Humidity, Condensation, Dew Point and Atmospheric Stability
This domain links moisture measures to cloud base and to stability criteria. The exam asks for a dew point calculation, a cloud base from a temperature-dew point spread, or a stability classification from a stated lapse rate.
Measuring water vapour
- Vapour pressure (hPa) is the partial pressure exerted by water vapour; saturation vapour pressure rises steeply with temperature.
- Relative humidity = actual vapour pressure ÷ saturation vapour pressure × 100. It falls when air is warmed and rises when air is cooled with unchanged moisture.
- Dew point is the temperature to which air must be cooled at constant pressure to saturate. It is a direct measure of moisture content. The dew point depression is temperature minus dew point; small depression means high humidity.
- Mixing ratio (g/kg) is conserved during vertical motion and is used for following moisture aloft.
Condensation
- Requires saturation (or slight supersaturation) plus condensation nuclei.
- Cooling routes: ascent and adiabatic expansion, radiation, contact with a cold surface, mixing of two moist air masses.
- Fog forms when the cooling reaches the dew point at the surface; a spread of 1°C or less with light wind and nuclei gives fog rather than cloud.
- Rule of thumb: cloud base (ft AGL) ≈ 400 × the spread in °C (125 m per °C).
Stability criteria
- Compare the ELR with the adiabatic rates.
- ELR less than SALR: absolutely stable; stratiform cloud, steady drizzle or rain, haze, smooth air.
- ELR between SALR and DALR: conditionally unstable - the normal case; showers possible if the air is lifted to its level of free convection.
- ELR greater than DALR: absolutely unstable (superadiabatic), found only just above strongly heated ground.
- Stability is altered by surface heating and cooling, advection of warm or cold air, lifting (destabilising) or subsidence (warming aloft, stabilising), and moisture (more moisture lowers the SALR and favours instability).
Worked example
Surface temperature 24°C, dew point 16°C: spread 8°C, so the lifting condensation level is about 3,200 ft. A parcel rising dry-adiabatically reaches 24 − 3 × 3.2 = 14.4°C at that height. If the measured ELR is 2.5°C/1,000 ft, it lies between the SALR and DALR, so the atmosphere is conditionally unstable: cumulus forming at 3,200 ft and showers possible with further heating.
Traps that cost marks
- Giving a dew point above the temperature, or assuming relative humidity is constant with temperature.
- Applying 400 ft per °C above the freezing level where the constant is unreliable.
- Calling air stable when the ELR only exceeds the SALR.