Frances Burton, age 9, of New Bedford, Mass., for her question:

WHY DO WE SEE CONTRAILS IN THE SKY?

Contrails, or more properly condensation trails, are the filmy white plumes we see following many planes flying at great altitudes. The trail is composed of a thin cloud of water vapor or ice crystals and it is created by a drop in pressure and temperature behind a fast moving plane.

Contrails are also called vapor trails. They are present in the sky when the air at the plane's altitude is nearly saturated with water vapor so that any pressure and/or temperature drop will reach the dew point and form a fog or cloud.

The trails usually disappear in a short time, as the cloud evaporates again.

Contrails are definitely not smoke or exhaust fumes and they have nothing to do with the operation of the plane.