Cumulonimbus clouds, commonly known as thunderstorm clouds, are powerful atmospheric formations. They indicate unstable atmospheric conditions and can generate intense, hazardous weather. Understanding their characteristics and the weather they bring helps individuals prepare for and respond to severe conditions.
Recognizing Cumulonimbus Clouds
Cumulonimbus clouds have distinct visual characteristics. They are notable for impressive vertical development, appearing as dense, towering structures that extend high into the atmosphere. Their bases are typically dark and can stretch for several kilometers, sometimes lying only a few hundred feet above the Earth’s surface.
The top of a mature cumulonimbus cloud often flattens into an anvil shape. This shape is caused by strong upper winds or an inversion layer acting as a ceiling for rising air. Composed of ice crystals, the anvil can spread for many kilometers ahead of the main cloud, signaling the storm’s movement. An “overshooting top,” a dome of cloud poking above the anvil, indicates a vigorous updraft within the storm. The overall appearance, from its dark, heavy base to its fibrous, towering top, provides visual cues that forewarn of potential severe weather.
Weather Phenomena Associated with Cumulonimbus
Cumulonimbus clouds are the source of thunderstorms, which occur when warm, moist air rapidly rises in an unstable atmosphere. This upward movement, an updraft, fuels the cloud’s growth and leads to precipitation. As the cloud matures, both updrafts and downdrafts develop, creating a dynamic environment for various severe weather events.
Lightning
Lightning is a common byproduct of cumulonimbus clouds, resulting from the separation of electrical charges within the cloud. Ice particles and water droplets collide in the cloud’s turbulent air, building positive charges in the upper cloud and negative charges in the lower cloud. When the charge difference becomes too great, a rapid discharge of electricity occurs. This is visible as lightning, striking within the cloud, between clouds, or from cloud to ground. The intense heat generated by a lightning strike, which can be hotter than the sun’s surface, causes air to expand rapidly, producing the sound of thunder.
Heavy Rain
Cumulonimbus clouds also bring heavy rain, often falling with high intensity and leading to significant accumulation in a short period. This precipitation can quickly overwhelm drainage systems and low-lying areas, increasing the risk of flash flooding. Flash floods can develop rapidly, turning small streams into torrents and making roads impassable.
Hail
Hail, frozen precipitation, forms within cumulonimbus clouds. Hailstones develop as ice crystals are carried up and down by strong updrafts and downdrafts, accumulating layers of ice until they fall. Hail can range in size from small pebbles to larger stones, capable of causing considerable damage to crops, vehicles, and structures.
Strong Winds
Strong winds are frequently associated with cumulonimbus clouds, manifesting as downdrafts, microbursts, and gust fronts. Downdrafts are powerful columns of air that descend from the cloud, sometimes reaching speeds of 20 to 90 km/hr. Microbursts are intense, localized downdrafts that produce damaging straight-line winds upon impact with the ground. The leading edge of these outflowing winds is known as a gust front, a boundary where cooler, dense air spreads out from the thunderstorm.
Tornadoes
Cumulonimbus clouds can also lead to tornado formation. While not all cumulonimbus clouds produce tornadoes, their presence indicates the atmospheric conditions necessary for these violent rotating columns of air to develop. Tornadoes emerge from supercell thunderstorms, a specific type of powerful cumulonimbus cloud with a rotating updraft.
Safety During Severe Weather
When cumulonimbus clouds are present, safety precautions are important due to the potential for severe weather.
Lightning Safety
If thunder is heard, lightning is close enough to strike. Seek immediate shelter indoors. A sturdy building offers the best protection, but a hard-topped vehicle with closed windows can also provide a safer alternative than being outdoors.
Once indoors, stay away from windows and doors, as strong winds can cause flying debris. Avoid contact with corded telephones, electrical equipment, and plumbing fixtures, as lightning can travel through these pathways. Remain inside for at least 30 minutes after the last sound of thunder to ensure the storm has passed.
For those caught outdoors during a thunderstorm, finding immediate shelter is important. Avoid open fields, isolated tall objects like trees, and bodies of water, as they increase the risk of a lightning strike. If no sturdy shelter is available, crouching low to the ground in a ditch or low-lying area, while minimizing contact with the ground, can slightly reduce risk.
Hail Safety
In the event of hail, seek sturdy shelter. Indoors, stay away from windows and close drapes or blinds to protect against shattered glass. If driving, safely pull over and angle the vehicle to allow hail to hit the reinforced windshield, which is stronger than side or rear windows. Protecting one’s head is also advised, as hailstones can cause serious injuries.
Wind Safety
To prepare for strong winds, secure outdoor objects such as patio furniture, trash cans, and trampolines to prevent them from becoming dangerous projectiles. During high wind events, it is safest to be inside a sturdy building, ideally in a basement or an interior room away from windows. Mobile homes are particularly vulnerable to high winds; occupants should seek more robust shelter in advance of a storm.