Richard, Here is a message I left about a month ago on the ComAirScan messge board...with a few changes. Depending on the temperature and humidity, contrails will form starting at around FL310 to FL350 (definitely not carved in stone.) . In summer, they may not form at all. In the northern part of the US, contrails on a cooler day, may be around FL280 or lower, but while it can be cold enough for a contrail, there may not be enough moisture in the air. Also, except for military aircraft that operate with fuel additives to prevent contrails, a Learjet can leave a contrail as a 747. Also in the Arctic, you may have seen photos of Twin Otters and C-130s leaving trails from start-up, to taxi, thru takeoff. Contrails form from the hot exhaust rapidly expanding as it hits the colder air...remember physics... as air expands, it cools...if it cools to the dewpoint it condenses...thus a trail of moisture is formed. (in this case, due to the extremely cold temps...the vapor sublimates directly into ice crystals) To answer your Qs. Yes...there can be a narrow band of altitude where contrails will form...depending on temperature and dewpoint. Walter DCA ----- Original Message ----- From: "Richard A Whitenight" <rum.runner@juno.com> To: <AIRLINE@LISTSERV.CUNY.EDU> Sent: Saturday, December 29, 2001 20:58 Subject: Contrails > Anyone who watches the skies sees the contrails emitting from a jet which > is obviously in rather cold air <grin>. Why is it that normally only see > contrails in a narrow band of cold air? Is there a high and low altitude > where contrails don't form, or is it strictly dependent on the > temperature of and/or moisture in the air. >