What is a cumulonimbus cloud?

A cumulonimbus is the thunderstorm cloud: a heavy, dense cloud of great vertical extent whose upper part usually flattens and often spreads into an anvil.

The pillar guide on how to read weather radar explains what the radar measures. This page is about the object being measured, and about the gap between the cloud you can see and the echo the radar returns.

The definition

The World Meteorological Organization’s International Cloud Atlas defines cumulonimbus as “heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers.” At least part of its upper portion is “usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.” Under its often very dark base there are frequently low, ragged clouds and sometimes precipitation in the form of virga.

The Atlas describes the cloud as water droplets and, especially in its upper portion, ice crystals, along with large raindrops and often snowflakes, snow pellets or hailstones. The water can be substantially supercooled. That mix of large particles is what matters for radar, below.

Calvus and capillatus

The Atlas recognizes two species, separated by the texture of the top:

• Cumulonimbus calvus: the sproutings of the upper parts are “indistinct and flattened” and look like “a whitish mass without sharp outlines.” No fibrous or striated parts are visible yet. It usually produces showers.
• Cumulonimbus capillatus: the upper portion has “cirriform parts of clearly fibrous or striated structure,” frequently in the shape of an anvil, a plume or “a vast disorderly mass of hair.” It is usually accompanied by a shower or thunderstorm, often with squalls and sometimes hail.

An anvil-topped storm is, in Atlas notation, cumulonimbus capillatus incus. Incus is a supplementary feature, not a species.

From towering cumulus

The Atlas says cumulonimbus most commonly evolves from cumulus congestus, the cloud the National Weather Service glossary calls towering cumulus: “a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb.” NSSL’s Severe Weather 101 places that tower in the developing stage of a thunderstorm, when an updraft is pushing the cloud upward and there is little to no rain.

The change of name is a change of phase. The Atlas attributes the transformation to ice particles forming in the upper part of the cloud, and gives the observer’s test: the cloud is cumulonimbus as soon as part of its top loses the sharpness of its outlines or takes on a fibrous or striated texture. When that cannot be judged, lightning, thunder or hail settles it.

Why the anvil spreads

NSSL gives the mechanism plainly: an anvil forms when the updraft reaches a point where the surrounding air is about the same temperature or warmer, so cloud growth “abruptly stops and flattens out.” The rising air is no longer buoyant, and it spreads sideways instead.

The level where that happens in a deep storm is near the tropopause. NOAA’s JetStream explains that temperature increases with height in the stratosphere, which prevents convection, and that the bottom of that layer is “readily seen by the anvil-shaped tops of cumulonimbus clouds.” The same page gives the height of the troposphere as 18–20 km at the equator, about 9 km at 50° latitude and about 6 km at the poles.

Wind shapes the result. The NWS glossary notes that anvils may spread hundreds of miles downwind and sometimes upwind, and that a back-sheared anvil, spreading against the flow aloft, “often implies a very strong updraft.” The Atlas adds that when wind increases strongly with height, the top spreads only downwind as a half anvil or a plume.

The overshooting top

The NWS glossary defines an overshooting top as “a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm.” The glossary notes a persistent or large overshooting top often sits on a supercell, while one that forms and collapses in cycles may mark a pulse storm. In a 2012 NASA validation study, Bedka and colleagues identified overshooting events in CloudSat radar profiles as cloud tops penetrating above both the model tropopause and the surrounding anvil.

Satellites can detect them. Dworak, Bedka and colleagues (2012, Weather and Forecasting) applied an overshooting-top detection method built for the GOES-R Advanced Baseline Imager, using 11 µm infrared brightness temperatures, to GOES-12 data over the eastern United States. They found:

• An overshooting top often corresponded with the highest radar echo top and the reflectivity maximum aloft.
• The detection false-alarm ratio against composite reflectivity was 16%.
• Severe weather was often associated with detections in April through September and over the southern United States, and the relationship strengthened by 15% when GOES ran in rapid-scan mode.
• In 75% of cases the earliest detection came before the associated severe weather report.

The authors’ conclusion is measured: a detected overshooting top suggests the storm is likely producing heavy rain and possibly severe weather. It raises confidence. It does not confirm a severe storm.

Mammatus

The Atlas lists mamma among cumulonimbus supplementary features, observed frequently on the underside of the anvil’s projecting portion. They are the best-known thing about an anvil and the most misread: what mammatus clouds are, and why they are not a warning sign.

What radar sees of a cumulonimbus

Radar does not see the cloud. It sees the precipitation inside it. The return from a sample volume depends steeply on particle size, so cloud droplets contribute almost nothing next to raindrops, graupel and hail. The wall cloud guide sets out that argument, and the beam-height problem, in full. For a cumulonimbus the consequence is that the radar outline of a storm is the outline of its precipitation, not of the towers and anvil you see. The updraft structure that shows up in reflectivity, such as the weak echo region and echo overhang, is covered in the supercell, on radar.

The vertical measure is the echo top. The NWS glossary defines it as “the height above ground of the center of the radar beam using the tilt, or scan, that contains the highest elevation where reflectivities greater than 18 dBZ can be detected.” NOAA’s Warning Decision Training Division notes that its 18 dBZ multi-radar echo top corresponds to the WSR-88D Enhanced Echo Tops product and gives only the approximate height of the cloud top. The same guide says rising echo tops often indicate a strengthening updraft, falling ones a weakening updraft, and that echo tops above the equilibrium level can identify overshooting tops.

Satellite and radar, side by side

The table summarizes the sources above. Dworak and colleagues make the last row’s point directly: a satellite overshooting-top product may be particularly useful where radar coverage is insufficient.

WeatherStory Radar reads full-resolution Level II reflectivity and GOES-19 true color, clean infrared and cloud-top height imagery from NOAA’s public archives.