S'COOL: Observing Contrails
What are contrails?
Contrails are clouds of ice particles formed around the small particles (aerosols) which
are in aircraft exhaust. When these persist after the passage of the plane
they are of great interest to researchers. Under the
right conditions, clouds initiated by passing aircraft can spread with time
to cover the whole sky. See an article by
CERES researcher Dr. Pat Minnis.
Where do contrails form?
Contrails are human-induced clouds that only form at very high altitudes
(usually above 8 km - about 26,000 ft) where the air is extremely cold (less than -40°C).
Because of this contrails form not when an airplane is taking off or landing,
but while it is at cruise altitude. (Exceptions occur in places like Alaska and Canada, where
such very cold air can sometimes be at or near ground level.) Thus, people who live under major
air traffic routes, not those who live near major airports, are those who will see the most contrails.
(However, some major airports are also under major
air traffic routes, which can lead to confusion.) You can use
Appleman chart to predict contrail formation for your area. Of
course, a contrail cannot form if no airplane passes through.
If the air is very dry, a contrail will not form behind the plane. If the air is
somewhat moist, a contrail will form immediately behind the aircraft and
make a bright white line that lasts for a short while (a short-lived contrail). Persistent
contrails form immediately behind the airplane in very moist air.
These long-lived contrails will usually grow wider and fuzzier as time
passes. You may wish to review the GLOBE Contrail Formation Guide (available in several languages).
Sometimes contrails will actually take on the characteristics of a
natural cirrus cloud and no longer look like contrails after only a half
hour or so. Persistent contrails can exist long after the airplane that
made them has left the area. They can last for a few minutes or longer
than a day. However, because they form at high altitudes where the winds
are usually very strong, they will often move away from the area where they were
born. When we look up into the sky, we may see old persistent
contrails that formed somewhere else but moved overhead because of the wind. An
example of several very persistent contrails is shown in the
S'COOL cloud chart.
Persistent contrails are those most likely to affect climate.
NASA could use more data on contrails. Thus, when cloud amount is
estimated, it would be good to know:
- Is it possible to see contrails? That is, can the high altitudes be
seen from the surface, or are there too many low clouds in the way?
- If it is possible to view upper levels of the atmosphere, are contrails
- If contrails are seen, are they persistent or short-lived?
- If persistent, how many were seen?
- If persistent, were natural-looking cirrus clouds also in the sky?
- If persistent and possible, how much of the sky contained contrails?
These observational details can be reported in the comments section of the
S'COOL report form. However, we do now ask for a count of the number of short-
lived and persistent contrails visible during every observation.
This information, if taken regularly, will help us learn where and how
often contrails occur. By matching the surface observations with the
satellite data, we will then know if we are using the satellite data
correctly to identify contrails and determine how they affect climate.