There are lots of things outside weather and inside conditions can do to any instrument. Wood instruments are most susceptible. If the humidity is too low, the wood will shrink and when it's high it will swell. Extreme cold and heat will have adverse affects as well. Winter can be especially problematic . All these conditions can first be heard in how off the tuning is and how hard is to tune the instrument properly. The intonation will be off. If the conditions continue for an extended period of time cracks can occur and glue joints will fail. You may even notice note buzzing as well.
As an example. A student of mine had played at a farmer's market in the late fall. It was fairly chilly out and the guitar (a Taylor) had been in the cold for 4-5 hours. The day after was her lesson and she complained the guitar could not be tuned and she didn't know what to do, fearing the instrument was a loss. Tuning was a pain and only an approximate tuning could be done. I told her to leave the guitar in its case for a few days and check it again. Thankfully, after those few days of resting the guitar acclimated and all was fine.
Rain will be a big issue since the humidity levels around the guitar will be 100%. For the most you want the humidity levels to be around 40%-50%. In the winter I use both damp-its and a room humidifier. I also have a hygrometer to monitor the actual levels.
For the most, a short period of exposure to any extreme will be fine IF you manage it well. Like I said, winter can be a problem so keeping a damp-it in the guitar and watching the gauge on it will help. Direct sunlight is an issue and will heat up and dry a guitar out in no time - especially if its a dark or black color.
I've been playing outdoors for years and not had any real problems. It seems that I would probably have to retune but nothing else. I have worked in the rain but never let my guitar get soaked..... I draw the line there.
What are signs that the guitar is getting too much, or not enough humidity? Thanks for the comments by the way, What I'm getting for your points is, that its pretty much the type of guitar you have, are their actually guitars that are specifically designed for outside weather? Mike, when you play outside, doesn't your Guitar lose its color? I was always afraid, the suns really hot rays that are banging on my guitar can get it cracked or something, even though I never heard of the guitar , it just looks like it should.
I also noticed that guitar dealers, in their store, they have their really expensive, mostly Taylor and Martin guitars in rooms because of the humidity levels. That's why I thought that playing outside can damage the guitar in some way.
First signs: Bad intonation is the first sign. Neck bowing.
These can be easily corrected be re-/de-humidifying the guitar. BUT, you have to catch this early on because...
Over time: Cracks appears. Failed glue joints. Neck separating from body (failed glue or warping)
These are permanent issues that will require repair ($) after the guitar is re-/de-humidified.
Winter is the worse time for acoustic instruments because the air becomes very dry, especially indoors. So it's best to monitor the humidity of the guitar and the room(s) they are stored.
I use Damp-Its in every guitar. Damp-Its (and other humidity devices for guitars) have a hygrometer built-in to tell you what the humidity level is. In general, the humidity level for instruments should be between 45%-55%.
In the winter I also use room humidifiers because untreated, the humidity in any room can get extrememly low. I found this out when I purchase a hygrometer gauge from Radio Shack. The room was 17%! So, by keeping the room humidity up and using a humidity device for the guitar will help immensely.