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 coming off jet aircraft are called contrails, which is short for condensation trail. Contrails are similar to clouds and are created by the aircraft. Contrails are formed two different ways.

The most common type of contrail is caused by a jet's exhaust. The exhaust adds water vapor to the air and may cause the air to become super-saturated. Because there is so much water some of it can condense and form the cloud you can see from the ground.

The other type of contrail is called an "aerodynamic contrail" and is caused by an aircraft's wings instead of its engine. When the air moves past a wing it creates a low pressure area (which is how planes fly). The lower pressure causes the air to cool and possibly reach saturation. This type of contrail (seen in the picture) doesn't last very long; it gets disrupted by the aircraft's wake.

Exhaust contrails are the ones you can see from the ground. They will last until the water in the clouds evaporates. During the summer the air is usually more humid than in winter. This means that evaporation will be slower and contrails will last longer.

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

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