You occasionally see very pretty photos of contrails, like this one:

Very pretty.  But what is it?  It’s clearly not a regular exhaust contrail, as the trail seems to start actually ON the wing, and it has a weird rainbow effect you don’t find in exhaust contrails.

It’s actually an aerodynamic contrail.  It’s formed by the reduction of pressure in the air as it moves over the wing.  When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator).  The reduced temperature cause small drops of water to condense, which then may freeze.  The (frozen) drops get larger as more water condenses on them.  The different sized drops (or ice crystals) have different optical properties, which affect different wavelengths of light, which accounts for the “rainbow” effect.

A rather more scientific explanation (also explaining the exact colors) can be found here:

http://contrailscience.com/files/Gierens_Aerodynamic_poster_060625.pdf

The interesting thing about this type of contrail is that they are actually more common in warm weather.   They need a lot of moisture, and cold air is generally dry, so they are more common in the summer months, and in warmer climes. See:

As aerodynamic contrails are independent of the formation conditions of jet contrails, they form an additional class of contrails which might be complementary because they form in predominantly in layers that are too warm for jet contrail formation.

There is actually surprisingly little work being done on the formation of aerodynamic contrails.   A lot of the time they only show up as wingtip contrails (which you can see are very dense in the above photo).  Conditions need to be just right for the full wing to generate a contrail, and it typically does not last very long at all.

You’ve probably seen aerodynamic contrails on landing planes, like:

Here you can see the wing contrails vanish almost immediately.  The long persisting contrails (which won’t persist very long) are from the outer end of the lowered flaps - where air is compressed and then expanded very rapidly, causing a lot of moisture to condense.   You can also see there’s a lot of moisture in the air, it’s quite misty looking.

This fantastic photo set on Flickr has some nice photos of both exhaust and aerodynamic contrails, you can clearly see the difference:

http://flickr.com/photos/jpro747/sets/72157594548787911/with/511377939/

In this shot, you can see the jet seems to be underneath a layer of clouds, suggesting it’s at a low altitude, where exhaust contrails would not normally form.  Although with this image, it’s little hard to see what is in front of what.  I suspect that the shadow you see near the tail is actually the shadow of the plane on the clouds, meaning the plane is just above them, or in them - in a region of high humidity, either way.

From the same set, we can see that the two types of contrail are not mutually exclusive.  You can have both at the same time:

Note you have the thick white contrails coming from the engine, and the aerodynamic contrails coming from the wings.

(side note here: most of the photos in jpro747’s set were taken FROM THE GROUND with a Canon 350D ($400) attached to a 1200mm 6-inch Dobsonian telescope ($300-$1000, depending on quality).   In most of the shots, you can clearly see the type of plane, and usually the airline markings.   Now, why has NOBODY in the supposed 2 Million “chemtrail” community managed to take a closeup photo of a jet spraying chemicals.  Considering you can do it with $700 worth of equipment, it seems rather odd).

This video shows an aerodynamic contrail forming in patchy air:

You can see the trail looks very similar to the photos, especially in the final segment of the video.  There are few wide shots, so it’s hard to tell how long the trails is lasting for, but at 0:50, the camera pulls back, and the provious trail is either off screen, or has already evaporated.    Note this video was shot in Torino (Turin), Italy,on August 16th 2008 - during the summer.  That’s when these type of contrails are more likely, as they need very humid air.

Reference:

Aerodynamic Contrails Phenomenology and flow physics  - Gierens, et al.
Aerodynamic contrails Microphysics and optical Properties - Gierens, at al.