow onto the higher and much brighter cloud bank, is it not? No! The camera was facing in the opposite direction almost towards the Sun, which was to the right and just outside the field of view. And the Sun’s rays almost never shine upwards! So what is happening?

Dense cumulus congestus can tower up to 6km (20,000 ft). Here it is casting its shadow downwards onto thin translucent cloud (possibly altocumulus stratiformis) at lower altitude and closer to the camera. A trick of perspective makes the nearer and lower cloud, together with the shadow projected onto it; appear to be higher in the sky. We see the shadow from the other side of the cloud, just like looking at the back of a movie or cinema screen.

Hires adams_cowley_diagram

Shadows are in fact three-dimensional. Here in this wider-angle view we see them at left as diffuse shadows coursing (again downwards!) through the hazy air. These shadows are 3D tubes of dark air.

Hires secondary image adams cowley

The nearby stratus is so bright because it is thin and its tiny droplets diffract sunlight with little deviation mostly forwards towards the camera. The thin edges of the distant cumulus do the same to form the familiar silver lining. There is a hint of color too, cloud iridescence, because the diffraction is wavelength dependent.

Thanks to Ian Loxley of the Cloud Appreciation Society for the cloud identifications.

Photo detals: TOP - Camera Maker: OLYMPUS IMAGING CORP.; Camera Model: SP570UZ; Focal Length: 56.5mm; Aperture: f/8.0; Exposure Time: 0.0013 s (1/800); ISO equiv: 64; Software: Adobe Photoshop 7.0. Bottom - Same except: Focal Length: 62.9mm; Exposure Time: 0.0016 s (1/640); Software: Adobe Photoshop CS3 Windows.