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 an Appleman chart to predict contrail formation for your area. Of course, a contrail cannot form if no airplane passes through.

Contrail Formation

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).

Contrail Evolution

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:

1. 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?
2. If it is possible to view upper levels of the atmosphere, are contrails seen?
3. If contrails are seen, are they persistent or short-lived?
4. If persistent, how many were seen?
5. If persistent, were natural-looking cirrus clouds also in the sky?
6. 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.