9th September 2005
Cumulonimbus Incus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide, except Antarctica; common in the tropics. Height: 2000 to 35,000 feet. Cause: Powerful convection assisted by atmospheric instability. Associated Weather: Heavy rain or hail, strong winds. Hazard Warning: Severe turbulence in cloud, strong winds, lightning, hail, and even tornadoes at ground level. A mature cumulonimbus incus is definitely the "King of Clouds", a mighty mountain of moisture often considerably taller than Mount Everest, and sometimes reaching 60,000 feet in tropical and subtropical areas.
In its full magnificence, it is crowned with a huge, wedge-shaped mass of high cloud resembling a blacksmith's anvil (incus is the Latin word for anvil). This part of the cloud, often referred to as a hammerhead or thunderhead, is a clear sign of a fully fledged thunderstorm.
Cumulonimbus incus can begin early in the morning as cumulus humilis, then proceed through the stages of mediocris and congestus. For the cloud to continue developing at this point, the convective process must combine with atmospheric instability to produce a powerful updraft.
As long as the air in the vicinity of the updraft remains unstable, the cloud continues to rise and expand. Eventually the cumulonimbus cloud reaches the top of the troposphere, where the air temperature levels off and begins to increase with altitude. This change in temperature has the effect of placing a lid on the updraft, and the cloud can rise no further. However, the momentum of the air below continues to push upward, and spreads the cloud out in a radial fashion at the tropopause, forming the characteristic anvil shape. The position of this formation therefore indicates the height of the troposphere in the area.
Because the anvil is situated well above the level where the air temperature drops below freezing, this part of the cloud is composed of ice crystals, which forms a crown of cirrus above the main cloud mass. These ice crystals may be blown about by strong, high-level winds, producing a streaky appearance.
Warning Signs
In rare cases, the updraft associated with the cloud is so powerful that it punches through the tropopause and carries a parcel of cloud into the lower levels of the stratosphere, before losing momentum and falling back. This produces an upward bulge on the otherwise flat upper surface of the anvil -- a good indicator of a particularly severe storm, which may produce hail, strong wind gusts, and even tornadoes.
A cumulonimbus incus formation must always be regarded as a significant aviation hazard because of the powerful air currents involved in its formation and the potentially damaging effect of the large hailstones it may produce. Fortunately, because of the precipitation generated, the cloud is easily located by onboard and ground-based radar, and elaborate procedures are in place to steer aircraft around any such activity.
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.
