Why are there high flying white jets leaving long expanding white smoke trails? What is their purpose?

The trail left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with air of low vapor pressure and low temp.

Tags: trailair 
Wednesday, February 01 2012
Source: http://cimss.ssec.wisc.edu/wxwise/class/contrail.html