Cloud Base Calculator
Enter the air temperature and the dew point to estimate the base of cumulus clouds above the ground, by the FAA spread rule and by Espy's lifting condensation level.
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The cloud base calculator estimates how high the base of cumulus clouds sits above the ground from two surface readings: the air temperature and the dew point. Choose Celsius or Fahrenheit, enter both values, and the result gives the cloud base in feet and metres above ground level, the temperature-dew point spread, and a second estimate by Espy’s lifting condensation level for comparison.
The estimate applies to cumulus formed by surface air that is lifted. It does not give the base of stratus, fog or frontal cloud. For those, use the ceiling reported in a METAR or on the ATIS.
How to use the cloud base calculator
- Choose Celsius or Fahrenheit before you type, because the values are read in the unit you pick.
- Enter the surface air temperature.
- Enter the dew point. It cannot be higher than the air temperature.
- Read the estimated cloud base in feet and metres above ground level. This is the FAA rule.
- Read the spread in both units, the Espy lifting condensation level and the difference between the two methods.
- Read the warnings under the result. They say that the figure is for cumulus, point out a base under 1,000 ft, and warn when whole-degree values from a METAR could move the result.
- Open “How it was calculated” to see each step with your numbers, and use the table of common spreads under the result to compare your figure with other spreads.
- To check a reading from a weather report, enter the METAR temperature and dew point in Celsius and compare the answer with the cloud layers in the report.
The preset buttons load a summer afternoon (30 and 18 C), the FAA handbook example (85 and 71 F), a METAR reading of 18/09 and saturated air (5 and 5 C). To use a METAR, copy the two numbers before and after the slash into the fields in Celsius.
How do you calculate cloud base from temperature and dew point?
Subtract the dew point from the temperature to get the spread. Divide the spread in degrees Fahrenheit by 4.4 and multiply by 1,000 to get the approximate cloud base in feet above ground level. With 85 F and 71 F the spread is 14 F, 14 divided by 4.4 is 3.18, and the base is about 3,180 ft.
The 4.4 comes from the lapse rates in the FAA Pilot’s Handbook of Aeronautical Knowledge: rising unsaturated air cools 5.4 F per 1,000 ft while its dew point falls 1 F per 1,000 ft, so the two converge at 4.4 F per 1,000 ft. Clouds often form where they meet. The calculator shows 3,182 ft for the handbook example because it does not round 3.18.
What is the cloud base formula in Celsius?
Multiply the spread in degrees Celsius by about 409 to get feet above ground level, or by 125 to get metres. For 30 C and 18 C the spread is 12 C, so the FAA rule gives 12 x 409.1 = 4,909 ft and Espy gives 12 x 125 = 1,500 m, or 4,921 ft.
The 409 is a conversion of the handbook’s rate: 4.4 F is 2.44 C, and 1,000 ft divided by 2.444 is 409 ft per degree. The 125 m is Espy’s figure: dry air cools about 9.8 C per km and the dew point falls about 1.8 C per km, so they converge about 8 C per km, which is 125 m per degree. Both rest on the same lapse rates, so they always differ by about 0.25 percent.
| Spread (C) | FAA rule (ft AGL) | Espy (ft AGL) |
|---|---|---|
| 1 | 409 | 410 |
| 2 | 818 | 820 |
| 5 | 2,045 | 2,051 |
| 10 | 4,091 | 4,101 |
| 20 | 8,182 | 8,202 |
How accurate is the cloud base estimate?
It is only as good as the two readings and the cloud type. A METAR gives temperature and dew point in whole degrees Celsius, so each value can be off by half a degree and the spread by up to 1 C. That moves the result by up to about 409 ft. The METAR 18/09 gives a spread of 9 C, or 3,682 ft, but readings of 18.4 and 8.6 would give 9.8 C and 4,009 ft.
The figure is height above ground level. Add the field elevation if you need height above sea level.
Is the cloud base the same as the ceiling?
No. The calculator estimates the base of cumulus formed by rising surface air. The ceiling is the lowest layer reported as broken or overcast, or the vertical visibility into an obscuration, and it is an observation of what is overhead. Stratus, fog and frontal cloud follow other processes, so take their height from the reported ceiling.
Sources
FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C, 2023), chapter 12, Weather Theory, section Temperature/Dew Point Relationship: the 4.4 F convergence rate, the 85 F and 71 F example and the statement on fog and low cloud when temperature and dew point are close. Espy, Essays on Meteorology No. IV, Journal of the Franklin Institute 22(4), 1836, as described in the Wikipedia article Lifted condensation level. The Celsius factor of 409 ft per degree is a conversion made for this page. Checked October 2026.
Questions from readers
What does a small temperature-dew point spread mean?
The air is close to saturation. The FAA handbook says that when temperature and dew point are close together, fog, low clouds and precipitation are likely to form. A spread of 2 C gives a base of 818 ft, and a spread of 0 means fog or cloud on the ground.
Does the calculator need altitude or pressure?
No. It uses only temperature and dew point and returns a height above ground level.
Why are two cloud base figures shown?
The first is the FAA handbook rule, the second is Espy’s lifting condensation level. They agree within about 0.25 percent, so the second is a check on the first rather than a different answer.
Can I use the temperature and dew point from a METAR?
Yes. They appear as two numbers separated by a slash, for example 18/09, in degrees Celsius, with M for minus. The METAR decoder reads the whole report and links here with the two values filled in.
Will the answer match the reported ceiling?
Not always. The calculator estimates cumulus base, and the reported ceiling can come from another layer. When the two differ, the reported ceiling is the one to use.
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