Since occasionally we have had brides with beautiful ceremonies in inclement weather, here are the limitations of our instruments (and of most wooden instruments).
Since our instruments are made of wood, we will need cover in the case of rain or drizzle. Rain/drizzle can damage our instruments. We can still play from a porch cover... or other covered area provided.
In extreme humidity (the only case I’ve ever seen that made this happen was a very dense fog), it is possible that our instruments (or any wooden instrument) may become difficult to play.
Below about 50 degrees, a heater will be required to keep our fingers nimble enough to perform. Also, our instruments may go out of tune at these temperatures. Below 40 degrees, playing may be impossible without two heaters provided by the client, and the duration of the ceremony (including seating guests) should be no more than 30 minutes.
Direct sunlight can warp the harp, and so partial shade (at least) is necessary... especially during the summer.