How this calculator works
A rising unsaturated parcel cools faster than its dew point falls. Their initial temperature spread estimates the height at which saturation begins, called the lifted condensation level. This tool uses 125 m per kelvin of spread and a dry parcel cooling rate of 9.8 K/km to estimate the cloud-base temperature. Add station elevation for mean-sea-level altitude. The approximation concerns surface-based convective clouds, not every cloud layer or a measured aviation ceiling.
Formula
h_AGL = 125 (T-Td) m, temperature spread in K or C degrees; z_base = z_station+h_AGL; T_base = T-0.0098 h_AGL.
Worked example
Air at 20 C with dew point 10 C gives a base 1250 m above the station. A station elevation of 100 m makes that 1350 m above mean sea level, with parcel temperature about 7.75 C.
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Frequently asked questions
Is the answer above ground or sea level?
Both are shown. The station-relative height is added to the entered station elevation for mean-sea-level altitude.
Does this predict all clouds?
No. Elevated layers, inversions, terrain and mixing can produce clouds unrelated to a surface parcel.
What if temperature equals dew point?
The estimated lifted condensation level is at the station, consistent with saturation or fog.