A contrail or a vapor trail. See the related link for confirmation.
If you are asking about contrails it is dependent on the temperatures aloft. At colder temperatures the water vapor in the exhaust condenses and causes the contrails you see. The blue angels and other aerobatic-performance aircraft inject a special oil "smoke oil" into their exhaust in order to leave the smoke trail you see at airshows. But the "smoke trail" you see behind civilian and commercial aircraft isn't smoke at all, it is water vapor formed as described above.
DUST!!!!!
heroin trail are dangerous. they are drugs, heroin trail causes diseases & and they are really bad for your health
When water vapor is cooled it turns into liquid.
Comets commonly develop two tails, a vapor trail, which we can see, and a gas/dust trail, which we may or may not be able to see. The tail develops when the heat of the sun causes the ices in the cometary nucleus to vaporize off. The tail always points directly away from the sun.
The sickness that they had during the trail was mountain fever
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Heat causes the ice to melt into water and the water evaporates into vapor.
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Condensation
A comet usually has two tails, though only one - the vapor trail - is commonly seen from earth (the other is a gas trail). It is made of vapor blown off from the comet by the solar wind - it is mostly water vapor, but contains many other molecular substances, too - ammonia, cyanide, methane, etc.