The promoter has a duty to keep you safe. Start with the presumption that you are never required to perform under conditions that place you in physical danger (unless you agreed to those conditions and waived the right to later sue about it). Contract or not, the promoter must act in good faith, in a reasonable manner, and at the standards of the industry. It would not be reasonable for a promoter to require a performer to do something dangerous or illegal (yes, we know there are plenty of bad promoters ... )
Get it in writing. That said, it's still best to get the standards on paper so that way the purchaser/promoter knows where you're coming from. Typically this is done with a statement in the performance agreement rider. We looked at a range of them (from DJs to chamber musicians) and put together a sample provision with a lot of optional choices.
Outdoor Performance: In the event of outdoor performances (or "rain or shine" events):
[ ] Artist's compensation shall in no way be affected by inclement weather.
[ ] Purchaser shall provide overhead shelter for setup and performance area.
[ ] Artist and Artist's equipment cannot be subjected to any form of precipitation and must be protected from direct sunlight when the temperature exceeds __ degrees F.).
[ ] Artist reserves the right, in good faith, to stop or cancel the performance should the weather pose a potential danger to Artist, Artist's equipment, or audience. Every effort will be made to continue the performance. However, safety is paramount in all decisions. Artist's compensation will not be affected by such cancellation.
[ ] In the event of circumstances that present a threat of injury or harm to Artist or Artist's equipment, Artist reserves the right to stop the performance. If the Purchaser is able to resolve the situation in a reasonable amount of time (maximum of __ minutes), Artist shall resume performance in accordance with the original terms of this agreement. Purchaser shall be responsible for payment in full, regardless of whether the situation is resolved or whether Artist resumes performance.
This will help ... and in addition, ideally, you'd want a person with your group to be knowledgeable about stage construction and/or electrical setups to verify and document your position. We sure wish we had this provision in our contract when we performed on the side stage at Lollapalooza back in the day. The stage was hotter than a George Foreman grill