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Contrails
The Controversial Cloud
by Tiffany Means
While you may not recognize contrail clouds by name, you've
likely seen them many times before.
The trail of cloud seen behind a passing jet plane, the messages
and smiley faces drawn in the summer sky at the beach; these are all examples of
contrails.
The word "contrail" is short for condensation trail, which is a reference to how these
clouds form behind the flight paths of aircraft.
Contrails are considered high-level clouds.
They appear as long and narrow, but thick, lines of clouds,
often with two or more side-by-side bands (the number of bands is determined by
the number of engines (exhaust contrails) or wings (wing tip contrails) a plane
has).
Most are short-lived clouds, lasting only several minutes before
evaporating.
However, depending on weather conditions, it's possible for them
to last hours or even days.
Those that do last tend to
spread into a thin layer of cirrus, known as contrail cirrus.
What Causes Contrails?
Contrails can form in one of two ways: by the addition of water
vapor to the air from a plane's exhaust, or by the sudden change in pressure
that occurs when air flows around a plane's wings.
· Exhaust contrails: Exhaust
contrails are the most common contrail type. As a plane uses up fuel during
flight, exhaust exits out of the engines, releasing carbon dioxide, water
vapor, and soot into the atmosphere.
As this hot, moist air mixes with the cold air aloft it cools
and condenses onto soot and sulfate particles to form a local contrail cloud.
Because it takes several seconds for the exhaust air to
sufficiently cool and condense, the contrail usually forms a short distance
behind the aircraft.
This is why a gap is often seen between the aircraft tail and
the start of the cloud.
· Wingtip contrails: If
the air aloft is quite humid and nearly saturated, the flow of air around
aircraft wings can also trigger condensation.
Air flowing over the wing has a lower pressure than that flowing
underneath it, and because air flows from high to low-pressure areas, a current
of air also flows from the wing's bottom to its top.
These movements combined create a tube of circulating air, or
vortex, at the wing's tip. These vortices are areas of reduced pressure and
temperature and can thus lead water vapor to condense.
Since these contrails require a relatively moist atmosphere
(higher humidity) to start with, they usually occur at lower altitudes where
the air is warmer, more dense, and capable of holding more water vapor.
Contributing to Climate Change?
While contrails are thought to only have a minor impact on climate,
their influence on daily temperature patterns is much more significant.
As contrails spread and thin out to form contrail cirrus, they
promote daytime cooling (their high albedo reflects incoming solar radiation
back out into space) and warming at night (high, thin clouds absorb the Earth's
outgoing longwave radiation).
The magnitude of this warming is thought to outweigh the effects
of cooling.
It should also be noted that contrail formation is associated
with the release of carbon dioxide, which is a known greenhouse gas and global warming contributor.
A Controversial Cloud
Some individuals, including conspiracy theorists, have their own
opinions about contrails and what they really are.
Instead of condensation, they believe them to be mists of
chemicals, or "chemtrails,"
deliberately sprayed by government organizations onto unsuspecting citizens
below.
They argue that these substances are released into the
atmosphere for the purposes of controlling the weather, controlling population,
and for the testing of biological weapons and that the idea of contrails as
harmless clouds is a cover-up.
According to skeptics, if contrails appear in criss-cross,
grid-like, or tic-tac-toe patterns, or are visible over locations where no
flight-patterns exist, there's a good chance it isn't a contrail at all.
Tiffany
Means
Introduction
Studied
atmospheric sciences and meteorology at the University of North Carolina
Former
administrative assistant for the National Oceanic and Atmospheric
Administration
Member
of the American Meteorological Society
Experience
Tiffany
Means is a former writer for ThoughtCo who contributed articles about weather
for five years. She has interned with the domestic and international weather
departments at CNN, written monthly climate reports for NOAA’s National Centers
for Environmental Prediction, and participated in a number of science outreach
events, including the Science Olympiad Competition. Means has personally
experienced such weather greats as the Blizzard of 1993 and the floods of
Hurricane Francis (2004) and Ivan (2004).
Education
Bachelor's
degree in atmospheric sciences and meteorology from the University of North
Carolina at Asheville
ThoughtCo
and Dotdash
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