Cloud base calculator
The spread between temperature and dew point shrinks as air rises. Where it reaches zero, cloud forms.
ft
degrees C per 1000 ft
Cloud base above ground
4000ft
- Cloud base above sea level
- 4200 ft
- Cloud base in metres
- 1219
- Temperature and dew point spread
- 10.0 degrees C
- Freezing level
- 11 200 ft
- Temperature at cloud base
- 10.0 degrees C
- Relative humidity
- 53.1 %
Show the working
- Surface temperature = 22
- Surface dew point = 12
- Surface elevation = 200
- Environmental lapse rate = 2
- Temperature and dew point spread = 22 - 12 = 10
- Cloud base above ground = 10 * 400 = 4000
- Cloud base above sea level = 4000 + 200 = 4200
- Cloud base in metres = 4000 * 0.3048 = 1219.2
- freezing_agl = 2 > 0 ? 22 / 2 * 1000 : 0 = 11 000
- Freezing level = 11 000 + 200 = 11 200
- Temperature at cloud base = 22 - 3 * 4000/1000 = 10
- Relative humidity = 100 * (exp(17.625*12/(243.04+12)) / exp(17.625*22/(243.04+22))) = 53.0622
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- cloud base in feet = 400 x (temperature - dew point) in degrees Celsius
- the 400 comes from the difference between the dry adiabatic lapse rate of 3 degrees per 1000 feet and the dew point lapse of 0.5
This gives the convective cloud base for cumulus. Stratus and frontal cloud form by different mechanisms and this does not apply.
This is a planning and study tool. Use certified charts, an approved flight manual and official publications for anything operational.