Cloud base
Convective cloud base: height where rising air cools to its dew point and condenses.
Unsaturated parcel rates:
| Parcel | Quantity | Change | Rate per 1000 ft | Ref |
|---|---|---|---|---|
| Rising | Temperature | Falls | dry-adiabatic rate | - |
| Rising | Dew point (rule of thumb) | Falls | ~0.5 °C | - |
| Rising | Spread (T − Td) | Closes | ~2.5 °C | - |
| Descending (compresses) | Temperature | Warms | dry-adiabatic rate | FAA AWH 12.2 |
| Descending (compresses) | Dew point | Rises | ~0.5 °C | FAA AWH 12.2 |
| Descending (compresses) | Spread | Widens | - | FAA AWH 12.2 |
- Cloud forms when spread reaches zero.
cloud base (ft AMSL) = elevation + (T − Td) ÷ 2.5 × 1000
Unit trap:
- Spread must be in °C.
- °F temperature and dew point: spread ÷1.8 for °C (difference, no 32° offset).
- Elevation in metres: ×3.28 for feet.
Trap: divide spread by 2.5, not 3; dew point falls too, so spread closes slower than temperature alone.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)