What exactly is the white we see coming off an airplane in the sky. Sometimes in the summer you can lookup and see many streaks in the sky. They are much longer and last longer in the summer than winter. Please tell me what causes this and why they last longer in the summer.

The white streaks you see coming off airplanes are called contrails, which is short for condensation trail. Contrails are actually clouds made by the airplanes. There are two types of contrails.

Most contrails people see are caused by a jet's exhaust. The exhaust has some water vapor in it which mixes with the air when it leaves the jet's engine. Sometimes this mixture has too much water vapor for the air to hold and it condenses into a cloud. This is the most common kind of contrail and it lasts the longest.

The other kind of contrails are form by an airplane's wing. Air moves over the wings really fast and cools because of the low pressure. The cool air can't hold as much water so some of it has to condense. This kind of contrail isn't as long-lived as the other kind. The picture to the left is an example of this kind of contrail.

Contrails last longer when there is a greater amount of water in the air. During the summer, the air generally has more water vapor in it than during the winter. You might notice that it's usually more humid in the summer. Because it's more humid in the summer, contrails take longer to evaporate. That's why they last longer than in the winter.

Submitted by Stephanie (Ohio, USA)
January 16, 1998

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