Atmospheric Moisture, Humidity and Stability
Humidity measures
Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at that temperature. Dew point is the temperature to which air must cool to become saturated. A small temperature–dew point spread means high relative humidity, a low cloud base and a greater risk of fog and carburettor icing. Mixing ratio and specific humidity express the mass of water vapour per mass of air and do not change with temperature unless condensation occurs.
Lapse rates
Three rates must be distinguished. The dry adiabatic lapse rate is about 3°C per 1000 ft. The saturated adiabatic lapse rate is smaller, roughly 1.5–2°C per 1000 ft, because latent heat is released as vapour condenses. The environmental lapse rate is the actual measured profile.
Stability
Air is absolutely stable when the environmental lapse rate is less than the saturated adiabatic rate, absolutely unstable when it exceeds the dry adiabatic rate, and conditionally unstable in between, which covers most of the real atmosphere. Stability is increased by cooling from below (night radiation, cold ground), by warming aloft, or by subsidence. It is decreased by surface heating, by cooling aloft, and by forced ascent over hills or fronts.
Traps
Do not confuse dew point with wet-bulb temperature; wet-bulb lies between temperature and dew point. Remember that relative humidity changes with temperature even if the moisture content is constant, so RH often peaks near dawn. Conditional instability is the normal state, not an exceptional one.
Rule of thumb
Cloud base in feet ≈ (temperature − dew point in °C) × 400. A 5°C spread suggests a base near 2000 ft.