Air Mixture Humidity

Air mixture – dry air vs water vapour

Box shows a gas mixture: N₂, O₂, Ar and H₂O as moving balls. Water vapour fraction changes at fixed temperature and pressure – you can see how density changes.

Water fraction in air

Water molecules

Component counts

Total molecules

N₂ molecules

O₂ molecules

Ar molecules

H₂O molecules

Mixture & density (standard p, T)

Mixture molar mass M

Air density ρ

ρ / ρ(dry)

N₂ (≈ 78%, 28 g/mol)

O₂ (≈ 21%, 32 g/mol)

Ar (≈ 1%, 40 g/mol)

H₂O (water vapour, 18 g/mol)

At the same temperature and pressure, replacing heavier N₂/O₂/Ar molecules with lighter H₂O molecules lowers the mean molar mass M and therefore the density ρ = p·M / (R·T). Humid air is slightly less dense than dry air.