Humidity Ratio from Vapour Pressure

This is the fundamental psychrometric variable, and if you only learn one humidity quantity properly, make it this one. The humidity ratio $W$ is kilograms of water vapour per kilogram of DRY air — not per kilogram of the mixture. That denominator is deliberate and it is the whole reason the quantity is useful: run air across a cooling coil and condense water out of it, and the dry-air mass is unchanged. It is the one thing in the airstream that survives the process untouched, so it makes a stable bookkeeping unit. Divide by mixture mass instead and your denominator moves under you every time moisture is added or removed.

The 0.62198 is not a fudge factor. It is $M_{water}/M_{air} = 18.015/28.964$, the ratio of molar masses. It appears because Dalton's law counts MOLECULES — partial pressures are proportional to mole fractions — while the humidity ratio counts KILOGRAMS. That ratio is exactly the exchange rate between the two counts, and it is the same number that makes moist air lighter than dry air. Note the denominator too: $p - p_v$, not $p$, because it is the dry air's own partial pressure that $W$ is a ratio to.

Here is the property that makes $W$ worth reasoning with, and that relative humidity does not have. Heat air without adding a drop of moisture and $W$ does not move. Its relative humidity falls sharply, because the saturation pressure in the denominator of RH has climbed, but the actual water content is identical. This is the complete explanation of dry winter indoor air, and it is worth working through: outdoor air at −10 °C and 80 % RH is nearly saturated and feels raw, but the saturation pressure at −10 °C is only about 260 Pa, so it is carrying roughly 1.6 g/kg. Bring that same air inside and heat it to 21 °C. It still carries 1.6 g/kg — nothing was removed — but saturation at 21 °C is about 2 490 Pa, so it now reads near 11 % RH. Your skin cracks and your furniture shrinks not because the heating "dried" the air but because the air was never carrying much water to begin with.

North American practice quotes $W$ in grains per pound, which converts cleanly: there are 7 000 grains in a pound, so one gr/lb is exactly one seventh of a g/kg. The 9.28 g/kg of the standard 24 °C / 50 % RH chart point is 65 gr/lb. And the 0.68 rule for latent load multiplies a CHANGE in this quantity — $\Delta W$ in grains — which is why grains, not percentages, are what a coil selection is actually written in.

One more thing the humidity ratio makes obvious that relative humidity hides: mixing. Blend two airstreams and the resulting humidity ratio is the mass-weighted average of the two — genuinely linear, so a mixing box carrying 25 % outdoor air at 2 g/kg into return air at 10 g/kg lands at 8 g/kg, and you can do it in your head. Relative humidity does no such thing; averaging two RH readings gives a number that means nothing at all. The same holds for enthalpy, which is why every mixed-air calculation in this shard is written in $W$ and $h$ rather than in percentages.