It happens when air with a certain dew point temperature finds a surface that's at or below that temperature. (energyvanguard.com)
If the air temperature is cooled below the Dew Point Temperature then a reverse phase change occurs called condensation and water vapor becomes liquid. (drtimball.com)
A Contrail is exhaust from an aircraft engine cooling very rapidly below the dew point temperature and condensing into a visible trail of microscopic droplets. (drtimball.com)