Cloud Base Altitude Calculator
Calculate cloud base height above ground (AGL) and mean sea level (MSL) from surface temperature and dew point using the LCL formula.
Includes fog indicator.
The cloud base altitude is the height at which rising air cools to its dew point temperature and water vapor begins to condense into visible cloud droplets. This height is called the Lifted Condensation Level (LCL) and can be accurately predicted from two surface measurements: temperature and dew point.
The Physics Behind the Formula: Rising air (in a thermal or forced ascent) cools at the Dry Adiabatic Lapse Rate (DALR):
- DALR = 5.4°F per 1,000 ft (3.0°C per 1,000 ft)
The dew point also changes as air rises, but much more slowly:
- Dew point lapse rate = approximately 1°F per 1,000 ft (0.55°C per 1,000 ft)
The air temperature and dew point converge at a rate of about 4.4°F per 1,000 ft. They meet — and clouds form — at the LCL.
LCL Formula (in Fahrenheit): LCL height above ground (ft) = (T − Td) × 227.3
Where T = surface temperature (°F), Td = surface dew point (°F)
LCL Formula (in Celsius): LCL height above ground (ft) ≈ (T_C − Td_C) × 400.9 Or equivalently, LCL (m) ≈ (T_C − Td_C) × 122
Cloud Base Above Sea Level: Cloud base MSL (ft) = LCL height AGL (ft) + Surface Elevation (ft)
The 4°F / 2.2°C Rule — Fog Indicator: When the temperature-dew point spread is less than 4°F (2.2°C), the LCL is at or very near the ground level. This means fog, stratus, or drizzle is likely. Pilots use this rule constantly when evaluating whether conditions are safe for visual flight.
Aviation Significance: The Federal Aviation Administration (FAA) requires a ceiling of at least 1,000 feet AGL for standard VFR (Visual Flight Rules) flight near airports. When the cloud base drops below 1,000 ft AGL, airspace transitions to Instrument Meteorological Conditions (IMC), and pilots need an IFR rating to fly legally. Cloud base forecast accuracy is critically important for aviation planning.
Standard Atmosphere Lapse Rates:
| Lapse Rate | Value | When it Applies |
|---|---|---|
| Dry Adiabatic (DALR) | 3.0°C / 1,000 ft | Unsaturated rising air |
| Saturated Adiabatic (SALR) | 1.5°C / 1,000 ft | Inside clouds (moist air) |
| Standard Environmental | 2.0°C / 1,000 ft | Average atmospheric profile |
Worked Example: Surface temperature: 75°F, Dew point: 55°F, Surface elevation: 5,280 ft (Denver, CO) Spread = 75 − 55 = 20°F Cloud base AGL = 20 × 227.3 = 4,546 ft above ground Cloud base MSL = 4,546 + 5,280 = 9,826 ft above sea level
Thunderstorm Context: The LCL is the base of cumulonimbus (thunderstorm) clouds. The top of an active supercell thunderstorm can reach 40,000–60,000 ft. On hot summer afternoons, the LCL rises as surface heating widens the temperature-dew point spread — which is why afternoon thunderstorms often have higher cloud bases and are more isolated than morning stratus events.