Guide

What is specific humidity?

Specific humidity is the mass of water vapour per kilogram of total air — the vapour counted in the denominator as well as the numerator. It is nearly identical to the mixing ratio, which divides by dry air only; the two differ by under 2% in ordinary conditions, and both are conserved when air is heated or lifted.

The distinction, in one line

Both figures count the same grams of water. They disagree about what to divide by.

Mixing ratio (w) — grams of water per kilogram of dry air. The water is not in the denominator.
Specific humidity (q) — grams of water per kilogram of total air. The water is in the denominator too.

Since the water is always a small minority of the air, dividing by the total rather than by the dry part makes the denominator slightly larger and the answer slightly smaller. That is the whole difference.

q ≈ w / (1 + w/1000) Both in g/kg. Exact enough for anything short of research work.

How small is small

The gap between the two, across the full range of surface conditions.
Conditions	Mixing ratio	Specific humidity	Difference
Cold, dry	2 g/kg	2.00 g/kg	0.2%
Mild	8 g/kg	7.94 g/kg	0.8%
Warm, humid	15 g/kg	14.78 g/kg	1.5%
Tropical	25 g/kg	24.39 g/kg	2.4%

For context, a good humidity sensor is accurate to about ±2%. So in almost every practical situation, the choice between these two figures is smaller than the error in the measurement feeding them.

When the difference does matter

Three cases, all of them technical:

Energy and mass budgets in atmospheric models, where quantities are tracked per unit mass of air and mixing the two conventions introduces a systematic bias.
Very humid air, where the 2%+ gap starts to matter against tighter tolerances.
Comparing datasets. Climate reanalyses generally publish specific humidity; forecast soundings and thermodynamic diagrams generally use the mixing ratio. Reading one as the other produces a small consistent error that is easy to miss.

Both are conserved

The property that makes them useful is shared: because both divide by a mass rather than a volume, neither changes when a parcel of air is heated, cooled or carried to a different altitude. Only adding or removing water moves them.

That is what separates them from absolute humidity, which is per cubic metre and therefore drifts as air expands, and from relative humidity, which moves with every degree. The mixing ratio guide has the full comparison table.

Getting the number

The mixing ratio calculator shows both figures for any temperature, humidity and pressure, so the size of the gap is visible rather than something you have to take on trust.

Common questions

What is the difference between specific humidity and mixing ratio?

The denominator. Specific humidity divides the water by the mass of all the air including the vapour; the mixing ratio divides it by the mass of the dry air only. Specific humidity is therefore always slightly the smaller of the two.

How much do they actually differ?

Under 2% in almost all real conditions. At a mixing ratio of 11 g/kg the specific humidity is about 10.9 g/kg. Even in extremely humid tropical air at 25 g/kg the gap is only about 2.4%.

What is the formula for specific humidity?

q = 622 × e / (p − 0.378e) in g/kg, or more simply q ≈ w / (1 + w/1000) where w is the mixing ratio in g/kg. The second form is exact enough for any practical purpose.

Is specific humidity conserved?

Yes, in the same way the mixing ratio is. Both divide by a mass rather than a volume, so heating, cooling and lifting a parcel of air leave them unchanged. Only adding or removing water alters either.

Which one should I use?

For anything outside atmospheric physics, whichever your source gives you — the difference is smaller than your measurement error. Climate datasets tend to publish specific humidity; forecasting and thermodynamic charts tend to use the mixing ratio.

Sources

WMO Guide to Instruments and Methods of Observation (WMO-No. 8)

Published 31 July 2026