What chemicals are being sprayed by jets in the skies over Dothan, Alabama? How long will this continue?

A: The white trails, or contrails, that jets leave in their wake are caused by water vapor in the jet exhaust.

According to Art Morris, airport director at Dothan Regional Airport, any aircraft that is powered by hydrocarbon gasses (aviation gasoline or jet fuel) contains water vapor in the exhaust. Automobiles have water vapor in their exhaust.

“The temperature at high altitudes is quite low which causes the water vapor in the exhaust to freeze and form ice crystals which leave a ‘trail’ behind the aircraft,” Morris wrote in an email. “These are called contrails. They are usually more visible in the winter than in summer but they can occur at any time.”

Morris said Dothan is located underneath several airway corridors.

“These corridors are analogous to highways in the sky,” Morris wrote. “They are the designated travel lanes for commercial and military aircraft to fly in as part of the air traffic control system in the United States.”

Morris said the aircraft that overfly Dothan on any given day come from all segments of the aviation community.

The aircraft seen by the reader who asked the questions could be military aircraft from any branch, commercial airliners or privately owned business aircraft.