Drag

I thought I was getting better mileage on humid or foggy days.

 

Vehicle drag is just one of many variables that effects mileage. For instance, how does the humid air effect combustion?

 

I always thought that a humid day would cause more drag,not less with the water vapor in the air. Just think if you could get the graph to slope upwards in a more straight line,the Bonneville Prius would have turned in a higher top speed

 

All I know is in NHRA Drag Racing,the Pro Stocks slow down in high humidity.

 

I'm guessing here and maybe rick can correct me, but I think the Atkinson engine might actually benefit from higher humidity as it adds a little more compression. Of course for the Drag Racers, don't they use more alcohol based fuels? They might separate more under high humidity causing incomplete burns. I really reaching here as this is way outside my line of work.

 

Wait... what's this... "metric" system?

 

I don't think any engine would like high humidity. As for the Atkinson cycle,by a reference in the 1st gen NCF it meantions that it does not generate high output but is highly Thermal Efficient. As for the racers,the Pro Stock class is based on production cars with a max of 500CID and run on high octane racing gas. There is no alcohol in it. But in the fuel classes,dragsters and funny cars,you will find some.

 

actually high humidity would create a loss of power. water simply does not heat up very well. that is good for us because the abundance of water on this planet helps to regulate and moderate the extremes in climate this planet has to offer.

the amount of energy required to heat water is one of the highest of any substance so it would be a detriment.

 

Water vapor is less dense then air? Hmm? I guess I will have to pay more attention to my weather man, :lol:

 

from experiences of my life and this is all very unscientific, but i do remember noticing a large difference in the travel of a hit baseball during the high humid days of spring verses the low humidity days of summer and it seems that summer balls flew farther.

of course it could also have to do with spring being early in the season and perhaps we just hadnt regained our swing yet from the winter layovers.

 

I don't know about you guys, but I don't drive my car in just water vapor. Not only would the ICE not run, but I'd suffocate. While pure water vapor may be less dense than dry air, humid air is a mixture of dry air and water vapor. The more moisture in the air, the more dense it becomes.

And what are these "hp" units doing on a metric chart?

 

not always. for water to vaporize, it has to have a certain amount of kinetic or thermal energy. water turns to vapor when boiling because the molecules vibrate fast enough that they escape into the air. when vibrating fast like that, they push other molecules away from itself and also transfers the vibration to the air around it causing the molecules to spread out and thus lowering the density of the air.

colder air has less vibration so the air molecules can pack closer together thus making the air denser.

you also have to consider the dew point which is uses a different mechanism for controlling how much moisture is in the air when talking about colder air.

 

OK, empirical proof: When weatherman reports pressure dropping, take cover, storm's a brewin. The lower the pressure, the worse the hurricane, and the more moisture it has.

Chemistry proof: There are the same number of molecules in a gas no matter what the chemistry of that molecule, under the same pressure and temperature. Molecularly, a mole of water vapor is lighter than a mole of oxygen, the latter being displaced by the former.

 

Actually, gasses behave kind of unexpectedly. If you take a cubic foot of one gas, and a cubic foot of another gas and mix them together at the same pressure they started at, they won't occupy two cubic feet of space (unless they were the same gas).

Moist air is more dense than dry air.

 

Well, if you were right, I'd agree with you, but that's simply not true.

A mole of gas at a given temperature and pressure will take up a given volume. That volume will vary according to the temperature and pressure, but it will be the same for all gases.

So every mole of water vapor in a given atmospheric volume will displace one mole of air, presuming everything is in thermal equilibrium.

In other words, the more water vapor there is in the air, the less air there is in the air. In a cubic meter of air under fixed conditions, there is a fixed number of particles, and if more of them are H[sub:15049eb32c]2[/sub:15049eb32c]O molecules, then fewer of them can be N[sub:15049eb32c]2[/sub:15049eb32c] or O[sub:15049eb32c]2[/sub:15049eb32c] molecules.

None of this would matter if all the molecules had the same mass, but they don't. Water molecules are only 18 amu. Nitrogen and oxygen molecules are 28 and 32 amu respectively. Given the relative abundance of these two primary gases, and the trace amounts of others, the "average" dry air molecule works out to an effective mass of 29.

Since 18 is less than 29, humid air is less dense than dry air at the same temperature and pressure. Sure there are some adjustments that have to be made to the "ideal" gas behavior, but none of them are anywhere near enough to overcome the fact that water vapor is a whopping 38% lighter than the air it displaces!

This fact is well known to meteorologists, as a rudimentary Google search will reveal. I know it's counterintuitive, and you may find it hard to believe, but that is how things are in real life.

As the ol' man always said, "There's a lot of truth in what actually happens."

 

weight of gas

 

Thanks Gary.

I couldn't believe I had to explain that to my JHS science teacher who not only was my science teacher, but a metorology buff. He forgot that water as a liquid would be more dense than water vapor and thus you can't compare weight of liquid water to air.

But then, maybe he was just challenging me. He was a great teacher.

 

Hey, NiMH. The drag info was interesting. What can you tell us about lift?