Look at a summer sky and you will notice something neat: the flat bottoms of cumulus clouds often sit at nearly the same height. That height is the cloud base, and it is no accident. It marks the level where rising air has cooled enough for its water vapour to condense into visible droplets. Below that level the air is clear, and above it the cloud begins. This tool estimates that height from two ground readings: temperature and dew point.
The physics is simple. A bubble of warm air rises from sun-heated ground and cools as it expands, by about 9.8 degrees per kilometre while it stays unsaturated. Its dew point falls too, but more slowly, by about 1.8 degrees per kilometre. The two values meet after a climb that depends on their starting gap, the spread. Dividing that climb out gives the handy rule used here: cloud base in metres is about 125 times the spread in degrees Celsius.
Here is a worked example. Surface temperature is 25 degrees and the dew point is 15 degrees, so the spread is 10 degrees. Multiply 10 by 125 and you get a cloud base of about 1250 metres, or about 4100 feet. On a drier day, with a temperature of 30 degrees and a dew point of 10 degrees, the spread is 20 degrees and the cloud base rises to about 2500 metres. Dry air means high cloud bases, and moist air means low ones.
The formal name for this height is the lifted condensation level, often shortened to LCL. It is the level a parcel of surface air reaches saturation when lifted without mixing. Glider pilots live by the LCL, because thermals carry them up to the cloud base, and the best lift often sits just under it. Drone pilots and photographers care too: in many countries, flying above the cloud base or into cloud is restricted, so knowing the base keeps flights legal and safe.
Cloud bases tell you about the weather to come. A low, falling cloud base through the day often means moisture is increasing and rain may follow. A high cloud base on a hot, dry afternoon means strong thermals and, in fire country, fast-spreading wildfire conditions. Morning fog is the extreme case: when the spread is near zero at ground level, the cloud base is at the ground itself, and that is exactly what fog is.
The estimate has honest limits. The 125 rule assumes a clean, well-mixed surface layer and a bubble that does not mix with surrounding air. Real air mixes, hills force air up early, and sea breezes bring in moister air that lowers the base locally. Layer clouds, such as stratus, form by broad lifting rather than bubbles, so the rule fits them poorly. If the morning sky already has a solid grey deck, this tool will not explain it well.
You can check the tool against the sky. On a day with scattered cumulus, enter the nearest weather station temperature and dew point, then compare the answer with the flat cloud bottoms you see. Mountain watchers can cross-check with peaks of known height: if clouds sit just above a 1500-metre summit, the base is near that height. Agreement within a few hundred metres is a good result for such a simple rule.
To use this tool well, take temperature and dew point from the same station at the same time, ideally in the early afternoon when cumulus is forming. Remember that the spread changes through the day: as the sun heats the ground, temperature rises and the base climbs with it, which is why cumulus often appears late in the morning and grows taller by afternoon. If the spread is zero or negative, expect fog or low cloud, not a high base.