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[[Image:Cloud base.jpg|thumb|300px|Photo of a cloud base taken above the island of [[Sulawesi]], [[Indonesia]], in 2005]]

A '''cloud base''' (or the base of the cloud) 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 [[Atmospheric pressure|pressure]] level corresponding to this altitude in [[Pascal (unit)|hectopascals]] (hPa, equivalent to the [[Bar (unit)|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 [[Lidar|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, used by the U.S. [[Federal Aviation Administration]] and often named after Tom Bradbury,<ref>{{cite book|title=Meteorology and Flight|ISBN=0713668318|first=Tom|last=Bradbury|publisher= A&C Black Publishers Ltd}}</ref> 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.<ref name="Education 2022 k673">{{cite web | title=Cloud | website=National Geography | date=2022-08-09 | url=https://education.nationalgeographic.org/resource/cloud/ | access-date=2023-09-06|quote=The prefix "nimbo-" or the suffix "-nimbus" are low-level clouds that have their bases below 2,000 meters (6,500 feet) above the Earth. Clouds that produce rain and snow fall into this category. }}</ref>

In well-defined [[air mass]]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 top]]s, which can vary widely from cloud to cloud, as the depth of the cloud is determined by the strength of local [[Atmospheric convection|convection]].

Clouds greatly affect the transfer of [[radiation]] 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 the [http://www.grida.no/climate/ipcc_tar/wg1/index.htm 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.

==See also==

*[[Ceiling (cloud)]]
*[[Cloud cover]]
*[[Cloud height]]

==References==
{{Reflist}}

*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.
*[https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/14_phak_ch12.pdf FAA Pilot's Handbook of Aeronautical Knowledge, Weather Theory]  p. 14

==External links==
*[https://web.archive.org/web/20050420085336/http://www.nsdl.arm.gov/Library/glossary.shtml#cloud_base National Science Digital Library - Cloud Base]
*[https://web.archive.org/web/20060924170359/http://www.arm.gov/measurements/measurement.php?id=cloudbase Cloud base Measuring instruments]
*[http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Cloud_Heights Measuring 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}})<!--retrieved 13 Sept 2010-->

[[Category:Cloud base]]
[[Category:Aviation Weather]]
[[Category:Ceiling (meteorology)]]
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[[Category:Ceiling and Visibility]]
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[[Category:Barometric Pressure Effects]]
[[Category:Cloud Layer Analysis]]
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[[Category:Cloud Altitude Sensors]]
[[Category:Ceiling Obscuration]]
[[Category:Cloud Cover Estimation]]
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[[Category:Cloud Base Variability]]
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[[Category:Cloud Base Monitoring]]
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