Humidity, Dew & Phase Diagrams
How much water the air holds, and the map of solid, liquid and vapour.
Lesson at a glanceShow the key points
- Dew point: temperature at which the SVP equals the vapour pressure present.
- Absolute humidity: vapour mass per volume. RH = vapour present/needed to saturate = vapour pressure/SVP = SVP(dew point)/SVP(air) (24.16–24.18).
- Regnault's hygrometer finds the dew point by evaporating ether until dew dims a silvered tube.
- Phase diagram: liquid–vapour, solid–liquid and solid–vapour curves meeting at the triple point (water 273.16 K, 4.58 mm Hg; CO₂ 216.55 K, 5.11 atm → dry ice sublimes at 1 atm).
- Dew: surfaces cooled below the dew point; fog: the air itself saturated, droplets on dust.
Why grass is wet on a clear morning, why a humid day feels sticky, and why dry ice skips the liquid stage.
Dew point
Air at 20°C containing vapour at 8.94 mm Hg is unsaturated: it would take 17.5 mm Hg to saturate it. Cool it to 10°C, where the SVP is 8.94 mm Hg, and it becomes saturated.
Humidity
Absolute humidity is the mass of water vapour per unit volume of air, usually in g/m³. Relative humidity compares what is present with what would saturate the same air:
Air at 20°C has a vapour pressure of 14.0 mm Hg. Find the relative humidity.
Measuring humidity: Regnault’s hygrometer
Two test tubes, A and B, have silvered lower outer surfaces, each holding a thermometer. Tube A contains ether. Water drained slowly from a vessel draws air through the ether, evaporating it and cooling tube A. When A reaches the dew point, dew clouds its silver surface, while B stays shiny. Note A’s temperature; stop the airflow, let A warm, and note when the mist clears. The average is the dew point; B’s thermometer gives the air temperature. Then
Dew forms on the hygrometer at 15°C when the air is at 40°C. Find the relative humidity (SVP 12.67 and 55.1 mm Hg).
Phase diagrams and the triple point
The SVP curve, plotted against temperature, marks where liquid and vapour coexist. Above it only liquid is stable, below it only vapour. It is also the boiling point as a function of pressure. Similar curves mark where solid and liquid coexist (melting) and where solid and vapour coexist (sublimation). The three curves divide the p–T plane into solid, liquid and vapour regions and meet at the triple point, where all three phases coexist.
- Water: triple point at 4.58 mm Hg and 273.16 K (the fixed point of the kelvin scale).
- Carbon dioxide: 216.55 K and 5.11 atm. At 1 atm it can only be solid or vapour, so solid CO₂ turns straight to gas: dry ice.
Dew and fog
On winter nights, grass, leaves and windowpanes cool by radiation below the air temperature. The air touching them reaches its dew point and droplets of dew condense. If the whole air mass cools enough, droplets form on dust in the air and float as fog.
- 1Dew point: where the SVP equals the vapour pressure present.
- 2Relative humidity = vapour pressure / SVP at air temperature = SVP(dew point)/SVP(air temperature).
- 3Regnault's hygrometer finds the dew point by cooling a silvered tube until dew appears.
- 4Phase diagram: three coexistence curves meeting at the triple point (water 273.16 K, 4.58 mm Hg; CO₂ 216.55 K, 5.11 atm, hence dry ice).
- 5Dew forms on surfaces cooled below the dew point; fog when the air itself is.
Check yourself
Keep exploring
Hand-picked videos, simulations and readings that explain this idea from a different angle.