By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the July 2024 issue of EAA Sport Aviation magazine.
It was years ago when a group of us instructors and pilots stood outside our vehicles on the outskirts of a local airport, gazing at the clouded sky along the Front Range outside of Denver, Colorado. To the casual observer, the underside of the overcast appeared scalloped — small lobes of menacing gray hanging beneath the base of the clouds. These are known as mammatus clouds, and they rarely bring good news. Further out in the distance, first one tornado and then another dropped down from the looming darkness, threatening unimagined danger and destruction. It was an eerie sight that held our attention captive.
As the scene unfolded, we suddenly heard a small aircraft departing the runway. It seemed incomprehensible to me that someone would decide to depart the safety of the airport in these decidedly dangerous conditions. But there it was, struggling skyward, a lone single-engine Cessna, its pilot seemingly unaware, or at least undaunted. As I continued to watch, a sense of dread came over me. I recognized it. The airplane was one I owned in partnership with another instructor and leased back to a flight school. As I would soon discover, an instructor had decided to take a potential student for an introductory flight. “Unbelievable,” I thought. Tornadoes, mammatus clouds, and what appeared to me as clouded judgment.
One late afternoon last fall, I had another opportunity to gaze upon a magnificent display of mammatus clouds, this time passing through the Florida sky. Fortunately, I wasn’t flying at the time. I was simply roaming a nearby beach. A line of particularly nasty weather with heavy rain, lightning, and thunder along a fast-moving front had just passed, and my wife and I had stopped to watch the spectacle of nature unfolding before our eyes. Seeing these clouds brought back memories of that day in Colorado when a similar display of nature was punctuated by that little single-engine Cessna.
Mammatus clouds are not exactly uncommon. We would occasionally see mammatus clouds along the Front Range of the Rocky Mountains. We were well warned that they — along with a few other specialized cloud types — demanded serious respect and from which one should maintain a considerable distance. Not everyone, as I had found, was concerned about the potential risks such signposts heralded. But as I peruse the contemporary literature on mammatus clouds, it seems there is a bit of confusion not just about their nature and the dangers they may represent, but the very physics of their formation.
Matters of Formation
For those unfamiliar with cumulonimbus mammatus clouds, the name “mammatus” is derived from the Latin meaning “udder” or “breast.” They appear as lumpy masses or pouches protruding from the bases of a variety of different cloud types, including cumulonimbus, cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus clouds.
What exactly causes mammatus clouds to form is a difficult question to answer. Researchers and meteorologists have suggested a number of mechanisms. According to an article that appeared in the October 2006 issue of the Journal of the Atmospheric Sciences, the hypothesized mechanisms include “anvil subsidence, subcloud evaporation/sublimation, melting, hydrometeor fallout, cloud-base detrainment instability, radiative effects, gravity waves, Kelvin-Helmholtz instability, Rayleigh-Taylor instability, and Rayleigh-Bénard-like convection.”
Unfortunately, much of this terminology is beyond the scope of my limited knowledge and understanding. According to several sources, the characteristic pouches or lumps on the underside of these cloud types are caused by pockets of cold air sinking from the base of the clouds. Exactly how and why this occurs is where the mystery of the mammatus cloud persists.
According to sources at NOAA — perhaps one of the most authoritative sources on this and other meteorological phenomenon — mammatus clouds are often found beneath the anvil of large cumulonimbus clouds, as well as other types of clouds, where they appear as a pattern of pouches hanging from their base. The pouches are indeed created as cold, moist air sinks from the cloud mass into the warm, dry air below — pretty much the opposite of how most clouds are formed. This may occur, as one report suggests, when the top of the anvil has been blown off by upper-level winds, and the updraft stage of the storm has ended, allowing the cold air to sink.
Occasionally, we may hear or read that mammatus clouds are associated with, or perhaps caused by, tornadoes. While this occurrence is sometimes reported, there doesn’t seem to be any definitive links between the two phenomena to suggest tornadoes always form with mammatus clouds. However, there may be evidence that when mammatus clouds are associated with powerful cumulonimbus clouds and thunderstorms, the risk of encountering a tornado is higher. This might give us a bit to think about. There’s not much point to lurking about the sky where a tornado might appear. And as we all know, the clear air beneath the anvil is a good place to avoid since damaging hail can often be thrown into the clear air below a thunderstorm’s anvil.
Physical Characteristics
It’s difficult to estimate the size of the mammatus formations, but reliable sources report the individual lobes can range from half a mile to 2 miles in diameter, extending roughly a third of a mile beneath the cloud base. The appearance of the mammatus lobes can range from translucent to opaque, depending perhaps on their makeup, size, and the lighting in which they are observed. The lobes are usually made of ice, but they can also be mixed with water or made entirely of water. Each lobe may exist for as long as 10 minutes, but the cluster of lobes can remain for 15 minutes to a few hours in duration, and can extend in some instances for hundreds of miles.
A Clear Danger
If the precise mechanics of mammatus cloud formation seems a bit murky and complicated, the potential dangers that they represent are clear enough. Mammatus clouds can occur before, during, or after severe weather such as the thunderstorms mentioned above.
While not exactly uncommon, they are reported to occur roughly 10 times as frequently in summer months than in the winter. In addition to tornadoes, pilots have reported such dangers as severe turbulence, icing, heavy rain, and wild squalls in the vicinity of mammatus clouds. Depending on our direction of flight relative to the clouds, we might be moving toward dangerous conditions or away from them. For those of us equipped with uplink weather, or those lucky enough to fly with airborne radar, it may be possible to determine where the danger exists, but the best bet might be to steer clear of the clouds in favor of the clear air and avoid flying through or directly below the mammatus clouds.
A variety of clouds can signal significant danger, including the rotor cloud, virga (especially in hot and dry conditions), and our old friend the cumulonimbus cloud. And just as we steer clear of these meteorological marvels, we might want to give mammatus clouds a wide berth as well, especially when associated with cumulonimbus clouds.
Mammatus clouds may be a magnificent sight, but like many magnificent displays of nature, they can be a sign of serious danger for those of us wise enough to read the signs.
Robert N. Rossier, EAA 472091, has been flying for more than 40 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.