Cloud base

Convective cloud base is the height where rising air cools to its dew point and condenses.

Unsaturated air cools at the dry-adiabatic rate, 3 °C/1000 ft (TC AIM RPA 3.2.22.8), while its dew point falls ~0.5 °C/1000 ft, so the spread closes ~2.5 °C/1000 ft. Cloud forms when the spread reaches zero: cloud base (ft AMSL) = elevation + (T − Td) ÷ 2.5 × 1000.

Note: the ~0.5 °C/1000 ft dew-point fall and the 2.5 net closure are a rule of thumb, not in the CARs/TC AIM.

Unit trap: the spread must be in °C. A temperature and dew point given in °F convert to a °C spread by ÷1.8 (a difference, so no 32° offset); the elevation may be in metres (×3.28 to feet).

Trap: divide the spread by 2.5, not 3. The dew point falls too, so the gap closes slower than the temperature alone.

Practice