9th September 2005
Cumulus Congestus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide,
except Antarctica.Height: 2000 to 20,000 feet. Cause:
Convection, enhanced by atmospheric instability.Associated Weather:
Can produce moderate to heavy showers.Hazard Warning: Moderate turbulence at cloud level. Cumulus congestus represents the next stage in the vertical development of a cumulus cloud after cumulus mediocris
Powered by strong updrafts, this cloud may grow to an altitude of 15,000 to 20,000 feet. Congestus clouds are taller than they are wide, and have a flat base and a sharp outline. Congestus seldom forms as a result of convection alone. Normally, atmospheric instability is also required. This occurs when the temperature of the surrounding air mass drops more rapidly with height than is normal, often as a result of a cold air current sliding over the cloud.
Congestus may grow to the cumulonimbus stage if convection is strong enough or the surrounding atmosphere becomes yet more unstable. The time of day may be a deciding factor, as convection over land weakens late in the afternoon when ground temperatures begin to fall. If, by this time, the tallest cloud in the sky is congestus, then it is unlikely to progress to cumulonimbus.
Congestus is capable of generating heavy and prolonged showers of rain or snow. In fact, during winter in North America, congestus clouds forming downwind of the Great Lakes often produce significant snowfalls.
The vigorous convection that produces congestus creates significant levels of turbulence within the cloud. However, while this may result in a bumpy ride for aircraft passengers, it does not pose a serious threat to their safety.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
