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I have read and heard that the mammatus cloud formation presents a danger to aircraft and is often associated with funnel clouds and tornadoes. How is it formed and why is it dangerous?

Mammatus
Source: Wikipedia

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    $\begingroup$ Would probably get better answers in Earth Science. $\endgroup$
    – Carey Gregory
    Commented Aug 2, 2016 at 23:50
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    $\begingroup$ I want aviation-specific answers so I disagree. Thanks for the comment though. $\endgroup$
    – ryan1618
    Commented Aug 2, 2016 at 23:54
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    $\begingroup$ Anedoctal: The only two times I has seen this cloud formation was the only two times I witnessed tornados forming. Also try to find a video, the strong winds gives that clouds a terrifyng "burbling". I'm pretty sure no one wish to go straight or under it $\endgroup$
    – jean
    Commented Oct 10, 2017 at 19:33

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Nobody knows exactly how mammatus clouds are formed, it's been studied and there are several theories about it but none are accepted. They are most often seen below anvil clouds, ie cumulonimbus clouds with extremely powerful updrafts. One thing that the theories all agree on is that where they are formed on the underside of cumulonimbus clouds there are huge shifts in temperature, moisture and pressure within a short space of time. Although these clouds can form under other types of cloud they are mostly seen under the most powerful of storms.

So why are they dangerous?

  1. They are usually made of ice: ice accretion is dangerous, especially for light aircraft without de-icing systems
  2. They are almost always associated with severe low level wind shear. Wind shear is sudden changes in wind direction which is dangerous for aircraft. You can suddenly find that a 30kt headwind is a 30kt tailwind so you've lost 60kt of airspeed, this is especially dangerous during takeoff and landing
  3. They are almost always associated with severe convective activity, ie very strong updrafts and downdrafts which are dangerous for aircraft. You could get sucked up thousands of feet or pushed down as well, and there's be nothing you could do about it
  4. They are almost always associated with severe turbulence which is also dangerous for aircraft. Severe turbulence is at best extremely uncomfortable, at worst can cause a loss of structural integrity

So mammatus clouds are usually an excellent sign of an area to be avoided.

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    $\begingroup$ I was in a turboprop approaching Antigua for a honeymoon with my wife (we were passengers not pilot). And while I've experienced turbulence before had never been exposed to a downdraft. It was terrifying, we dropped a good 20 feet in the space of three seconds. And from that day on, I've made certain to keep my seatbelt buckled while in my seat (I actually lifted out of my seat during the drop.) $\endgroup$
    – Michael Brown
    Commented Aug 3, 2016 at 19:59
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    $\begingroup$ 20 feet / 3 sec = 400 feet / minute $\endgroup$
    – user13148
    Commented Aug 4, 2016 at 2:00
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    $\begingroup$ Maybe he meant 200 feet. 20 feet's nothing. $\endgroup$
    – GdD
    Commented Aug 4, 2016 at 7:31
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    $\begingroup$ You offer several assertions here, all without citation, several of which are questionable, and at least one of which is definitely false: these cloud formations are generally not made of ice, as they are generally a low altitude, warm weather phenomenon. Their association with severe weather is true, but not directly. These exact formations themselves may be benign, though found in the vicinity of severe weather. $\endgroup$
    – J W
    Commented Aug 15, 2019 at 18:44
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Mammatus clouds are very gentle downdrafts, typically seen beneath the anvil or along the flanks of a thunderstorm, though they may also be occasionally seen under stratus and altostratus clouds. The reason for the udder-like shape is not well understood, though there are several interesting theories. Aircraft flights through the udders has described them as "rain-filled sacks" with very light turbulence. They are not a sign of heavly turbulence or severe weather, though they are often found in the aftermath of a severe thunderstorm (or any thunderstorm). Their most distinctive characteristic is their attractive appearance.

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Based on the fact that mammatus clouds are associated with crazy air currents, it would very easily throw an airplane out of control.

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    $\begingroup$ This doesn't really answer the question. $\endgroup$
    – Simon
    Commented Aug 3, 2016 at 5:57
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    $\begingroup$ It says the same thing as the newer answer, just without any details. $\endgroup$
    – fooot
    Commented Aug 3, 2016 at 14:25

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