Cloud base and freezing level
An estimate of the base of convective clouds from the temperature and dew point, by the 400 ft per degree rule and by the lifting condensation level, and of the freezing level.
Planning and education aid. Not for primary navigation. Full disclaimer
Experimental: not yet fully verified. How results are checked
Convective clouds should form about 4,123 ft above the field; the rule of thumb says 4,000 ft.
- Cloud base, lifting condensation level
- Freezing level
Provenance
- Computed by
- aviation.atmosphere.cloud-base 1.0.0, core 0.1.0
- Model
- Rule of thumb: base ≈ (temperature − dew point) × 400 ft per °C. Lifting condensation level: Bolton (1980) eq. 15, T_L = 1/(1/(T_d − 56) + ln(T/T_d)/800) + 56 in kelvins, reached at (T − T_L) ÷ (g/c_p) above the surface, with g/c_p = 9.77 K per km. Freezing level: surface temperature ÷ lapse rate above the field (standard 1.98 °C per 1,000 ft)
- Accuracy
- Estimates for well-mixed surface air forming convective cloud: the LCL is within about 0.1 K in temperature by Bolton's fit; real bases vary with mixing and terrain
- Notes
- 1 shown with the answer
- Cites
- Bolton, D., Monthly Weather Review, The Computation of Equivalent Potential Temperature; Federal Aviation Administration, Aviation Weather Handbook, FAA-H-8083-28B
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: Rule of thumb: base ≈ (temperature − dew point) × 400 ft per °C. Lifting condensation level: Bolton (1980) eq. 15, T_L = 1/(1/(T_d − 56) + ln(T/T_d)/800) + 56 in kelvins, reached at (T − T_L) ÷ (g/c_p) above the surface, with g/c_p = 9.77 K per km. Freezing level: surface temperature ÷ lapse rate above the field (standard 1.98 °C per 1,000 ft)
Accuracy: Estimates for well-mixed surface air forming convective cloud: the LCL is within about 0.1 K in temperature by Bolton's fit; real bases vary with mixing and terrain
Worked example: 25 °C with a 15 °C dew point. Source: add-aviation-suite cloud-base scenario (about 4,000 ft AGL by the rule, the LCL alongside). It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.
You enter
- Dew point
- 15 °C
- Surface temperature
- 25 °C
You get
- Cloud base by the rule
- 4,000 ft
- Cloud base, lifting condensation level
- 4,123 ft
- Freezing level
- 12,626 ft
Review: Not yet independently reviewed by a flight instructor (CFI).
Last verified: 2026-09-22, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.0, core 0.1.0. See this tool in the verification report.
Checked against: 7 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- The Computation of Equivalent Potential Temperature, Bolton, D., Monthly Weather Review, Vol. 108, No. 7. pp. 1046-1053, equation 15 (temperature at the lifting condensation level).
- Aviation Weather Handbook, FAA-H-8083-28B, Federal Aviation Administration, FAA-H-8083-28B. Section 8.4 (altimetry), section 3.4.2.14 and tables 3-17 and 3-18 (flight categories), section 24.4 (METAR and SPECI), section 27.2 (FB winds and temperatures aloft), and section 27.4 (TAF). Read free.
Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.