Is it harmful to drive with windows down and air conditioning on in the hot dry weather of the great American west.
Nope.
In my tC I often drive on hot days with the sliding glass roof open, the windows down a few inches, and the AC on and blowing. I get the sun on my forehead, the wind through my hair, and keep fairly cool since the cool air has a tendency to want to drop so it doesn’t exit out the roof. The engine just has a bit nore load on it and I’m adding a bit of cooler air to the outside through the roof…offset by the fact that the condensor is adding exactly the same amount of heat to the air that the roof is adding cold. The condensor is where the cabin’s heat went to be dissipated.
The great and powerful Al Gore might say we’re wasting gas, but my conscience is clear.
It wouldn’t be harmful but it wouldn’t get a cool as it would with the windows closed.
But if you’re comfortable, that’s the whole point, isn’t it?
Whatever floats your boat…err… car.
It will cost you a little fuel and likely it will be a little warmer in your car than it would be with the windows up.
Personally I would not do it on the highway as I don’t like the extra noise.