While driving through the Pilbara on a warmish but dry day the other week I got to thinking about cooling. I was wondering if your engine would gain a cooling effect from a higher humidity day if the temperature was the same as a lower humidity day?
Eg: 30 degrees and 95% humidity compared to 30degrees and 35% humidity.
Would it be the same as us sweating to help cool down?

Should be no great/direct effect... as in this case the moisture in the ambient air is entering at the SAME temperature as the air itself AND is already in the GAS state.
"Water-injection" for intake cooling is quite different, as there is a significant physical cooling effect as the fine water droplets (liquid) are vaporised in the manifold. The latent heat of vaporisation of water is involved in this situation. [Let's see the hands go up claiming humid air is as good as water-injection!]

Humid air (under the same conditions) contains slightly less O2 than exactly the same air dry (and is slightly less dense), as the moisture displaces the other gases. So very humid air may cause slightly richer running -- which may cause very slightly cooler running -- but not directly due to the presence of the water! (But if you had the choice, go for the DRY air, as it contains more oxidant - allowing more efficient combustion, or more power.)

The specific heat of dry vs. moist air will be slightly different too, but this probably won't have a great effect on cooling -- if you're thinking about the cooling effectiveness of the air as it passes over the fins...(?)

A much greater affect on running temp might be caused by daily variations in density of the air (or air pressure); which will directly affect mixture ratio, in a carbied car...

Yes it does.
It is why evaporative coolers (swamp boxes) don't work in the tropics but you see them everywhere in Broken Hill.

There is a dry bulb and wet bulb temperature

Sydney is 33 DB and 23 WB
Darwin is 33 DB and 28 WB

But Darwin is way hotter ?

Dry and hot (like here) is bloody tough on motors, but wet and hot (like your joint) will give much reduced temps at the same load. The evaporative effect mentioned makes a big difference.

The effect on YOU of sitting in a Beetle for hours in the wet on the highway might be different though.........

yep, absolutely, the higher the humidity the larger the heat capacity of the air, ie can absorb more heat.

Even if the thermometer temps are the same, the moist air will cool an engine better.

This is the opposite though to how our bodies feel it because we rely on evaporative cooling, phase change cooling basically.
Heat is lost from the liquid when water turns from liquid to gas. So when the air is already saturated with water, evaporation happens at much slower rates thus reducing the ability to remove heat making us hot and sweaty in humid air.

Also for anyone who has done tuning(EFI wih knock sensor logging) or has boosted engine car notice the benefits from high humidity. You can increase your timing advance and/or boost on moist day due to moist intake air resisting detonation and the cooling effects on the charge.

water injection anyone?

I got to thinking once to. Was driving along in my Valiant once. They had large air vents each side under the dash for fresh air. I thought about if the air was cool even on a humid day. So I reached down over my passengers slim long stockinged legs toward the vent on her side.......never did get to find out the answer.

Thanks guys, Tweety I'm guessing it was hot and sticky?
Modnrod, I'm in a Kombi at the mo but it is no hotter or cooler than my beetle when the sun and the flies are out.
Have done my last dash up north, last leg is across the Nullarbor and trying to find a house north of Byron, so fingers crossed I won't see too many more 100% humidity days