Originally Posted by cyccommute

Yes, lots of bridges ice without being over water but it's not "car exhaust"



Yes, some water from car exhaust will condense out but it's a minor component. Let's say you have a city that is 30 km square (19 miles). Now, go up 60 m (200 ft), that a volume of 5.4 x10^10 cubic meters. Let's assume that the air is at-18°C (0°F). It can carry 1.2 g per cubic meter or the total amount of water available is 64,800,000 kg assuming complete saturation of the air.

Now let's stuff 44,000 cars into that volume. Let's make each one burn 1 gallon of gasoline. That 167,000 kg of water that those 44,000 cars put out. The cars contribute 0.25% of the total water in the air. It's, almost literally, a drop in the bucket.

Now let's look at the ice that could form over that 30 km area. Let's assume 3 mm so that the ice is actually completely covering the craggy surface of the pavement. I'm also going to assume that the entire area is covered with 3mm of ice including all horizontal surfaces of the buildings. The total volume of a 900,000,000 square m x 3mm object is 270,000,000 mL. Water weighs in at just around 1g per mL which means that you have 270,000 kg of ice covering the area. You are about 100,000 kg short of the amount of water you need if every single car combusts one gallon of gas. And you'd have to wring every drop out of the exhaust.