I have never read anything about a MPG drop due to high humidity and or rain. I notice a 2 mpg drop on foggy mornings and a 3 to 4 MPG drop if raining on my 39.5 mile trip to work. Up to 6 MPG drop if heavy rain. Has anyone else notice this?
The rain effect is well documented and frequently mentioned. I haven't noticed the humidity effect. With your long commute you should be able to see such things better than most of us.
Any moisture on the road surface increases rolling resistance. Mitigate somewhat by ridge riding or by following in the tracks of the car ahead (at a safe distance).
I would be more concerned about the rain not the high humidity. Either way, you may end up using your AC and that may kill your mpg just a tad.
Hi Bushface, High non-condensing humidity (not fog) overcast is actually good for mileage. Traffic flows smoother because there are none of those anoying sun-in-the-face situations, and primarily because humidity reduces air density. As H2O is lighter than O2 and N2. So, for the same barometric pressure, there is the same number of molecules, and the higher the humidity, less of those molecules are O2 or N2. As soon as there is condensation every thing changes for the worse however. And in rain, especially with silcon/silcone (hydroedges/nokians etc) rain/low rolling resistance tires the mileage gets noticably worse. First off, your pushing the rain out of the way with your car. Second, the tires are pumping the surface water aside. And lastly the hydroscopic tires will hold onto to the water as the tire rolls further up before the hydroscopic forces cannot hold the water any more.
It could be that some of the "humidity effect" noted by the OP is actually reduced temperature. Fog coincides with lower temperature--when the temp. falls to the dew point fog developes. The sensitivity of the Prius to temperature is well known. A second aspect is that fog and rain often require some dehumidification of the interior window surfaces. These are energy intensive portions of climate control operation and will put a serious hit on mileage, all else being equal. When I first responded I did not address the dehumidification because it is something I would normally account for, but at looking back at the original post it occurs to me that it is probably not something that was factored in.
I think the fog cools the motor and radiator better. Just as in cold days, a foggy day would make the engine run cooler (unless you blocked off the radiator) and hence the decrease in mileage. I feel that rain effects the mileage due to the rolling resistance and moist air that is better at cooling the engine. Just my 2 cents.
Moisture added to the combustion returns greater power (water expanding). So when the air is moist and the roads are no more than just damp, MPG climbs. I see that routinely. It's something you'd never notice without having the constant feedback of the Multi-Display available. .
High humidity also reduces horsepower. Temperature and dew point can change your effective altitude or what we call Density Altitude (DA) and can effectively simulate driving at higher altitudes where there is less oxygen in the air and thus you get less HP for every CFM of air gobble down by your engine. I've never tracked this for MPG but it does have a big effect on 1/4mile times which is why racers love it nice cool days with very low humidity. So here in Sacramento we can have a DA of +2000' meaning even though our racetrack is only a couple hundred feet above sea level, we are racing in what feels like a 2000' elevation with regards to oxygen density. On good racing days (usually the Fall), we might experience -3000' DA. So it is like we are racing 3000' below sea level which would make the air MUCH more dense. It's almost like running a small shot of nitrous compared to a +3000' DA. Here is a link to a Density Altitude calculator and explains why you lose HP with high humidity. I would think this would affect fuel milage too but it gets a bit tricky and it is too early for me to try to think it through... <wipes the sleep from his eyes and heads off looking for coffee>
Thanks for all the info. I generaly try not to run the defroster for my mileage. If I do, I manually turn it on on off as needed. In sarcasm, I beleive that the NC DOT paves the roads to hold rain water instead of draining it off. There are some new roads that I feel I driving in a stream or shallow pond. One other thing, I wish that when you turn of the defrost, it would go back to the original setting instead of auto. Again thanks for the replies.
With fog (liquid water) the actual density of the fluid you are pushing thru increases, so drag increases.Same story with actual rain-you have to push air aside, and actual liquid water(tires and car body have to displace liquid water, so on the ground, some just hanging/floating in the air while it slowly falls or evaporates). I don't know how much weight/mass of water drops is in a cubic foot of fog, but air is pretty light- 29 grams in 22.4 liters-I think 30 liters is about a cubic foot. Charlie
