Sorry but the water/steam in the exhaust is not due to humidity in the air, it's just a byproduct of combustion!
A bit like us, cars take in fuel and oxygen and combine them to make energy, and produce CO2 and water as waste products.
Petrol and diesel are hydrocarbons - Long chains of Carbon atoms with Hydrogens to fill in the blanks:
Carbon atoms have 4 'links' that want to glom onto something; with hydrocarbon chains, 2 are usually joined to another carbon atom on either 'side', and the other 2 are joined to hydrogen atoms (Except the end ones, which have 3 H atoms).
Oxygen has 2 links and also wants to glom onto things, even more strongly than H and C.
It requires energy (i.e. heat) to break the bonds between all the Cs and Hs, but a lot more energy (i.e even more heat!) is released from them making new bonds with Oxygen atoms, so all the H's and C's get split apart, and joined to O's, so you go from lots of C's and even more H's, to lots of 2xH's filling in the O's 2 'links', and 2xO's (With their 2 links each) filling in C's 4 links.
(i.e. CO2 and H2O, water)