Anvil clouds, formally known as cumulonimbus incus, are the flat, spreading tops of powerful thunderstorms. While they may appear benign from a distance, the dangers they represent are significant and often extend far beyond the main storm column itself. The presence of this distinct structure indicates a storm has reached a high level of maturity and intensity, capable of producing atmospheric violence.
Anatomy and Formation of Anvil Clouds
Anvil clouds form when a thunderstorm’s powerful, vertical updraft of warm, moist air reaches its maximum ascent height. This atmospheric ceiling is the tropopause, the boundary layer between the troposphere and the stratosphere. The tropopause acts like a lid, preventing the cloud from rising further and forcing the cloud material to spread outward horizontally. This outflow consists primarily of millions of tiny ice crystals, carried downwind by upper-level winds, creating the characteristic flat, fibrous shape resembling a blacksmith’s anvil. The size and definition of the anvil directly correlate with the strength of the underlying storm’s updraft, signaling a highly energetic and potentially severe thunderstorm below.
The Threat of Distant Lightning Strikes
The expansive nature of the anvil cloud creates a unique and significant electrical hazard far from the storm’s core. Within a mature thunderstorm, lighter, positively charged ice crystals are carried to the top and spread out within the anvil. This accumulation creates the potential for positive cloud-to-ground lightning. This discharge is particularly dangerous because it can travel horizontally through the anvil’s base or surrounding clear air for long distances before striking the ground.
These strikes are often called “bolts from the blue” because they can occur up to 10 to 25 miles away from the main storm’s area of rain or thunder, where the sky may appear clear. Positive lightning is much more powerful and destructive than the more common negative lightning, with a peak charge up to ten times greater. A typical negative lightning strike might carry around 30,000 amperes, but a positive strike originating from the anvil can carry up to 300,000 amperes. This immense power, combined with the lack of warning, poses a serious risk to people engaged in outdoor activities. If you can see an anvil cloud, you are potentially within striking range.
Severe Turbulence and Aviation Risks
The region around a thunderstorm’s anvil presents severe hazards for aircraft due to extreme atmospheric instability. The intense vertical currents that power the storm create significant wind shear—a rapid change in wind speed or direction over a short distance. This wind shear is particularly pronounced at the edges and in the immediate vicinity of the anvil structure.
Thunderstorms can generate severe turbulence that extends 20 miles or more beyond the visible edge of the anvil cloud. This is often referred to as Convectively Induced Turbulence, which includes clear-air turbulence (CAT) that is invisible and undetectable by onboard weather radar. Aircraft encountering severe turbulence can experience violent buffeting, abrupt changes in altitude, and momentary loss of control. The powerful updrafts and downdrafts can displace an aircraft vertically by thousands of feet in seconds. Pilots are instructed to give thunderstorms and their associated anvil clouds a wide berth due to the risk of structural damage or serious injury to unbelted passengers and crew.
Anvils as Indicators of Ground-Level Hazards
The physical appearance of a large, well-developed anvil cloud serves as a direct, large-scale indicator of the violence happening in the lower atmosphere. The storm’s strength means that it is capable of producing the most destructive forms of severe weather at the surface.
This includes the production of large hail, as the strong updraft repeatedly lifts hailstones into the cloud’s cold upper layers, allowing them to grow significantly before falling. A strong updraft can also lead to intense downdrafts known as microbursts or downbursts, which are capable of generating sudden, damaging surface winds that exceed 40 to 50 knots.