So, have you ever noticed your dog intently sniffing the air, and you're just looking at him thinking, "What the heck are you doing? There isn't anything there." We can't smell what they smell, because dogs actually have almost 50 times more cells that are sensitive to smell than us humans have! Your dog has such a superior sense of smell that he can actually detect odors up to a million times less concentrated than a human can detect.
And a rather interesting tidbit is that a dog's sense of smell is at its greatest when he is moving. Maybe that is why so many dogs love to hang their heads out of car windows...they are literally taking in the scenery with their noses! (Be aware though, that dogs who hang their heads out of car windows are at significant risk of injury from airborne debris.)
It's good to know why they like to stick their heads out there, but that doesn't mean that we have to allow them to do it. The safest place for your dog when traveling is always in a crate. But we'll get to that another time...