Hygrometer
The definition
Take a kilogram of dry air. Ask how many grams of water vapour are mixed into it. That number is the mixing ratio, written w and expressed in g/kg.
The subtraction in the denominator is the important bit: p − e is the pressure contributed by the dry air alone, with the water's share taken out. That is what makes this a ratio to the dry air rather than to all of it.
Why conservation matters
Every other humidity figure fails at least one of these tests. The mixing ratio passes all of them.
| Process | Relative humidity | Absolute humidity | Dew point | Mixing ratio |
|---|---|---|---|---|
| Heating the air | ✗ | ✗ | ✓ | ✓ |
| Cooling (above the dew point) | ✗ | ✗ | ✓ | ✓ |
| Lifting / decompression | ✗ | ✗ | ✗ | ✓ |
| Adding or removing water | ✗ | ✗ | ✗ | ✗ |
That third row is the one that separates the mixing ratio from the dew point. Both survive an ordinary day's warming, but lift a parcel of air up a mountain and its pressure drops — which changes the vapour pressure, and so shifts the dew point, while leaving the mixing ratio alone.
For anyone tracking air across a weather map or up through the atmosphere, that is decisive. It is why the saturation mixing ratio lines are printed on every skew-T diagram.
Seeing it hold still
Take air with a 60°F dew point and warm it through a day:
| Temperature | Relative humidity | Absolute humidity | Mixing ratio |
|---|---|---|---|
| 60°F | 100% | 13.2 g/m³ | 11.02 g/kg |
| 70°F | 71% | 13.0 g/m³ | 11.02 g/kg |
| 80°F | 51% | 12.7 g/m³ | 11.02 g/kg |
| 90°F | 37% | 12.5 g/m³ | 11.02 g/kg |
Two columns move for no physical reason. One does not.
The practical version
For everyday purposes — is it muggy, will this dry, will that window fog — you do not need the mixing ratio. The dew point survives ordinary heating and cooling too, and it has the enormous advantage of also being a threshold you can act on.
Where the mixing ratio earns its place is anywhere pressure changes: aviation, mountain weather, convection, and any calculation that follows a parcel of air rather than sitting at one station.
The calculator works it out at any pressure you like, and shows specific humidity beside it for comparison.