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I have noticed that some jet condensation trails persist longer than others. What are the different atmospheric conditions that contribute to this?

Question submitted by Matt Clarke
(Vancouver, BC)

Hi there Matt,
Obviously you spend a bit of time watching the sky! Glad to hear it!

Contrails (short for "condensation trails") or vapour trails are visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.

Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours. For the contrail to last a significant amount of time, the airmass at 35-40,000 feet must be very stable. This usually occurs when the upper atmosphere is significantly ridging.

Here's some interesting facts about contrails:
Studies have found that vapour trails or contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). Therefore, the overall net effect of contrails is positive, i.e. a warming. However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known.

September 11, 2001 climate impact study
The grounding of planes for three days in the United States after September 11, 2001 provided a rare opportunity for scientists to study the effects of contrails on climate forcing. Measurements showed that without contrails, the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before; however, it has also been suggested that this was due to unusually clear weather during the period.

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