What is actually happening when you see two lines of cloud emitting from the engines of an airliner ?

Sometimes when you look up and see the airliner fly pass on a clear blue sky...a trail of two white cloud follows the airliner. What is actually happening? Is it humidity in the air or is the pilot testing the air density?What is actually happening when you see two lines of cloud emitting from the engines of an airliner ?
When conditions are right (temp. humidity, etc.) moisture in the exhaust from the engines condenses, just like water condenses to form clouds. These are called contrails. If you notice they stay around for awhile, just like a cloud, until the wind breaks them upWhat is actually happening when you see two lines of cloud emitting from the engines of an airliner ?
Condensation.