Cumulonimbus Capillatus: Identifying Australia's Hairy Thunderstorm Clouds

Cumulonimbus capillatus is a mature thunderstorm cloud with a fibrous, hairy-looking upper part. The top has glaciated, meaning supercooled water droplets have frozen into ice crystals. In Australia, this cloud type often appears during active storm periods and can point to strong convective activity, lightning discharge, hailstorm risk, and possible supercell development.

Key takeaways

• Cumulonimbus capillatus is a mature thunderstorm cloud with a wispy, glaciated top.
• The change from calvus to capillatus shows ice crystal formation higher in the troposphere.
• Its fibrous anvil cloud can sit above strong updrafts, heavy rain, hail, and lightning discharge.
• It may appear before a severe weather warning, but the cloud alone does not confirm one.

Identifying cumulonimbus capillatus in the Australian sky

The key clue is the top of the cloud. A cumulonimbus capillatus has a distinct fibrous anvil cloud or hairy cloud appearance above the main tower. The upper edge looks frayed or striated rather than smooth. That change usually means the cloud top has reached colder air in the troposphere and glaciation has begun. For a wider guide to storm cloud structure, see our cumulonimbus clouds guide.

The Bureau of Meteorology uses standard cloud terms that align with the World Meteorological Organization cloud classification. In practice, the capillatus stage tells you the storm is no longer a small cumulus cloud. It has become a mature thunderstorm with a deeper vertical structure. An example image on Wikimedia Commons shows cumulonimbus capillatus over Swifts Creek, Victoria, which highlights the dramatic structure of these cells.

From calvus to capillatus

Thunderstorm clouds change as they grow. In the calvus stage, the cloud top is still fairly smooth and rounded. As the updraft keeps feeding moist air higher into the cloud, the top reaches colder levels and glaciation starts. Ice crystals then spread through the upper cloud, which gives cumulonimbus capillatus its fibrous look. Reference the weather glossary for technical definitions of these convective stages.

This stage often sits near the cloud top, while the lower storm continues to draw energy from heat and moisture at the surface. If the top spreads into a flatter, more defined cap, the cloud may be classed as Cumulonimbus incus. That anvil shape is another sign that the storm cell maturation process is well advanced.

What is the difference between cumulonimbus calvus and capillatus?

Cumulus congestus has a smoother, cauliflower-like top, whereas Cumulonimbus calvus has indistinct, flattened summits that resemble a whitish mass without sharp outlines. Cumulonimbus capillatus has a wispy, fibrous top caused by glaciation. Both are thunderstorm clouds, but capillatus shows a later stage of growth and a more developed upper cloud. You can track these developments using a lightning tracker to see where electrification is most intense.

The role of the capping inversion in storm intensity

A capping inversion is a layer of warmer air above cooler air near the surface. It can suppress convection for a time and let heat and moisture build up below it. When the cap breaks, strong lift can trigger rapid storm growth. That is one reason a cumulonimbus capillatus can form quickly once the atmosphere is ready to release energy.

Convective available potential energy, or CAPE, is one measure of the energy available for rising air. When CAPE is high and the updraft is strong, the storm may grow fast through the freezing levels of the troposphere. This can speed up glaciation and produce the fibrous cloud top. For more on storm growth and rainfall signals, see Rainfall & Thunderstorm Forecast.

Severe weather risks: hail, lightning, and heavy rain

The fibrous upper section often appears in storms with strong vertical development. That does not mean every such cloud will produce severe weather, but it does show that the storm has a deep, cold upper part. In Australia, these storms can bring heavy rain, hailstorm risk, and frequent lightning discharge. A weather warning may be issued if the storm is expected to affect people or property.

Strong updrafts can keep hailstones suspended long enough for them to grow. As the stones cycle through colder and warmer layers, they can collect more ice before falling to the ground. Rain may also fall as virga if it evaporates before reaching the surface. For warnings and timing, follow the Bureau of Meteorology and local forecast updates. You can also review our thunderstorm rainfall guide and lightning safety guide.

Some mature storms can form supercell development if wind shear supports a rotating updraft. That means a cumulonimbus capillatus can be part of a supercell, but the cloud alone does not prove it. The storm’s full structure, movement and reading radar patterns matter as well.

Aviation safety and convective turbulence

For aviation, a cumulonimbus capillatus is a major hazard. The cloud can extend high into the troposphere and often sits near the tropopause. Aircraft should avoid the storm because of turbulence, hail, lightning, and sharp changes in wind and visibility.

The fibrous top is one sign that the storm has strong upper-level growth. The cloud may also produce nearby rain shafts and virga, which can make the area around it difficult to judge from the ground. Pilots and forecasters use radar, observations, and BoM advice to track storm cells and plan around them.