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How It Works
As air rises from the surface it cools at the dry adiabatic lapse rate (~5.4°F per 1,000 ft) while the dewpoint drops more slowly (~1°F per 1,000 ft). The net convergence rate is about 4.4°F per 1,000 ft, so dividing the temperature-dewpoint spread by 4.4 and multiplying by 1,000 gives the approximate cloud base altitude.
Example Problem
Surface temperature is 80°F and dewpoint is 58°F. What is the estimated cloud base?
- Temperature spread: 80 − 58 = 22°F
- Cloud base: (22 / 4.4) × 1,000 = 5,000 ft AGL
Frequently Asked Questions
How accurate is the cloud base formula?
The formula gives a reasonable estimate for cumulus clouds under convective conditions. It is less accurate for stratus layers, frontal systems, or terrain-influenced weather. Pilots treat it as a preflight planning tool, not a substitute for official weather reports.
Can I use Celsius instead of Fahrenheit?
Yes. In Celsius the constant changes to 2.5 instead of 4.4: Cloud Base (ft) = (T − Td) / 2.5 × 1,000. Both methods yield the same altitude.
What is a safe VFR cloud ceiling for flying?
In the U.S., basic VFR requires at least 1,000 ft of ceiling in controlled airspace and clear of clouds in Class G. A ceiling below 1,000 ft is considered IFR conditions. Always check official METARs and TAFs before flying.
Related Calculators
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- Lightning Distance Calculator — estimate how far away a thunderstorm is.
- Dry Adiabatic Lapse Rate Calculator — calculate temperature change with altitude.
- Heat Index Calculator — find apparent temperature from air temperature and humidity.
- Length Converter — convert between feet, meters, miles, and other distance units.