(1) A "condensation funnel" extending from the base of a towering cumulus or cumulonimbus cloud, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado).
A "condensation funnel" is a tornado, and not a funnel cloud, if it is either
A) In contact with the ground, and seriously sucking soil, or
B) A "debris cloud" or "dust whirl" (Wheeeee!) is visible beneath it. (Which pretty much means that it is in contact with the ground, eh? Kinda?)
(2) A tornado-like extension hanging down from the base of a towering cumulus or cumulonimbus cloud, and part of a rotating column of air not in contact with the ground. Once it is in contact with the ground or a debris cloud or dust whirl is visible beneath it, it is defined as a tornado. (Totally clear yet?)
(3) The supreme being's suckstraw, juicer of the gods.
Stay away from this one if you can. Don't go up to the railing for a peek.
This is the closest thing we have to an atmospheric black hole, and if it doesn't kill you the flying anvils might. The ones you see swirling around up there. Yes, those are really anvils. Didn't I just tell you not to try getting closer? Man, how dumb are you?
If you had any sense at all you would have turned your bung end toward it and begun moving away smartly, with vehicular assistance if at all possible. Which it usually isn't, if you are in the middle of a backpacking trip, which is why, every now and then, some J. Random Backpacker sort of puckers out of existence entirely, maybe to re-enter reality space near the far edge of a neighboring county, or maybe not. Usually no one cares, but...
The good thing about most funnel clouds is that, like, first, they're clouds, so you can see them, and second, they have this distinctive funnel shape so you can tell them from the other clouds that look like clowns or horsies and such. "Hey, I've heard all about tornadoes," you might say. But no. These are not tornadoes.
The sucking sound is the same, and the flying anvils and all, but funnel clouds are called that because they don't touch the ground. If one touches the ground, then you have a tornado along with all that dust and whatnot that gets in your eyes (in addition to the anvils).
But tornadoes can be surprisingly stealthy. Even invisible. Tornadoes don't need condensation to be tornadoes, which is the thing that makes funnel clouds visible. Which in turn is odd because most tornadoes begin as funnel clouds, in thunderstorms. Oh, life — so surprising at times, innit? Maybe it's time for another beer.
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Me? Still spinning in my grave.