What is a Cb? An operational perspective

Each day, thousands of routine aviation METAR and TAF reports contain information about cumulonimbus clouds (Cb) at airports, which is essential for aviation customers.

Although implicitly defined in aviation meteorology operations, it is sometimes hard to clearly identify Cbs and distinguish them from very well developed cumulus congestus clouds, especially during night time. At the same time, this distinction is of great importance for aviation. 

Cb is well defined in WMO’s Cloud Atlas as a “heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.”  Despite the official definition, there is no clear consensus within the MET community about strict distinguishing between Cb clouds and well developed cumulus congestus clouds. Possibly, the biggest misconception is that all Cb clouds develop lightning which is mostly, but not entirely true in mid-latitudes and to a much lesser extent in northern latitudes. 

Techniques that are mostly used for distinguishing include human observing, lightning detection, radar and satellite images, camera images, ceilometers and automatic derive from measured data and postprocessing.

Even though lightning is very often used as a means of detection of Cb clouds, it is of high importance to clarify that it is not the only nor the most dangerous phenomenon associated with these clouds. When it comes to observing Cb clouds, the right approach would be to identify the cloud as a Cb when its upper part starts losing the sharpness of its outlines which is a sign of glaciation rather than to wait for lightning to start.

Reporting Cb clouds in routine aviation reports is needed to meet regulatory requirements and user needs. Here we presented detailed challenges from an operational perspective - including the forecaster, observer and user point of view. Better observing reports surely contributes to better analysis, diagnosis, forecasts, verification and climatological series and trends.