From my location in Mobile, I witnessed a once-in-a-lifetime sky. A few miles above me, an airplane sliced a thin deck of altocumulus clouds nearly in half with a distrail that featured a sun dog square in the middle of it. The description is a mouthful, but the sight was beautiful:
Around the time I took the picture, the weather observation at Mobile Regional Airport (about one mile straight ahead of the camera) reported a few clouds at 20,000 feet. These clouds appeared in the form of an altocumulus cloud deck, which is seen above.
The airport in Mobile is frequently visited by high-flying aircraft using the airport as a navigational point on their flight paths, so it’s not unusual to see at least a few planes flying over the city at any given time. One of these aircraft traveling from east to west flew at just the right altitude to slice through the altocumulus deck, creating what is called a “distrail.”
Most people are familiar with contrails – short for condensation trails – as the thin strips of clouds that follow behind high-flying aircraft as their warm, moist jet exhaust condenses into a cloud that traces the plane’s flight path through the sky. These harmless contrails, simply consisting of condensed water vapor, last from a few minutes to a few hours depending on atmospheric conditions and are extremely common near busier airports.
Distrails – short for dissipation trails – are sort of the opposite of a contrail. As an airplane flies through a thin cloud deck made of supercooled water droplets, its jet exhaust can serve as a nucleus to for some droplets to condense and others to evaporate, leaving a stripe of dissipation – or the appearance of nothingness – through the cloud deck. As the supercooled water droplets condense or evaporate, it creates a domino effect that can spread through the entire cloud deck and dissipate the whole thing in a relatively short period of time.
I took three shots of the clouds at three different times that help to illustrate this point.
Here’s the first shot I took around 3:30 p.m., showing the distrail just starting to form after the aircraft flew through the clouds:
The next shot is the incredible sky that I witnessed around 3:50 p.m. as the domino effect took hold and the distrail began to spread:
Here’s a similar view of the sky about thirty minutes later from a different location (but same direction) showing the altocumulus deck almost entirely dissipated, leaving behind a thin wisp in what used to be the center of the distrail:
Distrails are very similar to their more common counterpart, the fallstreak hole. A fallstreak hole forms in pretty much the same process that a distrail forms and the results are spectacular.
My good friend and an excellent photographer Leesa Brown caught these beautiful twin fallstreak holes in Kissimmee, Florida, back in February 2011:
Distrails are uncommon to begin with, but a second phenomenon made the sky so unique. The little rainbow-colored shimmer inside the distrail is called a sun dog.
The science behind the formation of sun dogs is relatively easy to understand, as explained by the University of Illinois:
Sundogs are visible when the sun is near the horizon and on the same horizontal plane as the observer and the ice crystals. As sunlight passes through the ice crystals, it is bent by 22 degrees before reaching our eyes, much like what happens with 22 degree halos. This bending of light results in the formation of a sundog.
The sun was at the perfect angle in the sky relative to the location of the clouds and my location on the ground in order to create a sun dog right in the middle of the distrail. As their formation requires ice crystals in the atmosphere, sun dogs are a relatively common sight during the fall and winter months. CWG’s own Kevin Ambrose posted some beautiful sun dog pictures he captured back in September 2008.
The sky in Mobile that day was the chance combination of two weather events that created a scene unlike anything I’ve seen in my life. The “sun dog in a distrail” was breathtaking and justifies a quirk of mine: always have a camera within reach.