The cloud base is the lowest visible level of a cloud, the altitude at which rising moist air first cools enough to condense into visible droplets. On a convective storm it is the underside of the storm's main cloud deck, the surface that spotters and chasers watch for features like wall clouds, funnels, and scud.
What sets its height
The physical control on cloud base height is where a rising air parcel becomes saturated. As a parcel rises it cools, and once its temperature drops to its dew point it can no longer hold all of its water vapor and starts to condense. The altitude where that happens is called the lifting condensation level, or LCL, and for parcels that are being forced upward the LCL is a reasonable estimate of the cloud base.
Warm, humid boundary-layer air condenses at a low altitude and produces a low cloud base. Warmer, drier air has to rise farther before it saturates and produces a higher cloud base. This is why afternoon storms over a moist Gulf-air airmass typically have much lower bases than storms firing over dry high-plains air the same day.
How forecasters and chasers use it
A low cloud base, roughly 1000 meters or less above ground level, is a common ingredient in tornado environments because a low base keeps the level of any developing low-level rotation close to the ground and reduces the distance a spin-up has to reach to become a tornado. A high cloud base, common in the high plains, tends to favor microbursts and large hail rather than tornadoes.
In the field, the base tells you where to look. The dark, flat portion under the updraft is where organized rotation shows up. Precipitation cascades, ragged fragments, and lifted or lowered patches on the base all carry information about what the storm is doing at low levels.
Important limits
LCL-derived cloud base is an approximation. Real storms mix, entrain drier air, and lift parcels from different layers at once, so the actual visible base often differs from a sounding-derived number by hundreds of meters. Use the calculation as a guide to expected environment, not as an exact prediction.
The visible base can also be misread. Multiple cloud layers, thin scud beneath the true base, and heavy rain can all obscure what the storm base is really doing. Judging storm behavior from a single glimpse of the base is unreliable; watch it evolve.