Cloud base
Encyclopedia
This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons
, see Cloudbase
.
The cloud base (or the base of the cloud) is the lowest altitude
of the visible portion of the cloud
. It is traditionally expressed either in m or feet above mean sea level (or planetary surface), or as the corresponding pressure
level in hectopascal
(hPa, equivalent to millibar
).
and humidity
. Modern automated instruments to measure cloud base include specially designed laser systems called ceilometer
s.
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 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 base) on the climate system, see the 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 must follow Instrument Flight Rules
for take-off or landing.
Captain Scarlet and the Mysterons
Captain Scarlet and the Mysterons, often referred to as Captain Scarlet, is a 1960s British science-fiction television series produced by the Century 21 Productions company of Gerry and Sylvia Anderson, John Read and Reg Hill...
, see Cloudbase
Cloudbase
Cloudbase is the fictional skyborne headquarters of international security organisation Spectrum, from Gerry Anderson's science fiction television series Captain Scarlet and the Mysterons .-Appearance in Captain Scarlet and the Mysterons:...
.
The cloud base (or the base of the cloud) is the lowest altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
of the visible portion of the cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
. It is traditionally expressed either in m or feet above mean sea level (or planetary surface), or as the corresponding pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
level in hectopascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(hPa, equivalent to millibar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
).
Measurement
The height of the cloud base can be estimated from surface measurements of air temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
. Modern automated instruments to measure cloud base include specially designed laser systems called ceilometer
Ceilometer
A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere.-Optical Drum Ceilometer:...
s.
Weather and climate relevance
In well-defined air massAir mass
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or maritime...
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
Cloud top
The cloud top is the highest altitude of the visible portion of the cloud. It is traditionally expressed either in metres above the Earth surface, or as the corresponding pressure level in hectopascal .-Measurement:The cloud top is where the snow, rain and sleet come from.Cloud top...
s, which can vary widely from cloud to cloud, as the depth of the cloud is determined by the strength of local convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
.
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
Kirchhoff's law of thermal radiation
In thermodynamics, Kirchhoff's law of thermal radiation, or Kirchhoff's law for short, is a general statement equating emission and absorption in heated objects, proposed by Gustav Kirchhoff in 1859, following from general considerations of thermodynamic equilibrium and detailed balance.An object...
). 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 base) on the climate system, see the 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
Visual flight rules
Visual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
(VFR) or must follow Instrument Flight Rules
Instrument flight rules
Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
for take-off or landing.
External links
- National Science Digital Library - Cloud Base
- Cloud base Measuring instruments
- Measuring Cloud Heights : A lesson plan from the National Science Digital Library.archive - 13 Sept 2010