air density/humidity

[john davidson 1]

Received a copy of Model Aircraft Aerodynamics by Martin Simons from my daughter for Christmas and sat down after lunch with it. Soon had me puzzled, on page 29 he states "the moisture content of air, its humidity , also affects its density. Dry air is denser than moist air"   

Can this be correct, moist air is surely heavier so must be denser/heavier?

[Alan Hilton]

It is correct if counter intuitive .The molar mass of water is 18 the molar mass of oxygen is 32 and nitrogen is 28 so the water in the atmosphere dilutes its molar mass which reduces its density 

Alan

[J D 8]

       My old 1990 Land Rover 200tdi proves this. On a cold dry day it it has more oomph.

[john davidson 1]

Thanks both, certainly is counter intuitive

[Ace]

JD8 - Arr but water increases 1600 times in volume to become steam, so on a damp misty day the water vapor ingested by your IC engine creates a greater internal cylinder pressure as the explosion turns the water to steam - hence water injected performance engines.

Back to the OP topic. 

[Don Fry]

Arrrrr, but you are talking about mist, water droplets floating in air. Alan is talking about air with evaporated water in it, is done it’s expansion when it evaporated.

 

[Phil Green]

2 hours ago, john davidson 1 said:

"the moisture content of air, its humidity , also affects its density. Dry air is denser than moist air"   

 

Good job too, otherwise we'd be walking around waist-deep in clouds...  😀

 

Happy New Year everyone!

Edited January 1, 2023 by Phil Green

[Paul De Tourtoulon]

Is 'Scotch mist' any better than British fog ?.

[john davidson 1]

Two kinds of 'Scotch Mist' one appears indoors halfway  down the bottle, the other just affects sight outdoors

[Robin Colbourne]

Piston engined aircraft typically used 50/50 water methanol.  On the Vought Corsair, it gave a 20mph increase in true air speed, which is pretty impressive considering that drag goes up as a square of the speed.  Horsepower rose from 2,100hp to 2,450hp.  The extra power came from the increased supercharger boost permitted by the cooling effect of the water/methanol.

The benefit of water turning to steam, (increased cylinder pressure) is largely negated by the latent heat energy taken to turn water from liquid to gas (2 260 000 Joules/kilogram).

On a gas turbine, cooling the air passing through it increases the mass airflow and thrust. Water injection in gas turbines

[Engine Doctor]

13 hours ago, Paul De Tourtoulon said:

Is 'Scotch mist' any better than British fog ?.

That's best taken with little drop of spring water or ice 😉

[Peter Jenkins]

On 01/01/2023 at 17:11, john davidson 1 said:

Received a copy of Model Aircraft Aerodynamics by Martin Simons from my daughter for Christmas and sat down after lunch with it. Soon had me puzzled, on page 29 he states "the moisture content of air, its humidity , also affects its density. Dry air is denser than moist air"   

Can this be correct, moist air is surely heavier so must be denser/heavier?

John

 

If moist air were heavier then we wouldn't have any clouds.  Fog is really cloud on the ground due to local conditions.  I remember the first RAF station I was at sat at 300 ft above sea level and was only a mile from the sea.  We used to get force 9 fog there but if you were down on the nearby beach it looked like low cloud in a high wind.

[Don Fry]

Clouds float cos the are warmer and hence less dense than surrounding air. The water droplets are heavy, as water is, but are so small that they are kept up by convection currents (thermals). The opaque nature of clouds also make them better capturers of sunlight, keeping them warm, and up. Over simplified, but that’s how they work.

[J D 8]

55 minutes ago, Peter Jenkins said:

 

 

If moist air were heavier then we wouldn't have any clouds.  Fog is really cloud on the ground due to local conditions.  I remember the first RAF station I was at sat at 300 ft above sea level and was only a mile from the sea.  We used to get force 9 fog there but if you were down on the nearby beach it looked like low cloud in a high wind.

  Would that have been RAF Brawdy then?

[Martin Harris - Moderator]

19 hours ago, Robin Colbourne said:

The benefit of water turning to steam, (increased cylinder pressure) is largely negated by the latent heat energy taken to turn water from liquid to gas (2 260 000 Joules/kilogram).
 

I vaguely recall a not very convincing attempt to demonstrate the effect of the release of latent heat from rain producing lift near cloud base during a lead and follow cross country gliding course. 
 

Far more memorable was successfully thermalling in a snowstorm near the Black Mountains in South Wales on a cold day in late autumn. Quite surreal doing 360 degree turns with snow continuously streaming towards me from straight in front.  I’m not sure of the mechanism producing the lift but it was easy to gain and maintain height. 

[Peter Jenkins]

1 hour ago, J D 8 said:

  Would that have been RAF Brawdy then?

Yes. 

[kevin b]

On 01/01/2023 at 19:31, Phil Green said:

 

Good job too, otherwise we'd be walking around waist-deep in clouds...  😀

 

Where I live we regularly are !   🙃

[J D 8]

56 minutes ago, Peter Jenkins said:

Yes. 

 I live local and was present at every Brawdy air day when it was an RAF station and a good number of FAA open days before that.  Cheers, John.