Cloud Base Altitude Calculator
Calculate the altitude of cloud base (Lifted Condensation Level) from surface temperature and dew point.
The cloud base altitude — also called the Lifted Condensation Level (LCL) — is the height at which rising air cools enough to reach its dew point and water vapor begins to condense into cloud droplets. Knowing the cloud base altitude is important for aviation, weather forecasting, storm prediction, and outdoor activities like hiking and paragliding.
The Formula The LCL is estimated from the difference between the surface temperature (T) and the dew point temperature (Td):
LCL (meters) ≈ (T − Td) × 125
LCL (feet) ≈ (T − Td) × 227.3
Where T and Td are in degrees Celsius for the metric formula. The spread (T − Td) tells you how dry or moist the air is at the surface.
Why the Spread Matters The temperature-dew point spread is the key driver:
- Spread < 2°C (3.6°F): Air is nearly saturated. Clouds or fog will form at or very near the surface. Expect overcast skies, mist, or fog.
- Spread 2–5°C (3.6–9°F): Low cloud base, typically below 625 meters (2,050 feet). Stratus or stratocumulus clouds likely.
- Spread 5–15°C (9–27°F): Moderate cloud base. Mid-level clouds such as altocumulus or altostratus may form.
- Spread > 15°C (27°F): Air is quite dry. Cloud formation is unlikely at this level. Any clouds that do form would be high cirrus if upper-atmosphere humidity is present.
Cloud Classification by Altitude Once you have the LCL, you can classify the cloud type:
- 0–2,000 m (0–6,500 ft): Low clouds — stratus, stratocumulus, nimbostratus, fog
- 2,000–6,000 m (6,500–20,000 ft): Mid-level clouds — altocumulus, altostratus
- Above 6,000 m (20,000 ft): High clouds — cirrus, cirrostratus, cirrocumulus
Aviation Note Pilots use the LCL to estimate ceiling height — the lowest cloud layer that significantly reduces visibility. A cloud base below 1,000 feet (305 m) often limits visual flight rules (VFR) operations.
Worked Example — Metric Surface temperature: 28°C, Dew point: 15°C Spread = 28 − 15 = 13°C LCL = 13 × 125 = 1,625 meters → Low-to-mid level cloud base, likely altocumulus
Worked Example — Imperial Surface temperature: 82°F, Dew point: 59°F Spread = 82 − 59 = 23°F LCL = 23 × 227.3 = 5,228 feet → Mid-level cloud base