Contrails and our weekend weather forecast

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PHOENIX – Most days of the year our state, and much of the planet, is crisscrossed with contrails from high flying jet aircraft.

A contrail will form behind a jet if the humidity is high enough and the temperature low enough for liquid water to condense.

The air needs to be supersaturated and the temperature generally below -40°F, something that typically occurs miles up in the atmosphere where jets fly.  

Under those conditions, water vapor from the jet's exhaust and secondarily from the atmosphere condenses into water droplets. Very quickly these droplets freeze into the snow-white particles that bring the contrail to life.

How long a newly-formed condensation trail sticks around depends on the ambient humidity. If humidity is low, contrails will rapidly dissipate, looking like a comet's tail. The ice particles sublimate, meaning go straight from ice to vapor, and you're back to blue sky.

If humidity is high, however, contrails can persist and those are the ones that trouble climatologists.

One study showed the contrails from only six aircraft spread over 7,700 square miles.

We don’t really know how contrails impact our weather. One tantalizing study done for the three days after September  11, 2001, when there was no commercial aircraft traffic over the U.S., showed on average, daytime temperatures were 2 degrees warmer and nighttime temps were 2 degrees cooler. In other words, one impact of contrails may be decreasing temperature variability from day and night.

To learn more about contrails, check out this National Weather Service website: http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgz.