Wednesday, November 10, 2010

Contrails: worth a closer look

I found myself gazing at the sky this morning, taking particular notice of the contrails that marked the lanes of mid-air expressways used by commercial jetliners as they criss-cross the countryside. Above is a picture I snapped from the area near Memphis International Airport at about 8:30am. The contrails were aligned generally east-southeast to west-northwest, or along a "highway in the sky" that connects Memphis to Atlanta. I decided this might be a good opportunity to provide a little background on what contrails are and how they form and dissipate, since weather plays an important role in these processes.

Contrails, short for "condensation trails," are trails of millions of tiny ice crystals formed by the exhaust of jet engines mounted on aircraft. When the hot and humid air from a jet engine mixes with surrounding air that is much colder and of lower pressure, the exhaust forces the humidity of that air to rise and condense, similar to the way clouds form. It is similar to the process that causes one to "see their breath" on very cold days.

At very cold temperatures, a trigger is required for condensation to take place and exhaust particles serve as that triggering mechanism (ice nuclei), forming artificial clouds of condensed water vapor. Also at very cold temperatures, the water vapor that is created rapidly freezes, creating the ice crystals that are visible as contrails. It is interesting that contrails will generally only form above about 26,000 feet (almost 5 miles - an altitude at which most commercial jets traveling an hour or more to their destination easily reach) and when the air temperature drops to -40 F (also, -40 C) [CIMSS, UW-Madison]. At the time of the picture above, the temperature was -40F at an altitude of approximately 32,000 feet (6 miles) over Memphis, so the contrails above were created by aircraft flying at or above that level.

Another interesting contrail picture is shown below. This one was sent to by a SKYWARN spotter in northeast Mississippi, who happened to spot, and photograph, a Delta airliner flying overhead with one engine out. It is fairly obvious that this is the case when looking at the single contrail behind the airplane. (There were no reports of aircraft mishaps on that day, so it obviously landed safely.)

As for dissipation, contrails can last anywhere from just a few seconds to hours and can remain as narrow as a 6-mile high tightrope or spread out to become a blanket of cirrus clouds covering a large area. How long they last and whether they spread out or distort in shape depends on the humidity of the air they reside in and the wind direction and speed at the level of the jetliner producing them. The drier the air is at the level contrails are created, the shorter their duration. Drier, less humid air tends to "absorb" the humid contrail more quickly than humid air as it mixes with the drier environment, thus dissipating more quickly.

In very dry air, the contrail may disappear just after being created. If the air is moist and the contrail remains in the sky for a long period of time, then the wind has an opportunity to act on the contrail. If the wind at that level is calm, the contrail remains intact or drifts slowly without spreading. If the wind is blowing strongly, which is more typical at 30-35,000 feet, than the contrail will move more quickly in the direction the wind is blowing and likely spread out and become diffuse as it forms an artificial cirrus canopy. Changes in wind speed (turbulence) can be noted if the contrail snakes across the sky.

Check out the picture below, courtesy of NASA, of a satellite picture of contrails traversing the southeastern U.S. during ideal conditions. It's pretty easy to tell from this image where the higher moisture resides at the upper levels of the atmosphere. It's not that there are no planes over northern South Carolina/Georgia or Tennessee, the air is simply drier and they have dissipated. The presence of cirrus clouds (very thin light grey in the areas where contrails exist) also indicates upper-level moisture. This would explain why many times contrails tend to last longer when there are also cirrus clouds in the area. Crystal clear skies tend to not support long contrails as the air is typically drier.

The contrail topic is timely due to a recent "mystery contrail" off the coast of Los Angeles (see still image from video below) that is now being hypothesized to be that of a commercial jet. I'm not certain at this point, but it definitely is some sort of vapor trail (video of the contrail from KCBS in L.A.). The video at first appears to show a missile, rocket, or other projectile traveling up and away from the camera angle, however an excellent write-up, with many similar examples, on tends to support the jet contrail hypothesis - specifically that of America West flight 808 from Honolulu to Phoenix, which would have been traveling inbound to the L.A. Basin.

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