If you were employing someone to clean windows the traditional way and paying them £5.10 nmw per hour would you expect them to work in the rain
If you're prepared to work in the rain; then your staff will follow your lead. Don't make a fuss about it - or they'll think negative thoughts; just do it. I'd also prepare them with a couple of set answers on the off-chance they get quizzed about it by a customer. Not that customers do ask (I've not had one), but to give them the confidence to get stuck in there.
There's plenty of other occupations where working in the rain is the norm.
Personally, I won't work in the heavy stuff; but will in the lighter rain and drizzle.
And it rarely rains heavily all day.