Condensation Trails: What are they?

What is a condensation trail?

On a bright sunny day, have you ever noticed long white trails in the sky that look a bit like clouds? Have you ever wondered what they are and how they got there?

Those long white trails are known as condensation trails, (also referred to as contrails or vapour trails) and they are often visible behind jets flying at high altitudes. Contrails form when the hot water vapour produced by jet engine exhaust comes into contact with the much colder air present several miles above the earth’s surface. This rapid change in temperature causes the water vapour to form ice crystals. Contrails do not pose a health risk to humans.

What causes condensation trails to form?

A contrail will form if the humidity in the surrounding air is high enough to condense the water vapour into visible ice particles. Atmospheric temperature and humidity at any given location undergo natural daily and seasonal variations. As a result, there is a degree of variability in contrail formation, which explains why contrails are not witnessed in every instance of aircraft flight. The appearance and duration of contrails depends on many factors such as altitude, sunlight, temperature and humidity. Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours. If there is high humidity along the aircraft’s flight track, contrails will evaporate quickly. These trails are almost always present behind aircraft, although they are generally only visible from the ground when the sky is free of cloud cover.

Contrails form when the hot water vapour produced by jet engine exhaust comes into contact with the much colder air present several miles above the earth’s surface. Jet engine exhaust accounts for only a small portion of the water that forms ice in persistent contrails.

The ice particles in contrails do not reach the Earth’s surface because they fall slowly, and conditions in the lower atmosphere cause them to evaporate. Contrails are generally only visible on clear, sunny days, and are harder to see on cloudy, overcast days.

Why do some aircraft produce longer trails than others?

A contrail will form if the humidity in the surrounding air is high enough to condense the water vapour into visible ice particles. Atmospheric temperature and humidity at any given location undergo natural daily and seasonal variations. As a result, there is a degree of variability in contrail formation, which explains why contrails are not witnessed in every instance of aircraft flight.

The appearance and duration of contrails depends on many factors such as altitude, sunlight, temperature and humidity. Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours. If there is high humidity along the aircraft’s flight track, contrails will evaporate quickly. These trails are almost always present behind aircraft, although they are generally only visible from the ground when the sky is free of cloud cover.

Quick facts about contrails

  • The long white trails often visible behind jets flying at high altitudes are known as contrails, not chemtrails.

  • Contrails are almost entirely composed of water vapour and they do not pose a health risk to humans.

  • There is no scientific evidence to suggest that the trails often visible behind aircraft contain any dangerous chemicals.

  • Specifications and emissions standards for jet engines and jet fuel are very tightly controlled.

 

Did you know?

If there is low humidity along an aircraft’s flight track, contrails can persist quite a long period of time.

Persistent contrails often spread or widen because of air turbulence created by the passage of other aircraft, differences in wind speed along the flight track, and other factors. 

In fact, a persistent contrail can grow to several kilometres in width and up to 400 meters in height!

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