Iridescent contrails? Rainbow contrails? Something else?

View at EarthSky Community Photos. | Soumyadeep Mukherjee in Kolkata, India, captured these photos of contrails on July 19, 2023, and wrote: “I was lucky to capture this extremely rare phenomenon.” We agree! An unusual photo! Keep reading for an explanation from sky optics expert Les Cowley of the website Atmospheric Optics. Thank you, Soumyadeep!

Take a look at this contrail photo, which prompted a long discussion here at EarthSky. Soumyadeep Mukherjee in India told EarthSky he was photographing a sundog in the late afternoon of July 19, 2023, when the sun was low in the west, at an altitude of 12 degrees. Then, he wrote:

I noticed a contrail in my frame (I was imaging at 150mm focal length). I zoomed in (600mm) and noticed the colors on the contrail.

He said he couldn’t see the colors with the eye alone. But they were noticeable with the zoom. So, he captured around 20 images within a span of 30 to 40 seconds, he said, using a high shutter speed (f/9, 1/1600s, ISO 250).

And then he chose seven from which to make the collage you see at top.

But what is it, really?

They are rainbow colors, yes. But this isn’t a rainbow phenomenon. Rainbows take many forms, but, essentially, they are curved bows, seen on the opposite side of the sky from the sun.

And some people do capture photos of true iridescent clouds. We’ve seen some at our community photo page. Iridescent clouds also have rainbow colors. You can read about iridescent clouds here. But … they are clouds.

Soumyadeep’s photo does clearly show a contrail. And, at Les Cowley’s great website Atmospheric Optics, Les has photos and an explanation of iridescent contrails. That’s right, they do exist!

But we weren’t sure that’s what Soumyadeep had captured.

So we dropped a note to Les Cowley, and he graciously replied.

Not iridescent contrails … but beautiful and unusual!

Les wrote:

[You can] think of aircraft contrails as clouds, they have tiny water droplets just like nature’s clouds. These can diffract sunlight to form iridescent colors. More about them on my site here.

But I think the colors in the picture are not aircraft trail iridescence. There is some evidence for strips of thin cloud that are below the contrails. The contrails appear to be lit by a very low or setting sun. The lower cloud droplets [the clouds below the contrails] are diffracting the light of the bright contrails to form the colors.

The reason I support this explanation is that the colors are where the contrails are thick some distance from the aircraft. Look at the image on my site and you will see the colors close to the aircraft, where the trails are thin.

Those regions have droplets of just the right size formed by air passing over the wings and fuselage.

The thick trails are from moisture condensed from the engine exhausts.

An unusual picture!

Thanks, Les. An unusual picture indeed. Thank you again, Soumyadeep!

Here is Les Cowley’s enhancement of Soumyadeep Mukherjee’s photo, showing the clouds below. Les wrote: “The contrail brightly lit by the low sun was, in effect, acting as a long thin light source above the cloud layer. I have grossly ‘enhanced’ the image … you can see the thin patchy cloud [below the contrails].”

Bottom line: They look like iridescent contrails. But, are they? We asked the atmospheric phenomena expert Les Cowley, who gave a different opinion.

The post Iridescent contrails? Rainbow contrails? Something else? first appeared on EarthSky.