Anvil clouds are distinctive, flat-topped formations that signal the presence of powerful weather systems. They indicate a mature thunderstorm, often one capable of producing severe weather.
The Dangers Associated with Anvil Clouds
Lightning is a significant danger, as cumulonimbus clouds are prolific producers of both cloud-to-cloud and cloud-to-ground strikes. Lightning can travel miles from the main storm, posing a threat even when the sky overhead appears clear or only contains the distant anvil.
These storms also generate strong winds, primarily downbursts, which include microbursts and macrobursts. Downbursts are intense, localized downdrafts that spread outward rapidly upon hitting the ground, producing damaging straight-line winds. Microbursts affect areas up to 2.5 miles in diameter with winds potentially exceeding 100 mph, while macrobursts cover larger areas, sometimes over 2.5 miles, with winds that can reach over 130 mph. These winds can cause damage comparable to weak tornadoes, posing a threat to property and life.
Large hail is another common and destructive product of storms featuring anvil clouds. Hailstones form when raindrops are carried upward by strong thunderstorm updrafts into very cold atmospheric regions, where they freeze. These ice particles grow by colliding with supercooled water droplets, which freeze onto their surface. The strength of the updraft directly influences hailstone size; powerful updrafts can suspend hailstones longer, allowing them to grow considerably before gravity overcomes the upward force and they fall to the ground.
While anvil clouds themselves do not produce tornadoes, they are the upper part of supercell thunderstorms, which are the most common and often most dangerous type of storm to generate tornadoes. The presence of a well-defined anvil cloud signals the potential for tornadic activity from the parent storm. Tornadoes typically develop from a rotating updraft within the supercell, often beneath a feature called a wall cloud, which is associated with the storm’s base.
Identifying Anvil Clouds and Their Associated Storms
Anvil clouds are distinguished by their flat, spreading top, resembling a blacksmith’s anvil. This shape forms when strong updrafts within a thunderstorm push moist air high into the atmosphere. As this air reaches the tropopause, a stable layer where temperature stops decreasing with altitude, it can no longer rise and is forced to spread horizontally.
The direction in which the anvil spreads can indicate the direction of upper-level winds, and a back-sheared anvil (spreading upwind) can signify a very strong updraft and high severe weather potential. The anvil’s edges may appear fibrous or wispy due to ice crystals, but a solid, well-defined edge suggests a strong, active storm. Sometimes, a dome-like protrusion, known as an overshooting top, can be seen above the main anvil, indicating an exceptionally strong updraft that briefly punches through the tropopause.
Staying Safe During Severe Thunderstorms
The safest place to be during a severe thunderstorm is inside a sturdy building, away from windows and doors. Avoid seeking shelter under isolated trees or in open fields, as these locations increase the risk of lightning strikes. If indoors, stay away from plumbing and corded electronics, as lightning can travel through these pathways.
A common guideline for lightning safety is the “30/30 rule”: if you hear thunder less than 30 seconds after seeing lightning, the storm is close enough to be dangerous, and you should seek shelter immediately. Wait at least 30 minutes after the last clap of thunder before resuming outdoor activities. Staying informed by monitoring weather alerts, such as severe thunderstorm watches and warnings, provides crucial advance notice of dangerous conditions. If caught driving in a severe thunderstorm, reduce speed, turn on headlights, and pull over to a safe location if heavy rain or hail significantly reduces visibility.