Contrails: The Controversial Cloud

An aircraft contrail. © Mint Images/Art Wolfe/Getty Images

While you may not recognize contrail clouds by name, you've likely seen them many times before. The trail of cloud seen behind a passing jet plane, the messages and smiley faces drawn in the summer sky at the beach; these are all examples of contrails.

Contrails are clouds that form The word "contrail" is short for condensation trail, which is a reference to how these clouds form behind the flight paths of aircraft.

 

Contrails are considered high-level clouds. They appear as long and narrow, but thick, lines of clouds, often with two or more side-by-side bands (the number of bands is determined by the number of engines (exhaust contrails) or wings (wing tip contrails) a plane has). Most are short-lived clouds, lasting only several minutes before evaporating. However, depending on weather conditions, it's possible for them to last hours or even days. Those that do last tend to spread into a thin layer of cirrus, known as contrail cirrus.

What Causes Contrails?

Contrails can form in one of two ways: by the addition of water vapor to the air from a plane's exhaust, or by the sudden change in pressure that occurs when air flows around a plane's wings.

  • Exhaust contrails: Exhaust contrails are the most common contrail type. As a plane uses up fuel during flight, exhaust exits out of the engines, releasing carbon dioxide, water vapor, and soot into the atmosphere. As this hot, moist air mixes with the cold air aloft it cools and condenses onto soot and sulfate particles to form a local contrail cloud. Because it takes several seconds for the exhaust air to sufficiently cool and condense, the contrail usually forms a short distance behind the aircraft. This is why a gap is often seen between the aircraft tail and the start of the cloud.
  • Wing tip contrails: If the air aloft is quite humid and nearly saturated, the flow of air around aircraft wings can also trigger condensation. Air flowing over the wing has a lower pressure than that flowing underneath it, and because air flows from high to low-pressure areas, a current of air also flows from the wing's bottom to its top. These movements combined create a tube of circulating air, or vortex, at the wing's tip. These vortices are areas of reduced pressure and temperature and can thus lead water vapor to condense.

    Since these contrails require a relatively moist atmosphere (higher humidity) to start with, they usually occur at lower altitudes where air is warmer, more dense, and capable of holding more water vapor.

    Contributing to Climate Change?

    While contrails are thought to only have a minor impact on climate, their influence on daily temperature patterns is much more significant. As contrails spread and thin out to form contrail cirrus, they promote daytime cooling (their high albedo reflects incoming solar radiation back out into space) and warming at night (high, thin clouds absorb the Earth's outgoing longwave radiation). The magnitude of this warming is thought to outweigh the effects of cooling.

    It should also be noted that contrail formation is associated with the release of carbon dioxide, which is a known greenhouse gas and global warming contributor.

    A Controversial Cloud

    Some individuals, including conspiracy theorists, have their own opinions about contrails and what they really are. Instead of condensation, they believe them to be mists of chemicals, or "chemtrails," deliberately sprayed by government organizations onto unsuspecting citizens below. They argue that these substances are released into the atmosphere for the purposes of controlling the weather, controlling population, and for the testing of biological weapons, and that the idea of contrails as harmless clouds is a cover up.

    According to skeptics, if contrails appear in criss-cross, grid-like, or tic-tac-toe patterns, or are visible over locations where no flight-patterns exist, there's a good chance it isn't a contrail at all.