How Do Mammatus Clouds Form? The Complete Meteorological Guide

How do mammatus clouds form? They usually appear as pouch-like lobes beneath the anvil of a cumulonimbus cloud. Unlike most clouds, which grow as air rises and cools, mammatus form when air sinks, ice crystals sublimate, and the cooling helps pull the cloud base into hanging pockets.

Mammatus clouds over the Nepal Himalayas - By Anton Yankovyi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44258693

Key takeaways

• Mammatus clouds usually form under the anvil of mature thunderstorms, especially cumulonimbus incus clouds.

• Subsiding air, evaporative cooling and ice crystal sublimation help shape the pouches.

• The cloud lobes sit in and near the upper troposphere, where tropospheric stability changes quickly around the storm top.

• Mammatus clouds often appear near severe thunderstorms, but they do not by themselves mean damaging weather is at ground level.

The mechanics: how do mammatus clouds form?

A distant supercell thunderstorm showing a spreading cumulonimbus anvil with mammatus clouds on its underside.

Dark mammatus clouds over a rural field near a thunderstorm.

Pouch-like mammatus clouds beneath a thunderstorm anvil.

Most clouds form when warm air rises, cools and condenses. Mammatus work the other way. The cloud base shows what happens when air sinks beneath a storm anvil and the surrounding air is dry enough for the cloud edges to change shape quickly.

The World Meteorological Organization classifies mammatus as an accessory cloud feature, not a cloud type on its own. They attach to larger parent clouds, most often cumulonimbus clouds with a broad anvil top. As the air under the anvil becomes unstable, rounded pouches can form instead of a flat cloud base.

What is cloud pouching?

Diagram of sinking air beneath a thunderstorm anvil.

Cloud pouching is the rounded, hanging pattern seen on the underside of the cloud. The pouches form where subsiding air pockets become denser than the air around them. That negative buoyancy helps the cloud base sag into separate lobes rather than remain smooth.

Cloud pouching can look dramatic, but the shape itself is the result of local air movement and moisture changes. The same thunderstorm can produce a ragged base in one area and mammatus pouches in another, depending on the air around the anvil.

The Role of Evaporative Cooling in Pouch Formation

The role of evaporative cooling in pouch formation is central to the cloud’s shape. When precipitation falls from the storm anvil into drier air below, some of it evaporates or sublimes. That process uses heat from the surrounding air, which cools the pocket and helps it sink.

Ice crystal sublimation matters most near the top of a thunderstorm. In the anvil, much of the cloud is made of ice crystals. When those crystals change directly into water vapour, the cooling can be strong enough to make the air denser than its surroundings. The sinking motion then drags the visible cloud downward into the familiar pouch shape.

If you want the bigger picture, see our guide on how do thunderstorms form.

Why cumulonimbus incus clouds matter

Comparison of storm updrafts and the sinking air linked to mammatus.

Close-up view of mammatus lobes beneath a storm anvil.

Mammatus clouds are most often seen under a cumulonimbus incus, the anvil-shaped top of a mature thunderstorm. This anvil forms when the storm reaches the upper troposphere, where rising air meets stronger stability and can spread outward more easily than it can keep rising.

That spreading anvil often pushes into drier air around the storm. Under those conditions, the underside of the cloud can show the pouching pattern that marks mammatus. This is why the shape often appears along the edge of a severe thunderstorm structure rather than in ordinary fair-weather cloud.

For the storm side of the picture, see our guide on how do cumulonimbus clouds form.

How the updraft and downdraft work together

A severe thunderstorm structure depends on both updraft and downdraft. The updraft feeds the storm by lifting warm, humid air. The downdraft brings cooler air back down and spreads it outward.

Mammatus clouds are linked to sinking air aloft, not to the downdraft that reaches the ground. They can sit beneath the storm anvil while the main storm still produces heavy rain, hail or strong gusts elsewhere. The cloud itself does not cause those hazards, but it can appear in the same storm system.

What the storm environment tells us

The Bureau of Meteorology uses terms such as convective clouds, convective instability and severe thunderstorm warnings when discussing these storm systems. Mammatus usually form in an environment where buoyancy has changed sharply with height, and where air near the storm top is no longer rising in a smooth column.

That shift in buoyancy is why the top of a storm matters. When the air at the anvil level becomes more stable, the storm spreads sideways. When the air below the anvil is dry enough, the underside can break into pouches. This mix of convective instability below and stronger stability above helps explain why mammatus and anvil clouds often appear together.

FAQ

Do mammatus clouds mean a tornado is coming?

No. Mammatus clouds can appear near severe thunderstorms, but they do not mean a tornado is about to form. They are a sign of storm structure aloft, not a direct warning of tornado development at the ground.

Can mammatus clouds form without a severe storm?

Yes. They are most often linked with cumulonimbus incus clouds, but they can sometimes appear after a storm has weakened or moved away. The pouches can remain visible even when the most intense weather is no longer overhead.

Are mammatus clouds made of water or ice?

Mostly ice, especially near the anvil of a thunderstorm. Some water droplets can be present, but ice crystal sublimation is a major part of the process that shapes the cloud underside.

What is the main reason mammatus clouds look so smooth?

Evaporative cooling and sublimation change the air temperature quickly, which helps the cloud base settle into rounded lobes. The smooth look comes from the way the air sinks and the cloud material condenses around it.

Should I watch for warnings if I see mammatus clouds?

Yes, if you are near a thunderstorm. Check the Bureau of Meteorology warnings and radar, then move indoors if thunder is nearby. Mammatus clouds are not a warning on their own, but they often sit near active storm systems.

Sources

1. International Cloud Atlas (cloudatlas.wmo.int)

2. NOAA weather and atmospheric science reference (repository.library.noaa.gov)

3. Mamma | Atlas international des nuages (cloudatlas.wmo.int)

4. Cloud classification summary (cloudatlas.wmo.int)

5. A New Edition of the International Cloud Atlas (wmo.int)

6. 'Mammatus' cloud show captured by lucky scientists – Australian Antarctic Program (News 2020) (antarctica.gov.au)

7. Bureau of Meteorology weather reference (bom.gov.au)

8. Spectacular mammatus or 'mammary clouds' form over Sydney as storms swirl nearby (abc.net.au)

Last verified: 2026-08-02