The water is generally an invisible vapor. When you exhale, your breath contains a great deal of invisible water vapor as well. You may have noticed that on certain days in the winter your breath will form a cloud of condensation when you exhale. In the summer, however, you don't see your breath. In the winter the moisture in your breath hits the cold air.
Cold air can hold a lot less moisture than warm air, so the moisure condenses into a visible cloud.
If the temperature, winds and humidity in the upper atmosphere are right, long white contrails form when the moisture in the exhaust condenses. Inc. All rights reserved.