High humidity will make the AC work harder and compressor cycle more causing more heat to be put in to the radiator from the condenser. So in a round about way yes dry air cools a bit better.

 

I really like physics. Dry air cools humans better because we sweat, the sweat evaporates easier in dry air, and we feel cooler. Cars don't sweat (at least not on purpose), so dry air cools cars better because of higher density. Water vapor (humidity) is lighter than air, that's why clouds are usually overhead. Dry denser air has more mass and can absorb more heat and create better cooling. Driving through fog might provide the best cooling because at 100% RH, the water vapor condenses into tiny droplets of liquid. The liquid evaporates from the hot surfaces, creating an extra cooling effect.

My Uncle Ted taught me years ago the best way to cool down a radiator is to wipe it with a wet rag and let it steam off. Water changing state to steam takes away a lot of heat energy. I hope you like physics too!

 

there used to be a company that sold a "radiator mister" for this exact reason - not sure they really worked well but it was due to cheap parts no because the science was wrong

 

Anyone seen a random spike from 180f with it being like 65f outside then shooting up to 212 out of nowhere with normal driving, Then starting back up and going back to normal? Happened to me today for the first time and thought it was kinda weird. Was thinking maybe the thermostat was stuck for a moment

 

This is excellent point. Almost impossible for a vehicle to overheat when its raining because of the cooling effect of the liquid water on the radiator and the boil off of the water on the surface of the radiator. Some Subaru's actually came with a water sprayer that would spray water on the intercooler at high boost loads (very short term effect of course).

 

Could be stuck thermostat or could be a sign of the temp sensor flaking out. I keep one of these in the console to double check against the temp sensor because sometimes they do go bad and no need to panic if the truck is actually running cool. Check the manifold outlet pipe, oil pan, and cylinder heads/valve covers. The whole engine is really a giant block of aluminum thermal mass and aluminum conducts so well it is almost impossible for the the coolant to be hot and the rest of not to be.

 

So now this is getting interesting. On a recent trip to Moab (I didn't have the D2) hanging out with friends that drive VW Vanagon Syncros, this topic came up. On the long climbs & descents, it was common to have the temps rise due to low air flow. So we were curious if adding a system to spray water on the radiator might work as quick way to add temporary cooling. The Winder towing dude would dump water on the radiator of his jeep. Doing a quick Google, the pros say it would take more water than would be practical. A simple spray wouldn't be enough. I did a quick garden hose to the radiator test in the driveway with my UG saying about 186. Nothing happened. It actually went up a few degrees while idling. Unfortunate because building a water spray system would be a ton of fun!

 

In your driveway, the temp would go up because the bulk water from a garden hose would actually inhibit airflow. The goal is not to simply gather heat from the radiator surface into the water, but rather to add a ton of very small drops of water (vapor) and then to have that water evaporate. The heat energy required to evaporate the water (phase change from liquid to gas) is orders of magnitude more than the amount of energy required to raise the temperature of the bulk water just sitting on the radiator's surface. That's why the systems others have mentioned above used misters or sprayers to apply a thin film that is easily vaporized.

 

Excellent! My mister system project has hope! I will retest with mist vs water and see what happens. My truck was originally a Ft. Collins area truck btw.

 

Thank you! I had to go back and look at the historical weather report, but it was mid-70s for most of the drive along the route.

The tires are close to stock. I put 16" steel wheels on and attempted to make sure the overall diameter of the setup remained close to what it originally had. The tires are General Grabber AT2s. I would go check to confirm their size, but I unfortunately have to keep the truck in a storage unit when I'm not using it so the catalytic converters don't get stolen...again.

I'd be curious to see the balance of humidity vs altitude. We have nice dry air, but at ~5k feet there's some power loss and I'd imagine a decrease in cooling ability due to density.