Dunk tank and dropping temperature problems

[mykl]

I keep a thirty gallon pail for watering my orchids - especially the vandas.
It has proven quite successful all summer, but now that the temperatures are starting to drop, I'm concerned how long I can continue this practice into the winter.
Being a fairly large volume, the water can't cool all that much on any given day, and in zone 9B I don't expect very many days of frost/ freeze.

At what point would the temperature cause the plants to shock?
THNX