Cloud base
Clouds & visibility cloud-base height base of cloud
Cloud base is the height of a cloud’s visible underside. Always check whether the height is measured above the ground or above sea level.
Where the cloud begins
Rising air cools. When it reaches saturation, tiny droplets can form: invisible water vapour becomes a visible cloud. This is why many cumulus clouds have fairly flat bases. Other clouds form differently and have less regular boundaries.
How the height is measured
A ceilometer sends light pulses upwards and analyses the light scattered back. The travel time reveals the height. It measures above its own location; a nearby gap cannot be treated as a measurement of the whole sky.
Cloud base is not always a ceiling
In aviation, a ceiling is the base of the lowest layer covering more than half the sky, or a vertical visibility when the sky is obscured. One isolated low cloud therefore does not automatically create a ceiling.
Above which ground?
A cloud 500 metres above a plain may already touch a hillside. Metres above ground and metres above sea level are different references. Different model methods and multiple cloud layers can also produce different reported values.