Which is lighter? Moist or dry air? Why?

[arg]

Can anyone give me a good answer for this one?

 

[insdav3]

because water is heavier than air, so when you have a moist head of hair, you head is heavier. Dried hair doesn't have the added weight.

E= mc2

 

[minendo]

Dry air is lighter since moist air is a combination of water vapor and dry air.

Air

 

[arg]

huh?

 

[Atrail]

If mosit air was lighter.
Rain would never fall.

 

[arg]

Does anyone have a scientific answer? Cause my Hydrology prof said moist air is lighter. I got 5 out of 5 wrong on the exam question.

 

[jahawkin]

Moist air is less dense because the molecular weight of water vapor is less than that of other air constituents, such as nitrogen and oxygen.

 

[Capn]

density of water vapor is less than that of regular air.

Off the top of my head the density of air is about 1.22 kg/m^3, according to this chart you can see water vapor density is a good deal below that.

 

[arg]

>Moist air is less dense because the molecular weight of water vapor is less than that of other air constituents, such as nitrogen

b-i-n-g-o

 

[Atrail]

NM

 

[Capn]

">Moist air is less dense because the molecular weight of water vapor is less than that of other air constituents, such as nitrogen"

I think that justification only works if you assume water vapor is an ideal gas, since we're dealing of temps neat its saturation point it does not behave like an ideal gas afaik.

 

[dullard]

Do you know the ideal gas law? (Sometimes it is called the perfect gas law.) The ideal gas law states that a given number of gas molecules have a density that only depends only temperature and pressure in a simple manner.

Air behaves very closely like an ideal gas at room temperature and atmospheric pressure. In fact its components also behave like ideal gases (oxygen, nitrogen, water vapor, carbon dioxide, etc...) That means that 1 liter of oxygen has the same number of molecules as 1 liter of nitrogen and 1 liter of water vapor - at the same temperature and pressure. However each water molecule (18 atomic units) is lighter than the oxygen (32 atomic units) and nitrogen molecules (28 atomic units). Thus 1 liter of water vapor weighs less than 1 liter of oxygen or 1 liter of nitrogen.

Thus a given volume of dry air weighs more than the same volume of wet air if they are at the same temperature and pressure.

 

[arg]

I think that justification only works if you assume water vapor is an ideal gas

Are you saying that moist air isn't lighter in all cases?

 

[jahawkin]

<<
I think that justification only works if you assume water vapor is an ideal gas, since we're dealing of temps neat its saturation point it does not behave like an ideal gas afaik. >>



Water vapor being near saturation does nothing to it density. For most practical purposes, atmospheric gases are treated as ideal.

 

[dullard]

Assuming an ideal gas, dry air is about 61% heavier than an equivalent amount of water vapor.

There are many cases where the ideal gas law fails. Extreme temperatures and extreme pressures are examples.

The ideal gas law also fails when the molecules have attractions toward one another. One oxygen molecule really isn't attracted to another oxygen molecule since they are symmetric and thus have a neutral charge distribution. (Assuming the molecules aren't forced together by the extremes listed above and also ignoring the small Van der Walls forces). Similarly a neutral oxygen molecule isn't attracted to neutral nitrogen or carbon dioxide molecules.

Water is different, it has a dipole (one end is slighly negatively charged and one end is slightly postively charged). Thus if you have lots of water molecules, they will attract one another - leading to a higher density. So when you add more and more water vapor to the air it will become more dense. The key is will it ever reach the 60% more weight needed to surpass the weight of the dry air? Under normal temp and pressure ranges, this 60% increase in density doesn't occur.

 

[LiLRiceBoi]

Doenst temperature have something to do w/ it??

I dont know the answer, just bringing up another idea...

 

[fatalbert]

moist air is less dense because the water molecules are larger, therefore lighter

 

[CurtisBilly]

Capn is ABSOLUTELY correct. water vapor does not behave like an ideal gas anywhere in the vicinity of the liquid-vapor dome (applicable to most practical and engineering applications). This becomes extremely important when analyzing state changes.

Just for chits and giggles:
the specific volume of pure saturated water vapor @ 1bar 100 degrees C is (from steam tables):
v=1.696 m^3/kg

the specific volume of air @ 1bar 100 degrees C is (from Pv=RuT/molecular weight):
v=1.0704 m^3/kg

Therfore, at this state (and it actually works out at other states as well) that the density of water is less than that of air, so water is "lighter"

 

[telstar1]

Rule of thumb...If you get a question like this on a short quiz, take the less-obvious answer.

 

[silverpig]

The answer has been given, but here's a practical observation of the theory.


Weathergirl says "There's a large low pressure system moving into your area" means... RAIN


Weathergirl says "There's a ridge of high pressure sitting right on us" means... SUN

 

[SmackdownHotel]

Just look at how a barometer works. The barometer level drops when it rains or it's about to rain (less atmospheric pressure), and it rises when the weather is good (increasing atmospheric pressure). That should answer your question.

 

[GooberPHX420]

Hi my name is arg and I ride the short bus to school.

 

[Triumph]

From my environmental engineering book: "Ironically, although we refer to hot, humid air as 'heavy,' it is in fact less dense than cool, dry air." They go on to prove this through mole fractions, but I don't feel like summarizing it.