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 (age 27, Ohio, USA)
January 16, 1998
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