A cloud base (or the base of the cloud) is the lowest

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

of the visible portion of a cloud. It is traditionally expressed either in metres or feet

above mean sea level Height above mean sea level is a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level ...

or above a planetary surface, or as the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

level corresponding to this altitude in hectopascals (hPa, equivalent to the millibar).

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 Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

by calculating the

lifted condensation level The lifting condensation level or lifted condensation level (LCL) is the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to liquid water when it is cooled by dry adiabatic lifting. The RH of air increases ...

. One method for doing this, used by the U.S.

Federal Aviation Administration The Federal Aviation Administration (FAA) is a Federal government of the United States, U.S. federal government agency within the United States Department of Transportation, U.S. Department of Transportation that regulates civil aviation in t ...

and often named after Tom Bradbury, is as follows: #Find the difference between the surface temperature and the dew point. This value is known as the "spread". #Divide the spread by 4.4 (if temperatures are in °F) or 2.5 (if temperatures are in °C), then multiply by 1000. This will give the altitude of the cloud base in feet above ground level. Put in a simpler way, 400 feet for every 1°C dew point spread. For metric divide the spread in °C by 8 and multiply by 1000 and get the cloud base in meters. #Add the results from step (2) to the field elevation to obtain the altitude of the cloud base above mean sea level.

Weather and climate relevance

Rain clouds and snow clouds are clouds that have their bases below 2,000 meters above the ground. In well-defined

air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...

es, 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 Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

. Clouds greatly affect the transfer of

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

in the atmosphere. In the thermal spectral domain, water is a strong absorber (and thus emitter, according to Kirchhoff's law of thermal radiation); hence clouds exchange thermal radiation between their bases and the underlying planetary surface (land or ocean) 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 (and in particular the role of cloud bases) on climate systems, see th
IPCC Third Assessment Report
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 (VFR) or instrument flight rules for take-off and landing.

References

*Huschke, Ralph E. (1959) ''Glossary of Meteorology'', American Meteorological Society, Boston, Second printing-1970. *McIntosh, D. H. (1972) ''Meteorological Glossary'', Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.
FAA Pilot's Handbook of Aeronautical Knowledge, Weather Theory
p. 14

External links

National Science Digital Library - Cloud BaseCloud base Measuring instrumentsMeasuring Cloud Heights
– a lesson plan from the National Science Digital Library ({{Webarchive, url=https://web.archive.org/web/20071012172859/http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Cloud_Heights, date=12 October 2007) Clouds Gliding technology Meteorological quantities

Measurement

Weather and climate relevance

See also

References

External links