Cloud Base Calculator
This cloud base calculator uses the FAA rule of thumb: divide the temperature–dew point spread by 4.4 °F and multiply by 1,000 feet. At 85 °F and a dew point of 71 °F, the cloud base is about 3,182 feet above ground.
Example: Surface temperature 85 · Surface dew point 71 · Temperature unit: °F · Elevation unit: ft → Cloud base: Cloud base about 3,182 ft above ground (970 m). Source: FAA, Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 12 Weather Theory, p. 12-14 “Temperature/Dew Point Relationship”: convergence 4.4 °F per 1,000 ft, example 85 °F/71 °F → 3,180 ft AGL (read 10.10.2026). Updated: .
How it is calculated
Cloud base (ft AGL) = (temperature − dew point) ÷ 4.4 °F × 1,000. The FAA Pilot’s Handbook of Aeronautical Knowledge explains why: unsaturated rising air cools at 5.4 °F per 1,000 feet, while its dew point drops by 1 °F per 1,000 feet. The two converge at 4.4 °F per 1,000 feet, and clouds often form where they meet.
The handbook’s example: 85 °F and a dew point of 71 °F give a spread of 14 °F; 14 ÷ 4.4 = 3.18, so the cloud base is about 3,180 feet above ground. The calculator keeps the decimals and shows 3,182 feet.
Cloud base by spread
| Spread | Cloud base (ft AGL) |
|---|---|
| 5 °F | 1,136 |
| 10 °F | 2,273 |
| 15 °F | 3,409 |
| 20 °F | 4,545 |
In Celsius the rule is about 409 feet, or 125 meters, per degree of spread (1 °C = 1.8 °F).
When the rule works
- According to the FAA, it applies to clouds that form as moist, unstable air rises.
- Use temperature and dew point measured at the same place and time.
- Add the field elevation to compare with altitudes above sea level.
Find the dew point with the dew point calculator, and estimate air temperature higher up with the temperature at altitude calculator.
Frequently asked questions
How do you calculate cloud base?
Subtract the dew point from the temperature (in °F), divide by 4.4 and multiply by 1,000. The result is the approximate cloud base in feet above ground.
Where does the 4.4 come from?
Rising unsaturated air cools 5.4 °F per 1,000 feet and its dew point falls 1 °F per 1,000 feet, so they get 4.4 °F closer every 1,000 feet (FAA).
What is the formula in Celsius?
Spread in °C × 1.8 ÷ 4.4 × 1,000 ft, which is about 409 feet or 125 meters per degree Celsius.
Is the result above ground or sea level?
Above ground (AGL). Add your field or station elevation for the height above mean sea level.
What if temperature and dew point are equal?
A spread of 0 means the air near the ground is already saturated; the calculator then shows 0 feet.
Sources and legal basis
- FAA, Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 12 Weather Theory, p. 12-14 “Temperature/Dew Point Relationship”: convergence 4.4 °F per 1,000 ft, example 85 °F/71 °F → 3,180 ft AGL (read 10.10.2026)
- NIST SP 811, Appendix B.8: 1 ft = 0.3048 m (read 10.10.2026)
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