Cloud top

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

The cloud top is where the snow, rain and sleet come from. Cloud top height can be estimated from the ground by triangulation. However, this is often inconvenient as this is 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 (see this paper for an example using this approach).

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 instruments (ATSR-1, ATSR-2 and AATSR) here. An example of the estimation of cloud top height from temperature measurements is available from this paper.

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