Why do planes leave white trails?

What secrets do those mysterious white trails left by airplanes hold? 

Join us as we unravel the science behind contrails, those striking icy patterns in the sky that are more than just visual spectacles. 

Discover how the interaction between aircraft exhaust and the frigid air at cruising altitudes creates these fascinating formations, akin to seeing your breath on a cold day. 

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Episode Transcript

I Wonder Podcast (00:03):
Hey there, Curious Minds.
Welcome to an episode of IWonder, the show, where we
explore the questions you'vebeen wondering about.
Today's question comes fromEthan.

Ethan asked (00:13):
"Why do planes leave white trails in the sky?
That's a high-flying question,Ethan.
Those streaks in the sky mightlook mysterious, but they have a
lot to do with science, weatherand even jet engines.
Let's climb to cruisingaltitude and uncover the secret

(00:36):
of those white trails known ascontrails.
What are contrails?
Contrails, short forcondensation trails, are the

(00:59):
white lines you often see behindairplanes flying at high
altitudes.
They form when hot exhaust fromthe plane's engine meets the
cold, thin air in the upperatmosphere.
Airplane engines burn fuelwhich produces water vapor as
part of the exhaust.
When this warm, moist exhaustmixes with the frigid air, which

(01:25):
can be as cold as negative 40degrees, the water vapor freezes
into tiny ice crystals.
These ice crystals are what wesee as contrails.
You can think of contrails asbeing similar to your breath on
a cold winter day.

(01:45):
When you exhale, the warmmoisture from your breath
condenses into tiny droplets inthe chilly air, forming a mist.
In the sky, the process is thesame, but instead of mist you
get ice crystals, forming avisible trail.

(02:07):
Why do contrails look different?
If you've watched planes fly,you might have noticed that some
contrails disappear quickly,while others stretch out and
linger for hours.
The difference comes down tothe temperature and humidity of
the air where the plane isflying.
When the air is dry, thecontrails dissipate quickly

(02:29):
because the ice crystalsevaporate into the surrounding
air.
On the other hand, if the airis humid, the contrails can
stick around and even spread outto form thin, wispy clouds.
This process shows howcontrails are closely linked to
weather conditions in the upperatmosphere.
Pilots and meteorologists caneven use contrail behavior to

(02:54):
gather clues about the humidityand wind patterns at high
altitudes.
Are contrails harmful?
Contrails themselves aren'tharmful.
They're made up of ice crystalsand pose no direct danger to

(03:15):
people or the environment.
However, when contrails lingerand spread, they can trap heat
in the Earth's atmosphere.
This effect is similar to howclouds at night keep the Earth

(03:36):
warmer by acting like a blanket.
Scientists are studying howcontrails might contribute to
global warming over time, giventhe sheer number of flights
crisscrossing the globe daily.
To address this, engineers aredeveloping more efficient
airplane engines and alternativefuels to reduce emissions and
minimize aviation'senvironmental impact.

(03:59):
Do all planes leave contrails?
No, not all planes leavecontrails, and even those that
do, might not leave them onevery flight.
Whether a plane produces acontrail depends on several
factors, including altitude,temperature and humidity.
Planes flying at loweraltitudes where the air is

(04:22):
warmer, typically don't createcontrails because the exhaust
doesn't cool quickly enough toform ice crystals.
Most contrails occur atcruising altitudes, usually
between 26,000 and 40,000 feet,where the air is cold and thin.

(04:43):
Are contrails the same as smoke?
Contrails might look like smoketrailing behind a plane, but
they're very different.
Smoke is made up of tiny solidparticles from combustion, while
contrails are made up of icecrystals.
The white color of contrailscomes from the way sunlight

(05:06):
scatters through the icecrystals, just like it does in
natural clouds.
This scattering of light makescontrails appear bright and
white, even though they'reessentially frozen water.
How long have planes been makingcontrails?
Contrails aren't a modernphenomenon.
They've been around since theearly days of aviation.

(05:28):
During World War II, fighterplanes and bombers flying at
high altitudes often leftcontrails which could reveal
their positions to the enemy.
Pilots learned to adjust theirflight paths to avoid creating

(05:51):
contrails in sensitive missions,showing how even simple
atmospheric science could make ahuge difference in strategy.
Today, with thousands offlights crisscrossing the globe
every day, contrails have becomea familiar part of modern
aviation, often streaking thesky with their icy trails.

(06:12):
Here are some fun facts aboutcontrails.
Contrails aren't justfascinating to watch.
They can also help scientistsstudy the atmosphere.
By observing how contrails formand behave, researchers can
gather information abouttemperature, humidity and wind
patterns at high altitudes.
Occasionally, contrails formunusual shapes, like loops or

(06:35):
spirals, when planes make sharpturns or holding patterns.
These patterns, though rare,add a whimsical touch to the
science behind contrails.
So, Ethan, why do planes leavewhite trails in the sky?
It all comes down to theinteraction between hot exhaust

(07:01):
from airplane engines and thecold, thin air at high altitudes
.
These contrails, made up oftiny ice crystals, are like
nature's way of painting the skywith the byproduct of flight.
Whether they disappear quicklyor stretch out into wispy clouds
, contrails are a fascinatingreminder of the science at work
above our heads.

(07:24):
Thanks again to Ethan for such ahigh-flying question.
And remember, if you have aquestion you're curious about,
submit it on our website,iwonderpodcast.
com, and we might just featureit in our next episode of I
Wonder.
Until next time, stay curiousand keep wondering you.