Ever see the lines that follow planes high up in the sky? They are called contrails and for Weather Whys viewer Jim from Whitefish Bay asked why sometimes planes have them and other times they don't.

Contrails form when jet exhaust emits water vapor that condenses and freezes. Contrails don't form for every airplane. The atmosphere where the plane is flying needs to have low vapor pressure and low temperature.

There are three types of contrails. The short lived ones that only last a couple minutes after a plane passes. A persistent contrail lasts after a plane passes but stays in a thin line and a persistent spreading contrail lasts long after a plane passes and spreads out to a width wider than the width of your thumb if you held it up to the sky.

Weather Whys is a segment by Meteorologist Justin Thompson-Gee that airs during the CBS 58 News on WMLW - The M from 7 a.m. to 8 a.m. The segment answers viewer weather questions, explains weather phenomena and reveals interesting weather stats. To submit your question reach out to Justin on Facebook, Twitter or by emailing him at JThompson-Gee@cbs58.com.