Contrail is a shortened term for condensation trail. When the water from the exhaust after combustion mixes with local conditions (altitude/temperatures) it becomes visible as a cloud which can vary from horizon to horizon or be intermittent. Even a difference in altitude of a thousand feet can have totally different local temperatures. Forget the CT's about "chemtrails", it's just water vapour like the steam you see from the exhaust pipe when you start your car on cold days.