Cloud base height
The cloud base (or the base of the cloud) is the lowest altitude of the visible portion of the cloud. The height of the cloud base can be measured using a ceilometer. This device reflects a beam of light off the cloud base and then calculates its distance using either triangulation or travel time. Alternatively, the cloud base can be estimated from surface measurements of air temperature and humidity. In well-defined air masses, many (or even most) clouds may have a similar cloud base because this variable is largely controlled by the thermodynamic properties of that air mass, which are relatively homogeneous on a large spatial scale. This is not the case for the cloud tops, which can vary widely from cloud to cloud, as the depth of the cloud is determined by the strength of local convection. Cloud base is an important meteorological variable for aviation safety, as it determines whether pilots may use Visual Flight Rules (VFR) or must follow Instrument Flight Rules for take-off or landing.