Cloud top: Difference between revisions

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The cloud top (or the top of the cloud) is the largest altitude of the visible portion of the cloud. It is traditionally expressed either in m above the Earth (or planetary) surface, or as the corresponding pressure level in hectopascal (hPa, equivalent to the traditional but now obsolete millibar).

Measurement

Cloud top height can be estimated from the ground by triangulation. However, this is often inconvenient as this practically feasible only for isolated clouds in full view of (and some horizontal distance away from) the observers. Ground-based radars can be used to derive this cloud property (see this paper for a comparison of this approach to a satellite-based method).

An alternative (but also more expensive) approach is to acquire airborne observations either visually or using specific instruments such as a lidar. This technique is very appropriate to characterize individual clouds (and specifically to control or evaluate the accuracy of other methods) but becomes unmanageable to repetitively monitor clouds over large areas.

Cloud top height may be derived from satellite measurements, either through stereophotogrammetry (using pairs of images acquired at different observation angles) or by converting temperature measurements into estimations of height. An example of the stereo technique using the Multi-angle Imaging SpectroRadiometer (MISR) instrument can be found here, and using the Along Track Scanning Radiometer (ATSR) instrument here. An example of the estimation of cloud top height from temperature measurements is available from this paper.

Weather and climate relevance

In convective clouds, the cloud top is largely influenced by the strength of the convection activity, which itself may depend on surface properties, in particular the supply of heat and water vapor below the cloud. Cloud top height is often much more variable than cloud base elevation.

Clouds greatly affect the transfer of radiation in the atmosphere. In the solar spectral domain, cloud albedo is directly related to the nature, size and shape of cloud particles, which themselves are affected by the height of the cloud top. In the thermal domain, water is a strong absorber (and thus emitter, according to Kirchhoff's law of thermal radiation). Hence clouds cool down from the top through infrared radiation at the prevailing temperature: the higher the cloud top, the cooler the particles and the lower the rate of emission. For a synthetic discussion of the impact of clouds (and in particular the role of cloud tops) on the climate system, see the IPCC Third Assessment Report, in particular chapter 7.2.

References

Huschke, Ralph E. (1959) Glossary of Meteorology, American Meteorological Society, Boston, Second printing-1970.

External links

instruments]

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-The '''cloud top''' (or the top of the cloud) is the largest [[altitude]] of the visible portion of the [[cloud]]. It is traditionally expressed either in [[metre|m]] above the Earth (or planetary) surface, or as the corresponding [[pressure]] level in [[pascal|hectopascal]] (hPa, equivalent to [[bar|millibar]]).
+The '''cloud top''' (or the top of the cloud) is the largest [[altitude]] of the visible portion of the [[cloud]]. It is traditionally expressed either in [[metre|m]] above the Earth (or planetary) surface, or as the corresponding [[pressure]] level in [[pascal|hectopascal]] (hPa, equivalent to the traditional but now obsolete [[bar|millibar]]).
+==Measurement==
-In convective clouds, the cloud top is largely influenced by the strength of the convection activity, which itself may largely depend on surface properties, in particular the supply of water vapor below the cloud and the temperature of the underlying surface. Cloud top height is thus often much more variable than [[cloud base]] elevation.
+Cloud top height can be estimated from the ground by [[triangulation]]. However, this is often inconvenient as this practically feasible only for isolated clouds in full view of (and some horizontal distance away from) the observers. Ground-based radars can be used to derive this cloud property (see this [http://www.atmos.washington.edu/~qfu/Publications/ar.hollars.2004.pdf paper] for a comparison of this approach to a satellite-based method).
-Cloud top height may be estimated from [[satellite]] measurements, either through stereophotogrammetry (using pairs of images acquired at different observation angles) or by converting temperature measurements into estimations of height. An example of the stereo technique using the [[MISR|Multi-angle Imaging SpectroRadiometer]] (MISR) instrument can be found here [[http://www-misr.jpl.nasa.gov/gallery/galhistory/2004_sep_15.html]].
+An alternative (but also more expensive) approach is to acquire airborne observations either visually or using specific instruments such as a lidar. This technique is very appropriate to characterize individual clouds (and specifically to control or evaluate the accuracy of other methods) but becomes unmanageable to repetitively monitor clouds over large areas.
+Cloud top height may be derived from [[satellite]] measurements, either through stereophotogrammetry (using pairs of images acquired at different observation angles) or by converting temperature measurements into estimations of height. An example of the stereo technique using the [[MISR|Multi-angle Imaging SpectroRadiometer]] (MISR) instrument can be found [http://www-misr.jpl.nasa.gov/gallery/galhistory/2004_sep_15.html here], and using the Along Track Scanning Radiometer (ATSR) instrument [http://earth.esa.int/workshops/atsr_workshop_1999/Papers/Muller.pdf here]. An
+example of the estimation of cloud top height from temperature measurements is available from [http://ams.confex.com/ams/pdfpapers/81921.pdf this paper].
+==Weather and climate relevance==
+In convective clouds, the cloud top is largely influenced by the strength of the [[convection activity]], which itself may depend on surface properties, in particular the supply of heat and water vapor below the cloud. Cloud top height is often much more variable than [[cloud base]] elevation.
+Clouds greatly affect the transfer of radiation in the atmosphere. In the solar spectral domain, [[cloud albedo]] is directly related to the nature, size and shape of cloud particles, which themselves are affected by the height of the cloud top. In the thermal domain, water is a strong absorber (and thus emitter, according to [[Kirchhoff's law of thermal radiation]]). Hence clouds cool down from the top through infrared radiation at the prevailing temperature: the higher the cloud top, the cooler the particles and the lower the rate of emission. For a synthetic discussion of the impact of clouds (and in particular the role of cloud tops) on the climate system, see the [http://www.grida.no/climate/ipcc_tar/wg1/index.htm IPCC Third Assessment Report], in particular chapter 7.2.
-{{climate-stub}}
 ==See also==
@@ Line 18: / Line 28: @@
 * [http://www-misr.jpl.nasa.gov/ MISR home page]
+* [http://www.arm.gov/measurements/measurement.php?id=cloudtop Measuring
+instruments]