☁️ Cloud Base Calculator
Enter the surface temperature and dew point to estimate the height of the cloud base above ground.
Estimated cloud base (AGL)
4,091 ft
1,247 m above ground
Temp–dew point spread
18°F
A pilot's rule of thumb: cumulus cloud base ≈ (temperature − dew point in °F) ÷ 4.4 × 1,000 feet, or about 400 ft per °C of spread. It works best for fair-weather convective (cumulus) cloud on a well-mixed afternoon — it's an estimate, not for stratus, fronts or stable air, and not a substitute for aviation weather reports. 🔒 In your browser.
How the cloud base calculator works
As air rises it cools at about 5.4°F per 1,000 ft while its dew point falls at about 1°F per 1,000 ft, so the two converge at roughly 4.4°F per 1,000 ft. The height where they meet is where cloud forms. The tool applies this rule — cloud base ≈ (temperature − dew point in °F) ÷ 4.4 × 1,000 feet — to estimate the base of fair-weather cumulus.
This is a pilot's rule of thumb, best for convective (cumulus) cloud on a well-mixed afternoon. It doesn't apply to layered stratus, fog, or frontal cloud, and it's an estimate — not a substitute for aviation weather reports (METARs/TAFs). Roughly, 400 ft of base for every 1°C of temperature–dew point spread.
Frequently asked questions
How do I estimate cloud base height?
Use the spread between the surface temperature and dew point: cloud base (feet AGL) ≈ (T − dew point in °F) ÷ 4.4 × 1,000, or about 400 ft per °C of spread. A 77°F temperature and 59°F dew point gives roughly 4,000 ft.
Why does temperature and dew point predict cloud base?
Rising air cools faster than its dew point drops, so they converge with height. Where they meet, the air saturates and cloud forms — that's the cloud base. A bigger spread means a higher base; a small spread means low cloud or fog.
What is the temperature–dew point spread?
The difference between the air temperature and the dew point. A large spread means dry air and high cloud bases; a spread near zero means the air is near saturation, with low cloud, mist or fog likely.
How accurate is this cloud base estimate?
It's a good rule of thumb for fair-weather cumulus on a well-mixed day, typically within a few hundred feet. It doesn't apply to stratus, frontal or orographic cloud, and pilots should use official aviation weather for flight planning.
Does a small spread mean fog?
A temperature–dew point spread near zero at the surface means the air is saturated there, so mist or fog is likely. As the spread widens, any cloud base rises accordingly.