When organizing community gardening events, weather sometimes gets in the way. In most cases a little rain or cold doesn't hurt and the event goes on with dedicated gardeners doing their thing in the rain. In some cases though the weather is too severe, as is the case with thunderstorms or high >30mph sustained winds.
In terms of precipitation alone, how much rain is too much to garden in? For example, <5mm seems like fair weather to garden in, 10mm is probably OK too for people not too bothered by the rain, but >15mm seems like it's pushing it to even ask folks to show up. What threshold is used by gardening organizations to decide when to cancel an event due just to rain?