Rule 5b. Never use single quotation marks in sentences like the previous three. Incorrect: I had a visit from my 'friend' the tax man. The single quotation marks in the above sentence are intended to send a message to the reader that friend is being used in a special way: in this case, sarcastically. Avoid this invalid usage.
Why do jets leave a white trail in the sky? Jets leave white trails, or contrails, in their wakes for the same reason you can sometimes see your breath. The hot, humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapor pressure and temperature than the exhaust gas.
This led to the invention of jets, which are used by both the military and commercial airlines. A jet is an airplane propelled by jet engines. Jets fly much faster than propeller-powered aircraft and at higher altitudes, some as high as 10,000 to 15,000 meters (about 33,000 to 49,000 feet).