Atmospheric Moisture, Humidity and Phase Changes
Humidity measures
Understand the vocabulary precisely. Absolute humidity is mass of vapour per unit volume. Mixing ratio and specific humidity are mass of vapour per mass of air and are conserved when air rises or sinks, which is why they are preferred for tracking air masses. Relative humidity compares actual vapour content with the saturation value at that temperature. The key trap: relative humidity changes with temperature even when the moisture content does not. Cooling air raises relative humidity toward 100%; warming lowers it.
Dew point is the temperature to which air must be cooled at constant pressure and moisture to become saturated. Dew point depression (temperature minus dew point) is a quick humidity gauge: small depression means high humidity, fog risk and a low cloud base. A useful question rule is that the cumuliform cloud base in feet is about 400 times the temperature-dew point spread in °C.
Phase changes and latent heat
Water releases latent heat when condensing or freezing and absorbs it when evaporating or melting. Sublimation is ice directly to vapour, deposition the reverse. Latent heat release in rising saturated air slows its cooling: the saturated adiabatic lapse rate (about 1.5°C per 1000 ft) is less than the dry adiabatic rate (about 3°C per 1000 ft). This is the engine of cumulonimbus development and of the stability discussion in the next section.
Worked example
Surface temperature 20°C, dew point 14°C. Spread 6°C, so convective cloud base is roughly 6 × 400 = 2400 ft above ground. If the spread were 2°C, expect bases near 800 ft and a strong fog or low-cloud risk overnight.