First off make sure you understand how to ground them, especially in wet inclement weather. There is a recent thread on grounding &
some Pinned Sticky's if you haven't read them. Just to cover various scenarios: are you running it as a "Self-Derived System" (Standalone) versus powering a "House/Shop" through inlet box or transfer panel to electrical panel?
For "Enviromental Conditions" the answer is... Yes, they can be run in rain, sleet, snow, dark of night etc. as is. However, if you plan on making them last for a long time its best to run under cover or build a generator enclosure. Moisture & Sun exposure over time can weather the unit causing rust, corrosion & UV will take its toll on gauge faces, wiring insulation & paint etc. Also, ambient temperature swings induce tank breathing which and bring moisture into fuel system. Rich "Uncle Sam" could afford to send his generators in for a "Reset" makeover or auction, I can't.
As for me, I'm a Big Chicken when it comes to AC & standing in a puddle of water, snow, or ice even when running my gensets on covered trailers during inclement weather tied into house generator inlet box & properly grounded. I have a 2' x 3' foot rubber mats (Horse stall mat remnants) that I stand on in rain/sleet/snow while messing with control panel etc. It's my security blanket!
