What Cloud Base Height represents
The 125 m per degree Celsius rule approximates a lifted condensation level for a well-mixed surface parcel. It converts dew-point spread into height above the parcel's starting ground level.
Cloud Base Height begins with surface air temperature, surface dew point. Mark each value as observed, remotely sensed, modeled, assumed, or derived, and preserve its valid time and spatial support.
Formula and convention
The working relationship is Base ≈ 125 × (T − T_d). Cloud Base Height uses only the displayed entries and does not retrieve imagery, station observations, ephemerides, forecasts, or runway systems.
Keep percentages as percentages at entry and convert them to fractions only where the formula shows it. For Cloud Base Height, retain angle direction, optical threshold, height reference, and distance unit with the answer.
Checked numerical example
At 20°C with a 12°C dew point, the estimated base is exactly 1,000 m AGL.
Reset restores this Cloud Base Height example. Repeat it independently with the stated constants and rounding before replacing demonstration values with observations or scenarios.
Collecting compatible inputs
Use co-located, contemporaneous screen temperature and dew point representative of the parcel. Do not combine a hilltop temperature with a valley dew point or substitute relative humidity without first deriving dew point.
Record site, coordinates, elevation, timestamp, time zone, path direction, wavelength or sensor band, target definition, cloud-layer method, solar-time convention, averaging period, and quality flags when relevant to Cloud Base Height.
Interpreting Estimated cloud base height
A larger spread gives a higher estimated base; saturation at the surface gives zero. The result is metres above ground level, not elevation above mean sea level or an observed ceiling.
Compare Cloud Base Height outputs only after aligning reference height, sky footprint, path, illumination, contrast convention, and temporal average. Similar numbers can describe different optical or geometric quantities.
Boundary and monotonic checks
Dew point must not exceed air temperature in this unsaturated-parcel approximation.
For Cloud Base Height, change one input at a time and predict whether the answer should rise, fall, or remain fixed. Unexpected behavior can expose percent-fraction, degree-radian, metre-kilometre, or layer-order mistakes.
Continue the Cloud Base Height workflow with the related Weighted Cloud Cover Calculator, keeping the same observation support and conventions.
Where the model stops
The shortcut assumes dry-adiabatic cooling and typical dew-point change during lifting. Inversions, entrainment, terrain, fronts, precipitation, elevated cloud layers, and parcel selection can dominate the error.
Cloud Base Height is transparent educational arithmetic, not a cloud observation, weather forecast, solar-resource assessment, flight rule, runway visual range report, instrument calibration, or operational safety authority.
Continue the Cloud Base Height workflow with the related Cloud Oktas from Percentage Calculator, keeping the same observation support and conventions.
Uncertainty and sensitivity
Vary the least certain Cloud Base Height input over a credible range and retain the resulting spread in estimated cloud base height. Display precision cannot overcome observer classification, retrieval uncertainty, spatial variability, or an empirical coefficient.
A Cloud Base Height sensitivity range is not automatically a probability interval. It omits correlations, model-form error, sampling mismatch, sub-grid cloud structure, nonuniform visibility paths, and uncertainty outside the entered variables.
Visibility and contrast conventions
Visibility from Cloud Base Height depends on the target contrast threshold and extinction units. Meteorological optical range fixes a two-percent contrast convention; a general contrast range or runway screening estimate may use another threshold.
Do not equate Cloud Base Height optical distance with prevailing visibility, slant range, night visual range, or official RVR without the required observation and processing conventions.
Solar geometry and radiation conventions
Solar elevation is measured above the geometric horizon, while azimuth here is clockwise from true north. Cloud Base Height must retain whether radiation is normal to the rays or incident on a horizontal surface.
Civil time is not solar hour angle. Longitude, time zone, equation of time, atmospheric refraction, shading horizon, and surface tilt belong in Cloud Base Height only when the displayed model explicitly includes them.
Keeping an auditable record
Save raw Cloud Base Height inputs, conversions, constants, coefficient sources, formula version, unrounded output, rounded result, and quality flags. A reviewer should reproduce the number without guessing a cover, angle, contrast, or height convention.
If a source value or assumption changes, create a dated Cloud Base Height revision. Preserve the earlier output and label whether the change is a correction, a new observation, or a separate scenario.
Frequent clouds and visibility errors
Typical Cloud Base Height errors include mixing oktas and tenths, confusing AGL with MSL, adding overlapping layers, treating degrees as radians, using sea-level-reduced visibility, or applying a two-percent constant to a five-percent threshold.
Reject impossible Cloud Base Height combinations rather than forcing an answer. Keep a clear zero distinct from missing or obscured sky, preserve signs on hour angle, and verify every inverse distance and logarithm denominator before downstream use.
Cloud and visibility questions
How can I verify Cloud Base Height?
Repeat Base ≈ 125 × (T − T_d) with the recorded conversions, then test the checked example and a physical boundary.
Why could another source disagree?
Another source may use different cloud overlap, optical threshold, solar convention, coefficient, sensor path, height reference, or rounding than Cloud Base Height uses.
Is this an operational visibility or aviation value?
No. Cloud Base Height does not replace official weather observations, RVR systems, flight procedures, warnings, or qualified judgment.
How should the result be rounded?
Keep full precision inside Cloud Base Height, then round no more finely than the least certain measurement or model assumption supports.
When should I recalculate?
Recalculate Cloud Base Height when its site, time, path, layer, sky footprint, illumination, source observation, or model assumption changes.
What does Cloud Base Height report?
Cloud Base Height reports estimated cloud base height from the displayed inputs and formula.
Can forecast or scenario inputs be used?
Yes. Label the Cloud Base Height output as a scenario; the page does not fetch or validate a forecast.