Welding in the rain is not illegal, but you risk getting electrocuted while welding in an environment with high moisture. It is a bad idea.
Precautions for Job Site Safety Hazards in the Rain
Gas Monitor 101: Everything You Need to Know About Gas Monitors - Premier Safety
Lightning Safety When Working Outdoors
Warning Welding Area Eye Protection Required Sign
OSHA 30 Construction Final Test Answer Key, PDF, Occupational Safety And Health Administration
Full Brim Vented Hard Hats Construction OSHA Safety Helmet 6 Point Ratcheting System, Meets ANSI Z89.1
OSHA Welding Safety Requirements and Checklist
When welding, the goal is to not burn the workplace down
Safety reminder: OSHA's tips for working in cold weather - SMART Union
Preventing Falls From Slips & Trips” – Infographic
Hot work handout J. J. Keller® Compliance Network
Winter Safety Tips for Workers
When welding, the goal is to not burn the workplace down
Safety Tips for Welding in Wet Conditions – Baker's Gas & Welding Supplies, Inc.
Welding Sign, OSHA Danger Sign, 10x7 Inches, Rust Free .040 Aluminum, Fade Resistant, Indoor/Outdoor Use, Made in USA by Sigo Signs : Industrial & Scientific