Cloud base


A cloud base is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to this altitude in hectopascals.

Measurement

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 by calculating the lifted condensation level. One method for doing this, notably used by the U.S. Federal Aviation Administration, is as follows:
  1. Find the difference between the surface temperature and the dew point. This value is known as the "spread".
  2. Divide the spread by 4.4 or 2.5, then multiply by 1000. This will give the altitude of the cloud base in feet above ground level.
  3. Add the results from step to the field elevation to obtain the altitude of the cloud base above mean sea level.

    Weather and climate relevance

In well-defined air masses, many 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.
Clouds greatly affect the transfer of radiation in the atmosphere. In the thermal spectral domain, water is a strong absorber ; hence clouds exchange thermal radiation between their bases and the underlying planetary surface by absorbing and re-emitting this infrared radiation at the prevailing temperature – the lower the cloud base, the warmer the cloud particles and the higher the rate of emission. For a synthetic discussion of the impact of clouds on climate systems, see the , in particular chapter 7.2.
Cloud base is an important meteorological variable for aviation safety, as it determines whether pilots may use visual flight rules or instrument flight rules for take-off and landing.