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What a jet contrail says about the air up high

All year

A jet contrail is a thin cloud of ice crystals formed from a plane's exhaust. If it is short-lived the air high up is relatively dry, and if it lingers and spreads the air high up holds a lot of moisture.

Why this time of year

Jets cross the sky in every season, and the same kind of trail can look short one day and long the next, depending on the air they fly through.

How to notice it

  1. Find a jet's white trail and watch the end nearest the plane. Note whether it forms a line behind the plane at all.
  2. A trail that fades out quickly means relatively dry air aloft. A trail that stays and spreads wider means moist air aloft.
  3. Look again an hour later, or on another day, and compare. Spreading trails can merge and end up looking like natural cirrus cloud.
  4. If you want to guess at the weather, treat lingering spreading trails as a loose hint that a change or precipitation may be coming. They are not a forecast on their own.

Try this with kids: Can you find a jet's trail that disappears fast and another that sticks around and gets fuzzy? Which one says the sky up high is wetter?

Word to know: “Contrail” is short for condensation trail, a line of ice-crystal cloud left behind a high-flying jet.

Safety

  • Do not look near the sun to watch a trail.

Sources

  • globe.govNASA GLOBE/S'COOL contrail FAQ: persistent or persistent spreading contrail means upper atmosphere contains large amounts of moisture; short-lived means relatively dry; short-lived may indicate fair weather, persistent may indicate an approaching change in the weather or precipitation. Contrails will not form if the temperature is too warm or the air too dry; along the moist east coast more persistent contrails may be observed.
  • globe.govGLOBE: persistent contrails form long-lasting and sometimes extensive clouds that would not normally have formed. Persistent contrails can last hours to days and become indistinguishable from naturally occurring cirrus.
  • earthdata.nasa.govNASA Earthdata: exhaust water vapor saturates already moist air, condenses and freezes into ice crystals; jets cruise at 26,000 feet or higher; spreading contrails can merge and mimic cirrus layers.

Checked against these sources on October 9, 2026.

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