Updated: May 20, 2019
Mammatus are likely one of the more majestic and distinctive clouds you will ever witness. They are actually not all that rare. When they occur near sunrise or sunset, they can almost look unreal. So it's not surprising that they have appeared in many paintings over the last few centuries and are common to see plastered all over social media. Just go to #mammatus on Twitter and you'll certainly get your fill of amateur photographers that have captured these clouds. But what causes the distinctive shape to these clouds and what do they mean to you as a pilot?
Mammatus get their name from the Latin for mamma (meaning "udder" or "breast"). They are most commonly observed on the underside of cumulonimbus anvils so pilots tend to see these as a warning for severe or extreme convective turbulence. But, but they have also been observed under cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus, as well as in contrails. Perhaps the most awe-inspiring mammatus are those associated with pyrocumulus ash clouds from volcanic eruptions as was seen below after the eruption of Mount St. Helens in 1980.
Surprisingly, we still have a long way to go before we can understand the underlying properties of these clouds. There have been very few studies done with regard to these clouds with hanging protuberances, however, there are many untested theories. In the end, published research is very limited and often speculative and typically tangential of the primary objective of the study. Perhaps the most important omission is the lack of modeling of these types of clouds.
Some pilots have been taught these clouds only occur in conjunction with severe thunderstorms. In fact, it is well know that they are not directly related to significant weather events on the ground (e.g., tornadoes, severe hail or strong wind gusts). Consequently, they do not necessarily provide insight to forecasting severe convective storms. Certainly mammatus are observed in association with severe thunderstorms, but they don't appear with all severe thunderstorms.
Based on a 20 year study by a German scientist, mammatus were observed 10 times more often in the summer than in the winter months with a majority of the events occurring between 2 pm and 5 pm. More importantly almost half were associated with cumulonimbus clouds. From the current studies, it is thought that mammatus that occur with cumulonimbus clouds have a different physical process than those that occur with other cloud types making it even more challenging to model their behavior.
Generally it is thought that cumulonimbus anvil mammatus develop when the updraft within the cumulonimbus hits it's equilibrium level (the tropopause) and spreads out. Cumulonimbus anvil mammatus form when there's a strong vertical temperature gradient (either a very stable temperature inversion or a very unstable near superadiabatic layer), strong gradient in moisture, and strong wind shear. Therefore, the ascending air in the anvil is not likely to have the same temperature, moisture, and momentum as the surrounding environmental air.
Cooling due to subcloud evaporation or sublimation is the most common explanation for the hanging mammatus lobes. Essentially, hydrometeors such as ice crystals, snow aggregates, supercooled liquid water droplets, or a mixture of these fall from cloud base into subsaturated air and begin to sublime and evaporate. Evaporation is a cooling process and the air becomes heavy and begins to sink. At the point where the descending air in the lobe is no longer buoyant with respect to the static air, the air in the lobe may return upward and create a rounded shape that we see as the mammatus cloud.
Are mammatus a sign of severe or extreme turbulence? Yes and no. Yes, flying into the deep, moist convection generating these clouds would be unwise. However, aircraft penetrations through the hanging protuberances or mammatus lobes have not been conclusive. Some pilots have reported a smooth ride and others have reported turbulent conditions. For example, Jeffrey Stith of the University of North Dakota flew through two cumulonimbus mammatus lobes over North Dakota using an instrumented Citation jet. The lobes themselves, the were composed mostly of aggregated ice particles with some rounded ice particles. In other words, supercooled liquid water was not observed in large quantities. The main descending portions of the mammatus lobes not very turbulent (approximately 0.2–0.3g, just barely within the range that would be considered light turbulence). Again, while the air within the lobes themselves may be non-turbulent in most instances, the environment that these grow within can often be quite turbulent. Clearly, there is much that remains to be learned.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist