The heat from your exhaust causes some of the moisture in the air to condense
not quite, it is normal, but heat doesnt cause water to condense...
the water comes from burning a hydrocarbon fuel, (gasolene) when the carbon C and hydrogen H in the gasolene reacts with oxygen O2 it forms H2O water and carbonduoxide CO2 (other reactions also takes place but this is the main energy releasing reaction) when the exhaust system is not yet heated by extended running the water (in the form steam) condenses on the inside of the exhaust tubing and eventually drip out through the tailpipe
contensation will occur whenever a surface is colder than the dewpoint of the ambient atmosphere, so centerline, the next time your Cold beer becomes wet on the outside on a hot and humid day, its not because the bottle is leaking, its the vater vapour condensating on the COLD bottle
(no offence intended centerline, but i couldnt help it...