In reply to ianstevens:
> a long, long time - I'd be amazed if they come in this side of the next ice age
Hopefully this was said in jest and wasn't meant to be taken seriously, otherwise it's very misleading.
They don't take all that long, given the right conditions and assuming good groundwater/ surface flow prior to the cold spell. In the last, ahem, 'ice age' (2009-2010 and 2010 -2011 seasons), it only took about 12 days of severe frost (-10C to -15C overnight temperatures), with correspondingly low, ideally consistently sub-zero daytime temperatures, to bring Maesglasau into excellent, fat condition. It can be in climbable condition, but lean and with holes, after 8 days or so, when the RH variation is normally taken on pitch 3.
Pistyll Rhaeadr takes longer, on account of verticality and flow amounts I suppose. Quicksilver can take a good 3 weeks, but Chandelier has formed after 12 days or so of severe frost.
The situation's broadly the same with regards to Pistyll y Llyn, Rhaeadr Nant Ddeiliog and Ffrwd Fawr further south. The latter takes a good 3 weeks; both others about 10 days or so of severe frost.
If the current forecast's correct, I don't think it's going to be cold enough next week.
HTH
Dave