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.
It is at the bumping points where the jet streams begin. Because of their strong winds, jet streams play an important role in the economy of the aviation industry. When an airplane flies into a jet stream for any period of time, extra fuel must be used. Not only is the fuel expensive, but it takes up room where either passengers or cargo could be. Just the opposite occurs when an airplane flies with a jet stream at its back. Fuel and money are saved because the wind pushes the plane along. If you have ever flown in an airplane, you probably have experienced something called turbulence.