Halos & Shadows

  1. Peg Zenko (Tangent Photos) - Illinois
  2. Christopher Gerekos - Belgium
  3. Ian Brock - Atlantic Ocean

In Peg Zenko's (1) and Chris Gerekos's (2) images, bright contrails cross the sky accompanied by their dark shadow and a 22� halo. Are they connected? Yes and no. In both cases the aircraft is at cruising altitude and its contrail is above a thin and translucent layer of halo producing cirrus cloud. The dark trail is the shadow of the contrail cast downwards onto the cirrus layer. The contrail can be thousands of feet above the halo forming crystals.

Ian Brock (3) was fortunate to be between a contrail and its shadow on lower clouds.

The apparent position of contrail shadows relative to the trail itself can be counterintuitive. The sun's rays are everywhere parallel and perspective makes the shadow appear farther away from the sun than the contrail itself.   When a contrail passes in front of the sun or is very close its shadow can race ahead of it in the sky as a dark spike.

Are there more contrails than there used to be? Yes, aircraft movements have considerably increased.

But there is another reason - Paradoxically, modern aircraft with more fuel-efficient engines and airframes form contrails under a wider range of atmospheric conditions than did older airplanes.

Modern aircraft waste less of the fuel's combustion heat in the exhaust gases. The resulting cooler plumes have a higher relative humidity and their moisture condenses more readily to form contrails.

The lower image (4) is of a scientific trial (Aerosp. Sci. Technol. 4 (2000) 391-401 'Influence of propulsion efficiency on contrail formation'). On the left is a modern Airbus A340 - on the right is an old Boeing 707. The striped pole is the nose probe of the following research aircraft. The aircraft are flying under identical conditions. The Airbus produces contrails, the older 707 does not.


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