Air Density - Humid air is less dense?

[Phil1980's]

Sorry if this is dumb and I should really read my book...I know that humid air equals less dense air.
But when I'm out say in florida, the air feels thick and humid...so I used to think that the air is thus thicker...or am i right in saying that it's the water in the air that is making it seem thick when in fact the air is thin...but then the air would be carrying the moisture...

[gfunc]

This density versus humidity thing is due to the relative molecular weights of air and water. If you dig out your school chemistry notes and the periodic table, you'll find that water molecules (H2O) have a molecular weight of 18 units. I think O is 16 and H is 1. If you do the same thing with air, which is 70 ish percent Nitrogen and 20 ish percent it works out to be around 29 units.

So if you have a box full of pure air (no water vapour) and then you displace some air in the box with water vapour (maintaining the pressure and temperature) you've moved out molecules of 29 units each out of the box and put in molecules of 18 and decreased the total mass of gas in the box. Since density is mass divided by volume the density has decreased from the no water vapour case as we have fixed the volume (using our box).

I'm not sure what makes it 'feel' thicker when humid, it might be the loss of some molecular oxygen (although this is unlikely since water vapour makes up only a few % of air by mass even when it is really sticky). More likely it is the reduction of cooling by sweating, which makes anything physical much more difficult. maybe someone with a medical background could answer this bit!

I hope this helps.....

Gareth.

[Beefy_EMA]

Temperature and pressure affects density.

Low pressure, less dense.
High temperature, less dense.
High temperature can hold more moisture. Therefore more moisture, less dense.

But pressure has a greater affect on density, therefore when pressure decreases and temperature decreases, the air gets less dense.

I am sure someone else can explain it better.

[gfunc]

Yes, that's correct temperature and pressure have by far the largest effect on density via the ideal gas law (p=(ro)*Rgas*T). For all practical purposes these dominate the density.

It is true that warm air can hold more mositure at saturation (via the Clausius-Clapeyron relationship). The relationship is non-linear, but even at 40C saturated air can hold about 50 g per kg at sea level, which is about 5% of its total mass.

Couple o' links for those interested:

Clausius Clapeyron relationship.
http://en.wikipedia.org/wiki/Clausius-Clapeyron_relation#Meteorology
Calculate the saturation mixing ratio:
http://www.srh.noaa.gov/elp/wxcalc/mixingratio.shtml


Cheers,

Gareth.