Which air is lighter dry air or humid air?

Answer 1

Avogadro's law tells us that a given number of gas molecules (e.g., a mole) always occupies the same amount of space at a given temperature, regardless of the chemical composition of the gas. Thus, the density of any gas depends on the molecular mass of the gas molecules. Dry air is a mixture of 80% nitrogen molecules (N2 - molecular mass = 28) and 19% oxygen molecules (O2 - molecular mass = 32); it has an average molecular mass of 28.1g per mole. The molecular mass of water (H2O) is 18. Thus, while liquid water is much more dense than air, gaseous water is less dense than dry air. Therefore, humid air, which contains a mixture of air and gaseous water, is lighter (less dense) than dry air.

Answer 2

The amount of water vapor in the air effects it's density. Water vapor is a light gas compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor is increased, the amount of Oxygen and Nitrogen decreases per unit volume leading to density decreases because mass is decreasing.

Answer 3

Dry air is heavier than humid air. Newton discovered this in 1770 and Avogadro put forward the exact reasons for it. Equal volumes of different gases at the same pressure and temp will contain the same number of molecules. Hence water vapour molecules must displace air molecules in humid air. Water vapour has a molecular weight of 18 while air is approx 29 so the weight for a given volume ( ie density) is less. Don't confuse humid air, which is water vapour and invisible, with cloud or fog which is visible water droplets ( ie liquid water) suspended in air and settles in valleys only because it is colder than the surrounding air.

Answer 4

Humid air is lighter than dry air.

Any gas has a weight or mass that is a combination of its component gasses. Water, as H2O, weighs less in its gaseous form than air does normally.

At normal temperatures and pressures, such as people normally experience, one only needs to compare the masses of the various components of air to see how much each contributes to the mass of air.

Pure dry air, all over the Earth, consists of approximately 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, with the last few tenths of a percent being made up of other gasses that do not add much to the mass of air.

One really just needs to consider the masses of gaseous nitrogen, oxygen, argon and carbon dioxide in comparison with water. One can look these up in the Periodic Table.

Pure oxygen (two atoms of oxygen) has a relative weight of about 32.

Pure nitrogen (two atoms of nitrogen) has a relative weight of 28.

Pure carbon dioxide (12 for carbon and 2x16 for oxygen) is 44, much heavier than either oxygen or nitrogen.

Argon is an atom with Atomic Mass of almost 40.

Water, being 2 hydrogen atoms and one oxygen atom weighs in at 18. So, a water molecule actually weighs less than any other major component of air.

In dry air, to get the average mass of an air molecule, we take the appropriate proportions of the above masses. That comes out to be about 29 which is quite a lot more than the 18 for a water molecule.

When water vapor is mixed in with dry air, the relatively lighter water molecules displace some of the heaver nitrogen, oxygen, argon and carbon dioxide molecules making the humid air lighter. And the more humid the air is the more lightweight molecules in the mix and the lighter it is.

Caveats:

The above analysis assumes the gasses behave nearly as ideal gasses.

So, all that is said above assumes a reasonable temperature such as a temperature above zero degrees centigrade. Similarly, high pressure and temperatures near the boiling point of water require more careful discussion to assure accuracy.

One should note that if the concentration of water molecules gets very high, more than a few percent, then the water stops being a gas and forms water droplets. That results in rain.

In any case, for conditions that humans usually experience, humid air at a given temperature and pressure is less dense that dray air at the same temperature and pressure.

It should be noted too that weather forecasting that connects barometric pressure with a prediction of rain is indirectly connecting the lowering of temperature with and increase in air mass. It starts to get more complicated, but the point is that increasing air pressure is not the result of more water vapor in the air.

Answer 5

Yes, this statement is true that dry air is more heavier than moist air. I surely agree with this statement. It is because-

The amount of water vapor in the air affects it's density. Water vapour particles are lighter as compared to diatomic Oxygen and diatomic Nitrogen particles. Thus, when amount of water vapor is increased, the amount of Oxygen and Nitrogen decreases per unit volume leading to decrease in density because mass is decreasing.

Read more: Why_is_moist_air_lighter_than_dry_air

Answer 6

Dry air is lighter than humid air because water vapor molecules are heavier than the molecules that make up dry air. Thus, humid air is denser and therefore heavier than dry air.

Answer 7

Moist air is heavier than dry air, because of the water.