Andrew Burgess wrote:

An indoor outdoor digital thermometer has a long lead on the
outdoor sensor, and it should be waterproof, so just tie it to a measuring
pole and submerge to a depth and read.

I would expect it to be water resistant not waterproof.
I'd add a coat of epoxy...

It's just a metallic probe - there's no electronics in it. Adding a coat of
epoxy will insulate it (my outdoor thermometer probe is currently hidden
under the new wall of a building addition, and it reads 40C - so it's not
just a matter of waiting longer for the outside temperature to get through
the epoxy).
--
derek