Why a jet plane emits a white smoke in high flights?

Jet aircraft appear to emit white smoke when flying at high altitude because they are compressing the air and turning any water vapour in them to steam. There is also a lot of condensation into the hot trails left behind by the engines. Over time, as with any cloud, these trails disappear. At low altitude occasionally from a jet fighter or stunt aircraft you can often see white lines tracing the path of the wingtip across a sharp banking turn. This is because the air is being pressurized under the wings and depressurized above the wings and the pressurized air is harder to see through and appears white.