Why Planes Leave Trails in the Sky?

Contrails, chemtrails, or smoke: What are those white trails left by planes?

Have you ever looked up in the sky after a plane flew by and noticed the trail of what looks like it could be smoke, and wondered why some planes leave those trails while others don’t? Perhaps you have heard the misused term ‘Chemtrail’ used by those in some circles who believe in the conspiracy that those trails of what look like smoke from the ground are dangerous, purposefully emitted poisons from the government. In reality though, these benign cloud-like lines left in the sky by passing aircraft, are called contrails and they are made up mostly of water vapor, and some disappear faster than others based on weather conditions.

Weather and Temperature influence the forming of contrails

The word contrail is actually short for condensation, which means that those trails form when the water in the jet exhaust mixes with the cold, wet air in the sky and then condenses into ice crystals to form a type of cirrus cloud; or what we see from the ground as contrails. How long the contrail remains visible to us from the ground depends on a couple factors, including the type of aircraft, the weather, and the surrounding air temperature. So when the air around the engine is cold and wet enough, the water in the exhaust will mix with that air to leave contrails that linger in the air for a period of time after the aircraft passes. In other words, the colder the air is surrounding the engine, the more likely that cirrus cloud, or contrails will form, persist, and spread throughout the sky.

There are 3 different main categories used to describe the various types of contrails: (all contrails are basically the same in composition, what is different is the lifespan of the trail)

  1. Short lived contrails: Typically the slender white lines you see trailing an airplane, a short 
lived contrail will disappear almost as quick as it was created, partly due to the fact that there is minimal water vapor in the air through which the plane is passing so the ice crystals quickly return to vapor state.
  2. Persistent, non-spreading contrails: Since the air through which the plane travels is humid and contain a substantial amount of water vapor to make a long lasting trail, a persistent non- spreading contrail is a longer, white line that remains visible for a longer period of time.
  3. Persistent, spreading contrails: While similar to the previously mentioned contrail category, a persistent spreading contrail is one that is spread out over large areas due to weather conditions. It’s been suggested that this type of contrail has the strongest potential to affect surrounding climate.

Are contrails a cause for concern?

Since contrails (often misidentified as, ‘chemtrails’) are made up of mostly water vapor, the white trails left by airliners are not harmful to us, though some do argue that large contrails that linger have the potential to absorb a large amount of thermal energy, thus increasing overall heat in the atmosphere. It’s important to remember though that airliners of all types are more fuel efficient and energy conscious than ever before, meaning that they are meticulous when it comes to knowing the exact ingredients in the fuel that powers their aircrafts. The science is always being reevaluated to make sure that we know all there is to know about contrails.