Relative humidity, dewpoint, saturation & phase changes
How close air sits to its vapour capacity drives fog, cloud and precipitation.
| Term | Meaning | Cite |
|---|---|---|
| Saturation | Max water vapour a parcel holds at its current temperature and pressure | AWH 6.3 |
| Saturated | Holds all it can | AWH 6.3 |
| Unsaturated | Can hold more | AWH 6.3 |
| Relative humidity | Vapour in parcel / vapour it could hold at that temperature and pressure, as percent. 100% = saturated | AWH 6.4 |
| Dewpoint | Temperature air must cool to, at constant pressure and constant vapour pressure, for vapour to condense. Below 0°C: frost point | AWH 6.5 |
| Temperature-dewpoint spread (dewpoint depression) | Temperature minus dewpoint. Narrowing: RH rises. Zero: saturated, RH 100% | AWH 6.6 |
- Capacity depends on temperature, not content. Cooling parcel, no vapour added or removed: RH still rises, colder air holds less (AWH 6.4).
- Spread flags fog, not rain. Small or narrowing surface spread warns of fog. Says little about precipitation, which needs saturation through a thick layer aloft (AWH 6.6).
Four ways air reaches saturation (PHAK ch.12):
- Warm air moving over a cold surface
- Cold and warm air mixing
- Overnight cooling by contact with cooler ground
- Air lifted or forced upward
Phase changes of water:
| From | To | Process | Cite |
|---|---|---|---|
| liquid | vapour | evaporation | AWH 6.2.1 |
| vapour | liquid | condensation, at the dewpoint | AWH 6.2.5, 6.5 |
| ice | vapour | sublimation | AWH 6.2.3 |
| vapour | ice | deposition | AWH 6.2.4 |
Trap: dewpoint is the temperature that saturates the air, not how humid it feels.
References
- AWH 6.3-6.6
- PHAK ch.12 Methods by Which Air Reaches the Saturation Point