An “anvil head” cloud is the distinctive, flattened, spreading top of a cloud, often resembling a blacksmith’s anvil. This unique shape is a hallmark feature of the cumulonimbus cloud, the primary cloud type responsible for powerful and often severe thunderstorms. Its presence signals a highly developed storm system, indicating a significant stage in its lifecycle.
Characteristics of the Cloud
The cloud type that develops an anvil head is the cumulonimbus, a towering and vertically extensive cloud formation. These clouds are often called thunderclouds or thunderheads. Cumulonimbus clouds typically have dark, flat bases, often 1,100 to 6,500 feet above the ground. They exhibit significant vertical development, sometimes extending through the entire height of the troposphere, reaching altitudes of 39,000 feet or more.
The formation of these clouds requires an unstable atmosphere, abundant moisture, and a lifting mechanism. This lifting can occur through convection over a heated surface, along cold fronts where warmer air is forced upward, or by air rising over elevated terrain. Cumulonimbus clouds progress through developing, mature, and dissipating stages.
How the Anvil Head Forms
The formation of the anvil head is a direct result of the vertical growth within a cumulonimbus cloud. As the cloud’s strong updrafts carry moisture and water vapor higher into the atmosphere, they eventually reach a stable layer called the tropopause. This layer acts like a ceiling or a lid, preventing further upward movement.
Upon hitting this barrier, the rising air spreads horizontally. At these high altitudes, temperatures are below freezing, causing water droplets to transform into ice crystals. Upper-level winds, also known as wind shear, then encounter these spreading ice crystals. These winds drag the ice crystals downwind, creating the flattened anvil shape that extends away from the main cloud tower.
The Weather it Brings
An anvil-headed cumulonimbus cloud is a reliable indicator that a thunderstorm has reached its mature and often most intense stage. These clouds are capable of producing a range of severe weather phenomena. Common occurrences include heavy rainfall, which can lead to flash flooding, and frequent lightning and thunder.
The presence of an anvil also signals the potential for damaging winds, such as downbursts or straight-line winds. These storms can generate large hailstones and, in some instances, tornadoes. The direction the anvil extends can provide a clue about the storm’s movement, as it is shaped by upper-level winds. Observing an anvil-shaped cloud serves as an important visual warning of hazardous weather conditions.