Atmospheric Moisture, Humidity and Stability
What the exam tests
This domain tests moisture measures, dew point, saturation and how the environmental lapse rate determines stability.
Core rules
- Relative humidity compares actual vapour content with saturation at the same temperature; dew point is the temperature to which air must cool to saturate.
- A small temperature-dew-point spread means high humidity and a low condensation level; a large spread means drier air.
- DALR is about 3°C per 1,000 ft; SALR is about 1.8°C per 1,000 ft. Stability depends on the ELR compared with these rates.
- ELR less than SALR: absolutely stable; between SALR and DALR: conditionally unstable; greater than DALR: absolutely unstable.
Worked example
Surface temperature is 25°C and dew point 15°C, a spread of 10°C. Using the planning rule cloud base ≈ spread × 400 ft, convective cloud base is about 4,000 ft AGL. If the ELR is 6.5°C/km and a lifted parcel cools at about 9.8°C/km, the parcel becomes cooler than the environment and sinks: the layer is stable, so expect suppression rather than deep convection.
Traps
Treating relative humidity as an absolute moisture measure; using surface dew point for cloud bases without allowing for mixing; applying DALR to saturated air; calling conditionally unstable air absolutely unstable.