The clouds that jets form on certain days are called condensation trails or contrails. Some days, the contrails will form thin lines that cross the entire sky. Other days they will be much shorter, and on some days there will be none at all.
A contrail forms because one of the components of jet engine exhaust is water. Jet fuel is made of carbon and hydrogen (see this Question of the Day for details on fuel, and see How Gas Turbine Engines Work for details on jet engines). When jet fuel burns with oxygen, most of the exhaust consists of CO2 (carbon dioxide) and H2O (water). 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. Cold air can hold a lot less moisture than warm air, so in the winter, when the moisture in your breath hits the cold air, the moisture condenses into a visible cloud.
The same thing happens when a jet engine "exhales." If the temperature, winds and humidity in the upper atmosphere are right, long, white contrails form when the moisture in the exhaust condenses.
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