If you’ve ever wondered why planes leave trails, the answer is actually quite straightforward.

Planes flying at high altitudes frequently leave behind trails because of the difference in temperature between the plane’s exhaust and outside temperatures.

These trails, known as contrails, are harmless, but some conspiracy theorists believe that they contain dangerous chemicals.

The “chemtrails” conspiracy theory is widely discredited by both scientists and government agencies, but remains a popular claim. 

Why Planes Leave Trails

Airplanes leave trails because the water vapor in the plane’s exhaust condenses in the atmosphere and forms trails.

These trails are similar to why you’re able to see your breath in the cold sometimes. Jet exhaust also contains soot, which helps condense the water vapor. 

The thickness and duration of the trails mostly depends on the plane’s altitude and weather conditions.

You can even predict the weather based on the nature and persistence of the jet contrails. 

For example, a short-lived contrail implies low humidity at high altitudes, which indicates fair weather.

In contrast, a longer and thicker contrail implies humid air at higher altitudes, which indicates there may be a storm approaching. 

3 Types of Contrails Planes Leave Behind

There are three types of contrails. 

1. Short Lived Contrails

Short-lived contrails are bright white lines that immediately form behind a plane and last only a short while if the air is moist.

Some short-lived contrails disappear immediately upon forming, while others may last a few seconds to minutes.

2. Persistent Non-spreading Contrails

Persistent non-spreading contrails form in moist air and remain for longer times after a plane can be seen in the sky.

These contrails last anywhere from a few minutes to more than a day, and they’re always shaped like a thin line.

3. Persistent Spreading Contrails

Persistent spreading contrails also form in moist air, but they spread across the sky and become wide fuzzy lines over time.

Persistent spreading contrails may even appear like natural cirrus clouds over time, rather than looking like contrails at all.

Persistent spreading contrails also contribute to climate change. 

Planes Trails Aren’t Dangerous

Some people claim that contrails are dangerous, and they contain chemicals that damage human populations.

These people believe that these chemicals are used for multiple malevolent purposes like population control, weather modification, or sterilization. 

Chemtrail theorists claim that normal plane trails quickly dissipate, so longer-lasting contrails contain additional substances.

Chemtrail theorists aren’t confined to the United States either, almost 17% of respondents in an international survey confessed at least partly believing in a large-scale atmospheric program. 

Yet, in 2016, the Carnegie Institution for Science completed a peer-reviewed study. 76 of 77 of the leading atmospheric scientists stated they found no evidence of a chemtrail conspiracy.

Apart from the claims of chemtrail conspiracy theorists, there is no evidence that the trails that planes leave behind are dangerous.

Differences Between Contrails and Chemtrails

Chemtrail is short for ‘chemical trail’, and contrail is short for ‘condensation trail’.

Chemtrails are a conspiracy theory that states that long-lasting contrails contain undisclosed chemicals. These chemicals are supposedly sprayed over population centers and agricultural areas for malevolent purposes.

In other words, chemtrails are just contrails that supposedly contain additional chemicals for malevolent purposes. 

Despite chemtrail conspiracy theorists being relatively popular, there is no evidence to support that chemtrails exist.

International governments, scientists, and international agencies have repeatedly claimed that chemtrails don’t exist.

These experts claim that so-called “chemtrails” are, in fact, normal contrails made up primarily of water, in the form of ice crystals.

Why Some Planes Leave Trails and Others Don’t

Generally, planes that fly at higher altitudes and use turbojet engines leave contrails.

Conversely, planes with turboprop engines fly at lower altitudes and leave fewer contrails. 

This is because contrails most commonly form at altitudes near or above 35,000 feet and at atmospheric temperatures of -50°C or -58°F, which is the airspace and conditions that turbojet planes operate in.

Commercial and private airplanes, as well as military jets, use turbojet engines, so these aircraft are the most likely to leave contrails.

In contrast, turboprop aircraft fly at lower altitudes and use exhaust gasses to propel themselves.

As a result, these planes are unlikely to release enough exhaust or climb high enough for the temperatures to be best suited for creating contrails. 

Others Causes of Planes Leaving White Smoke Behind Them

There are three other possible causes of planes leaving behind white smoke.

  • Stunt Planes Using Smoke Oil

Stunt aircraft often leave smoke trails as they perform stunts during shows.

These smoke trails are produced using smoke oils.

These planes will use a fuel pump to shift oil from the tank to nozzles connected to exhaust points. The oil produces smoke when released from the nozzles.

  • Crop Dusting or Fire fighting

Often, planes are used for releasing chemicals for practical tasks like crop dusting or putting out fires.

Often, these planes will release white smoke while doing so.

Crop dusting and fire fighting planes are most often seen leaving behind white smoke while working.

  • Partially Burnt Fuel/ Engine Malfunction

Sometimes planes suffering from engine failures will leave behind white smoke.

Engine smoke indicates a potentially dangerous emergency, and is sometimes mistaken for contrails by on-ground observers. 

In conclusion, airplanes leave behind contrails because of the difference of temperature between airplane exhaust and the atmosphere.

Contrails are most often released by military jets or other aircraft with turbojet engines, since they fly at higher altitudes and at lower temperatures.

Although contrails are not dangerous, there is a chemtrail conspiracy theory that inaccurately claims that long-lasting contrails contain dangerous chemicals.

Helen Krasner holds a PPL(A), with 15 years experience flying fixed-wing aircraft; a PPL(H), with 13 years experience flying helicopters; and a CPL(H), Helicopter Instructor Rating, with 12 years working as a helicopter instructor.

Helen is an accomplished aviation writer with 12 years of experience, having authored several books and published numerous articles while also serving as the Editor of the BWPA (British Women Pilots Association) newsletter, with her excellent work having been recognized with her nomination of the “Aviation Journalist of the Year” award.

Helen has won the “Dawn to Dusk” International Flying Competition, along with the best all-female competitors, three times with her copilot.