Transverse bands and billows


Aircraft flying through transverse bands will cross volumes of air that are moving vertically at different speeds. Often there are features in the satellite images that suggest the presence of CAT, although its altitude cannot be determined accurately.

A characteristic pattern of cirrus clouds, known as billows, signals the presence of CAT. Billows are an indication of a turbulent flow described by the Kelvin-Helmholtz instability

Fig. 3.18: Conceptional Model of Transverse bands and Scallops [WMO Aviation Hazards]

Fig. 3.19a (left): Conceptional Model of Transverse bands [WMO Aviation Hazards]
Fig. 3.19b (right): Satellite image with Transverse Bands, Scallops, Billows [WMO Aviation Hazards]

Fig. 3.20: 9 November 2009, Eumetsat Image Gallery: Transverse bands over South Africa and Namibia [EUMETSAT]

The Airmass RGB, Dust RGB, WV6.2 and IR10.8 channel images above give examples of transverse cirrus bands. Large transverse cirrus bands can be seen over southern Namibia and smaller ones over the central parts of South Africa. Since there are few airplane observations from southern Namibia or the northwestern part of South Africa, it is difficult to verify whether there was turbulence in the area of the transverse cirrus bands or not. [from Jochen Kerkmann, Eumetsat]

See the loop of this Airmass RGB from 9 November 2009


Locate the jet stream in this Airmass RGB by clicking on it.


Which particular cloud formation signals potential clear air turbulence?

The correct answer is a), b), c), and d).

All 4 cloud formations give you a signal for potential Clear Air Turbulence.