Water leaves an annular gap as a thin film, and surface tension pulls it into a closed dome that lands on a circle. It is optically beautiful — the film is thin enough to show interference colour in raking light — and it is the most fragile effect in the catalogue.
The gap is typically 1 to 3 mm and is adjustable, because the dome geometry is set by the ratio of flow to gap rather than by pressure alone. A 0.2 mm error around the annulus, or a single hard water deposit on the lip, shows up as a tear in the dome. In hard-water regions we specify a softened top-up supply for bell nozzles specifically, and we tell the client that they will be descaling the lip monthly.
It fails at 2 m/s. Not degrades — fails, the dome collapsing on the windward side and slapping down inside its own landing circle. It belongs indoors, in an atrium, or in a genuinely sheltered courtyard, and anyone who offers you one for an exposed plaza has not measured the plaza.